kiran.docx

7/25/2019 kiran.docx

1/45

Glare And Night Time Driving

CHAPTER 1

INTRODUCTION

1.1 CHARACTERIZING GLARE

Glare is a sensation caused by bright light in ones field of view. Glare can reduce

ones ability to see, create feelings of discomfort or both. The term headlamp glare is

defined herewith as visibility reductions or discomfort caused by viewing oncoming

vehicle headlamps or headlamps in rear view mirrors.

Glare reduces seeing distance because it causes light scatter in the eyes, which in turn

reduces the contrast of roadway objects. This effect is known as disability glare. The

greater the intensity of the glare light and the closer the glare light is to where one is

looking, the greater the disability glare will be. isability glare can lead to the following

effects!

ecreasing visibility distance. The distance at which an object can be seen is

known as the visibility distance. This distance is reduced when disability glare

is present.

"ncreasing reaction times. #s the intensity of oncoming headlamps increases,

drivers reaction times to objects in and along the roadway become longer.

"ncreasing recovery time. #fter drivers pass an oncoming vehicle, the glare has a

lasting effect that increases the time it takes for the drivers eyes to recover their

ability to detect objects. uring that time, the visibility distance is reduced and

reaction times are increased.

#s described above, headlamps can produce a sensation termed discomfort glare.

This is the feeling of annoyance or even pain that is possible when viewing a bright light.

$%periencing discomfort can distract drivers from the driving task, cause them to slow

down, and cause drift slightly in their lane.

Department of Civil Engineering 1SVNCE


2/45


1.2 POSSIBLE FACTORS CONTRIBUTING TO GLARE

&ehicle headlamps are necessary for sufficient nighttime visibility and safety, and

headlamp glare has long been recogni'ed as a potential problem. (owever, concerns from

the public about nighttime glare have increased in recent years.

)everal factors associated with recent vehicle and lighting developments may

e%plain the driving publics passion regarding nighttime glare. These include increasing

light intensity levels, headlamp mounting height on larger vehicles, novel headlamp color

appearance, smaller headlamp si'e, variations in headlamp aim on the *.). vehicle fleet,

and an increase in the presence of au%iliary lighting, such as fog lamps. The increasing

age of the driving public may be another factor. )ince all of these factors have changed inparallel, assigning the cause of these concerns to any single factor is difficult.

1.3 VISIBILITY AND GLARE A TRADEOFF

+ith low beam headlamps, there is a tradeoff between visibility and glare,

because the illumination from headlamps that contribute to visibility for one driver can

make them more troublesome with respect to glare for another. resent regulations for

low beams re-uire them to produce sufficient light levels in locations that contribute to

forward visibility, but limit the light levels they produce in locations where they would

contribute to glare to other drivers. ecause these two locations are sometimes the same

due the varying geometry of roadways, the illumination desired for detecting road

ha'ards needs to be limited to minimi'e the adverse effects on other drivers

1.4 ASSOCIATING GLARE WITH DRIVING PERFORMANCE AND CRASH

RISK

+hile greater seeing distance and shorter reaction times have an intuitive

connection to safety, it is very difficult to directly relate crash risk to nighttime glare. "n

part, this is due to the multiple driver, vehicle, and environmental factors that contribute

to a crash. "t is also due to the difficulty that crash investigators have in determining the



3/45


role of glare and low visibility in crashes. (owever, nighttime glare cannot be ruled out

as a causal factor in crashes. "n addition to influencing safety, glare may cause some

drivers, particularly older drivers, to restrict their nighttime driving.

1.5 NIGHT TIME GLARE AND INCREASED RISKS TO DRIVERS ON TWO

LANE HIGHWAYS

Two/lane highways, as compared to multi/lane roadways and e%pressways, may

present the worst/case scenario for nighttime glare. The reasons for this include!

0ower light levels. 1ighttime glare on unlighted roadways is more problematic

because the visibility of an object is influenced by the overall light level. )incelight levels tend to be lower on two/lane highways, objects along these roads are

less visible in the presence of glare. "n addition to the reduced visibility, the lower

light levels on two lane highways increases discomfort glare because the relative

brightness difference between headlamps and the roadway background is higher

on these roads.

2ncoming traffic closer to drivers line of sight. ecause of the closer pro%imity

of oncoming glare on two lane roadways, the beam pattern is directing more light

at oncoming drivers, which produces more scattered light in their eyes. 3omple% roadway geometry. Two/lane highways often have complicated roadway

geometries, including sharper curves and steeper grades. Thus, drivers on two

lane roadways may be e%posed to a higher range of oncoming headlamp glare

than on multilane roadways.

0ess restricted roadway access. The greater potential for ha'ards to be found in

any number of locations along the roadway increases the comple%ity of the

driving task and makes glare more problematic and arguably less safe than it

would be under easier driving situations.

4ewer roadway markings. 5oadway markings in and of themselves have no direct

impact on the amount of light that oncoming headlamps produce toward a drivers

eyes. (owever, they make the driving task easier by providing visual guidance

about the geometry of the roadways and other roadway conditions.



4/45


3loser pro%imity of pedestrians. edestrians are not always easily seen. +hile

they may not be found fre-uently along two/lane roadways, they are in closer

pro%imity when they are encountered on these kinds of roads. The difficulty

seeing them at night can increase the difficulty of nighttime driving, and

therefore, increase the impact of glare on both visibility and comfort.

1.! NIGHT TIME GLARE AND INCREASED RISKS FOR OLDER DRIVERS

The visual system changes as people age, resulting in differences that are

important in the conte%t of nighttime driving and headlamp glare. These differences can

be categori'ed as optical and neurological.

#s people age, less light enters the eye due to smaller pupils and thicker, less

transparent optical media. This latter effect results in more scattered light in the presence

of a glare light for older drivers. This increased scatter results in lower perceived contrast

of a potential ha'ard against its background for an older driver than for a younger one.

2lder drivers also take longer for their eyes to recover their sensitivity after being

e%posed to glare.

The three major retinal neurological problems associated with aging are macular

degeneration, diabetic retinopathy, and detached retina. Glare is particularly problematic

for older drivers who have one or more of these neural anomalies, even in early stages of

disease development.

1." FACTORS THAT CONTRIBUTE TO GLARE AND CRASH RISK AND

CHANGES IN DRIVING PERFORMANCE

The following design and operational factors are considered as they affect glare

and associated risks.



5/45


(eadlamp mounting height. (igher mounting heights may produce improved

visibility for drivers but could also result in higher light levels at the eyes of

oncoming drivers, increasing their disability and discomfort glare.

0amp aim. "mproperly aimed headlamps can increase glare and cause objects to

remain invisible to oncoming drivers for longer durations. This effect is

intensified when headlamps with higher mounting heights are improperly aimed.

"mproper headlamp aim can also reduce visibility for a driver.

(eadlamp beam distribution. (eadlamps are optically designed to direct different

intensities of light towards different directions. 4ederal regulations set minimum

and ma%imum intensity levels that provide some control of the beam pattern to

limit glare and provide light for seeing the roadway environment. )ome locations

within headlamp beams are not regulated in terms of allowable light levels. "f

headlamps produce high light levels in these locations, oncoming drivers might

e%perience glare.

(eadlamp color. 5esearch indicates that people e%perience more discomfort when

e%posed to bluer high intensity discharge headlamps than when they are

e%posed to yellower halogen headlamps 6halogen headlamps are found on most

vehicles in the *nited )tates7.Thus, color can affect discomfort glare. 1o

evidence e%ists to link the oncoming headlamp color to visibility reductions.

(eadlamp si'e. +hile lamps of different si'es are statistically e-ual when seenfrom far away, smaller lamps might be more uncomfortable at closer distances.

(eadlamps and windshield cleanliness and condition. (eadlamps that are dirty or

damaged can produce higher light levels to oncoming drivers and also produce

lower light levels on the roadway, reducing forward visibility for drivers who

have dirty or damaged headlamps. +indshields that are soiled or damaged can

scatter light and potentially increase the effects of glare.

CHAPTER 2



6/45


PURPOSE

"n order to address the various issues that relate headlamp glare to driver

performance and risk,

1(T)# has initiated a study including the following tasks!

#nalyses to compare the relative effects of different headlamp characteristics,

such as mounting height, light source type, aim and beam intensity patterns, on

visibility and glare to oncoming drivers along two/lane roadways.

#ssessment of headlamp aim characteristics on a representative sample of

vehicles.

# field e%periment to study visual performance of older drivers following

e%posure to glare light.

evelopment and evaluation of a prototype adaptive headlamp system with the

potential to reduce glare while maintaining visibility by automatically decreasing

the level of light intensity when other vehicles are nearby.

#s the relevant agency within the *.). epartment of Transportation to undertake the

study authori'ed above, the 1ational (ighway Traffic )afety #dministration 61(T)#7

hereby submits the present report as a summary of its efforts to understand the issue of

headlamp glare and to address the instructions provided by 3ongress in )#4$T$#/0* to

study the 8risks associated with glare to oncoming drivers, including increased risks to

drivers on two/lane highways, increased risks to drivers over the age of 9:, and the

overall effects of glare on driver performance.8

CHAPTER 3



7/45


BASIC ISSUES RELATED TO HEADLAMP GLARE

3.1 GLARE

Glare is defined as a harsh uncomfortably bright light. .Glare is a sensation in

response to light that can take a number of forms. #lthough light is necessary for vision,

light ; usually bright light / can also reduce visibility of objects in the field of view

enough to make some objects invisible. #s this report will describe, bright light can

createscatter inside the eye not unlike that e%perienced when looking through fogged

glass or smoke, which makes objects harder to see. 0ight that is much brighter than the

rest of one. "n terms of the four

regions shown in this figure, headlamp beam patterns generally minimi'e light in the

-uadrant corresponding to potential glare for oncoming drivers, while producing



8/45


sufficient light in the other -uadrants to aid in forward and peripheral visibility. 2f

course, the boundaries between regions where light is likely to aid in forward visibility

and regions where light can result in potential glare vary between situations and

individuals. 3hanges in road curvature and in the locations of oncoming vehicles may

happen so that light in any region could enhance visibility of potential ha'ards or could

create glare.

5esponding to a ha'ard in a timely manner re-uires seeing it at distances of hundreds of

feet at most typical driving speeds.? To make objects hundreds of feet away visible,

vehicle headlamps need to produce and project high light levels, particularly in the

-uadrant labeled 8forward visibility8 in 4igure >.

F#$%&' 3.1(R'$#)*+ ,-'&' )*'+ ),* -'/0/+ /&' #' 6) 7)*6%6' 6) 9#+#8##6

/*0 6) $/&' 6) )*7)#*$ 0'&+. :A9#;#6 G-)+-< 2==4>

&ehicles are e-uipped with two beam patterns, typically denoted the low beam

and the high beam. The high beam pattern is designed to produce high light levels in both

upper regions of figure >= because this pattern would create unacceptable glare to

oncoming drivers, the high beam is generally not used when oncoming vehicles are



9/45


present. "n fact, most drivers use the low beam most of the time, even when the high

beam would be more appropriate for forward visibility.ecause the si'e of a headlamp is

generally restricted by the si'e of the vehicles and important aerodynamic factors that

influence a vehicle


10/45


relative to darkness. (owever, this reduction did not occur for all crash types. )ingle/

vehicle, run/off/the/road crashes were not affected by light or darkness, for e%ample. This

study does provide some evidence that good visibility has important safety implications

for some types of crashes. Therefore, by e%trapolation, one can infer that light, either

from the vehicle or on nearby poles, would support visibility and thus, safety

3.4 DIFFERENCE BETWEEN HEADLAMPS ON TODAY?S VEHICLES AND

EARLIER ONES

1ew headlamp technologies are changing the appearance of oncoming vehicles at

night on *.). roadways in several important ways. 1ewly developed headlamps, using

high intensity discharge6("7 light sources produce more light and appear 8bluer8 thanconventional headlamps, and are becoming more common.(eadlamps that produce more

light than earlier ones, but conform to the same standards, can result in beam patterns that

differ greatly from earlier headlamps, particularly in locations that are not regulated to

produce minimum or ma%imum light levels. These patterns often result in wider and

brighter distributions of light on and along the roadway. "n addition to their different light

distributions, (" headlamps last longer than conventional halogen headlamps, often for

the life of the vehicle, making them attractive from a maintenance perspective.

The lighted areas of some headlamps are smaller in si'e, which increases their

brightness. 4or e%ample, a tubular fluorescent lamp might produce the same amount of

light as a clear incandescent light bulb, but because the filament of the light bulb is much

smaller than the surface of the fluorescent tube, the filament is much brighter in

appearance. #dditionally, vehicles themselves have been increasing in height, resulting in

higher mounting heights for headlamps. This can be problematic because the headlamps

patterns of light might not account for the increased mounting height, and therefore will

project a bright part of the beam to the oncoming driver.These brighter headlamps might

be persistently present in rear view mirrors from vehicles located behind a driver for a

long period of time. righter headlamps with different beam distributions can reduce

visibility of objects along the road by increasing the amount and duration of light



11/45


reaching an oncoming driver


12/45


CHAPTER 4

EFFECTS OF GLARE ON DRIVER PERFORMANCE

#n important issue to remember in discussing the effects of glare on driver

performance is that evidence linking headlamp glare and crash risk is difficult to obtain.

# survey of crash reports from several *.). states conducted several decades ago found

that the percentage of accidents that could be at least partly related to headlamp glare was

no more than >D.# recent in/house 1(T)# evaluation shows that an average of :.?D of

nighttime fatal crashes list glare as a contributing factor. #ssigning a specific percentage

of crashes to glare is difficult when considering that potentially at/fault drivers might

claim in police reports that they were impaired by glare in attempts to seem less liable.

There could also be crashes for which glare was a factor, but which were unreported.

Thus, headlamp glare cannot be ruled out as a factor in driving safety. Glare might

also affect the probability of driving at all. 2lder drivers, for e%ample, sometimes report

headlamp glare as a cause for refraining from nighttime driving.

This report summari'es what is known about the links in the logical chain from

headlamp glare to reduced safety. #s discussed in this section, headlamp glare can and

does have measurable impacts on visibility, and may have other behavioral impacts.

0ogically, visibility is an important factor in determining safety,>: and much of the

research on glare and safety has used visibility 6in terms of distance at which potential

ha'ards can be seen, and of how -uickly and accurately they can be seen7 as a surrogate

for safety.

)everal types of e%periments and research studies on glare and drivingperformance are discussed! simulation or calculation analysis studies, in which effects of

glare are predicted using software or other tools rather than directly measured with

human subjects= laboratory studies, in which the e%periments are controlled carefully but

often do not capture all of the realistic conditions directly applicable to driving= field



13/45


studies, which are usually conducted outdoors along mocked/up conditions similar to

those along actual roadways= and naturalistic studies, which are not really e%periments,

but involve monitoring people driving in conditions as close to the 8real world8 as

possible

4.1 HOW DOES GLARE REDUCE VISIBILITY

Glare from vehicle headlamps has several effects. 1ot surprisingly, glare reduces

visibility, not only while the glare light is visible, but even after the glare light is no

longer in the field of view, that is, after an oncoming vehicle has passed by. These two

effects are discussed later in this report.

The eye


14/45


Caking difficult/to/see objects invisible. #ll objects re-uire a minimum amount

of contrast against their backgrounds to be seen= otherwise they will be invisible.

The amount of this threshold contrast differs for every object.?> 4or e%ample, a

small object such as a white golf ball re-uires a higher contrast to be seen than a

larger object at the same distance such as a white soccer ball. "f an object


15/45


4or this reason, vision is initially difficult when just entering a dark area such as a

dimly lighted tunnel from an open roadway on a sunny day, until a few seconds have

passed. 0ight from oncoming headlamps can have the same effect, creating a recovery

time after seeing the lights for the eyes to recover. uring this time, objects can be

invisible or difficult to see.

The recovery time from glare depends upon the intensity of a glare light and the

duration of e%posure to the light. "n a laboratory study sponsored by 1(T)#,

participants< recovery time depended on the dosageof light e%posure, which is defined as

the product of its intensity and the length of e%posure 6duration7.

4.2 OTHER EFFECTS THAT GLARE HAVE WHICH REDUCES VISIBILITY

The discussion above describes the ways in which glare can reduce visibility

6disability glare7 by making objects harder to see, either when a glare light is visible or

shortly afterward. Glare has other effects on people as well. erhaps the best known

effect is discomfort. iscomfort glareis the sensation of annoyance, or even pain, that we

e%perience when we see a bright light in our field of view.+hile it would seem that using

separate terms, disability glare and discomfort glare, to describe the effects of glare is like

splitting hairs,they really are different phenomena. 3onsider driving on a clear day. The

light entering the eyes from outside the car can cause disability glare for seeing the

dashboard instruments, even if it is not felt as uncomfortable. #nd, as will be discussed

below, two glare lights that have the same negative impact on visibility can result in

different amounts of discomfort.

iscomfort is most commonly studied in various e%periments by asking subjects to

provide a numerical rating using a subjective rating scale. The following is the mostcommonly used scale used in headlamp glare research 6known as the e oer scale7!

>. *nbearable

A. isturbing

?. just permissible



16/45


F. satisfactory

9. just noticeable

2ther types of measures of discomfort glare have been used in research studies.

5esponses like pupil si'e, muscle tension around the eyes, and even brain waves have

been measured in attempts to replace subjective rating scales, but none have been

satisfactory or consistent in the results yielded. #sking people to give numerical ratings

can however produce very consistent and repeatable results if e%periments are conducted

properly.

rivers are much more aware of how a glare light makes them feel than of how much

it might reduce their visibility. )ome research has e%plored how important the sensation

of discomfort is, even when not necessarily related to visibility while driving.

$%periments that measured visibility showed that subjects< ratings of discomfort or of the

-uality of lighting 6in terms of its ability to support visibility7 rarely agreed with their

actual visual performance.FA "n the 1(T)# supported study that showed that the dosage

6the product of intensity and duration of light e%posure7 of glare e%posure determined

recovery times. )ubjects< discomfort ratings were related to the peak intensity of the glare

source rather than the duration or the dosage.>H # glare light with a lower intensity for a

longer time resulted in e-ual recovery times as a glare light with a higher intensity for a

shorter time when the overall dosage was the same. (owever, subjects rated the higher

intensity light as more problematic, even if the dosage was lower.

erhaps a more important -uestion about the potential importance of discomfort glare

relates to whether discomfort glare causes people to change their behavior while driving.

#s discussed below, 1(T)#/sponsored researchAF has confirmed other workA?

demonstrating that the bluer color of (" headlamps makes them more uncomfortable

than conventional halogen headlamps. 2ther research supported by 1(T)# indicates that

drivers might look at oncoming (" headlamps for longer durations than conventional

headlamps, resulting in a larger dosage, which reduces visibility.



17/45


# 1(T)#/sponsored naturalistic study showed that drivers tended to slow down

following e%posure to oncoming headlamps.$vidence also e%ists that some drivers drift

slightly in their lane when e%posed to oncoming headlamps 6with about as many drivers

drifting toward the edge of the road as toward the center of the road7. F9 2ne could argue

that slowing down actually increases safety, because it is an appropriate response to

reduced visibility of the roadway.

(owever, glare that reduces visibility of the roadway can also make hard/to/see

objects, perhaps including pedestrians along the edge of the road, invisible. "f drivers

were not aware that such objects are not visible, they might not adjust their driving

behavior to reflect the inability to see these other objects. The safety implications of

slight drifts in either direction are also unknown, but again may reflect a loss in visibility.

1evertheless, the evidence is beginning to show that glare from headlamps can affect

driving behavior in reliable ways. $%periencing discomfort glare over longer periods of

time 6such as during a several/hour driving session7 might affect driving behavior

differently, but the evidence for this is contradictory

2utside of the headlamp glare area, many studies of the effects of fatigue, FH

stress,FE,9:,9>,9A restricted visibility9? and distraction9F on driving behavior have been

conducted. These studies have measured driving responses such as driving speed,

following distance behind other vehicles, brake pedal pressure when stopping, lane

position, eye blinking, and changes in steering wheel position. #gain, some of these

responses are easier to relate to potential driving safety, such as eye blinking, which

presumably means a driver


18/45


EFFECT OF GLARE ON TWO LANE HIGHWAY

Glare is an inherent aspect of driving at night because of the small si'e and high

brightness of headlamps. 4or a number of reasons, two/lane highways, where one lane of

traffic opposes another lane of traffic, present some of the worst conditions for nighttime

glare. These reasons described subse-uently include!

0ight levels can be lower than on other types of roadways

2ncoming traffic is more likely to be closer to a driver


19/45


The sensation of glare is much larger on unlighted roads because one of the

factors contributing to discomfort is the ratio in brightness between a glare light and the

surrounding area ; the greater the difference the greater the glare.Thus, seeing a pair of

oncoming headlamps during the day or on a lighted two/lane road would likely be less

disturbing than seeing the same oncoming headlamps on an unlighted two/lane road.

5.2 ONCOMING TRAFFIC CLOSER TO DRIVERS LINE OF SIGHT

Cany freeways and some undivided highways have more than two lanes of traffic

traveling in each direction. ecause the right/hand lane is usually used as the main

driving lane, a significant proportion of traffic on these roads is separated from oncoming

traffic by more than two lanes.

This separation results in oncoming traffic being seen further off/a%is than if it

were on an undivided, two/lane highway= see 4igure A. This has two effects! the light at

the drivers eyes from oncoming headlamps is lower in intensity because headlamps

produce less light further from the forward direction, and the oncoming headlamps are

further from the drivers direction of view down the roadway. oth of these factors

combine to lessen the negative impacts of the glare light on visibility and comfort. 2n

undivided, two lane roadways, both of these factors reduce drivers visibility and increase

discomfort. The visual angle 6I7 toward oncoming traffic can be more than three times

larger on a multiple lane roadway 6lower panel7 than on a two/lane roadway 6upper

panel7.



20/45


F#$%&' 5.1( E@@'76 )@ 0#9#0'0 9'&+%+ %*0#9#0'0 -#$-,/+ )* 9#+%/ /*$' :A9#;#6

G-)+-

Glare screens, mechanical barriers in the center of the roadway, can block the

view of oncoming traffic, thus reducing glare. #lthough these screens could perhaps be

used to advantage in multi/lane highways, they are not feasible for most two lane

highways.

5.3 COMPLE ROADWAY GEOMETRY

4reeways are designed to carry large amounts of traffic at very high speeds. J: they

therefore tend to be built without sharp curves or steep gradients that would make high/

speed travel dangerous. Two/lane highways also carry traffic at moderately high driving

speeds but can have sharper curves as well as hills and changes in elevation not found on

freeways. These more complicated roadway geometries e%pose drivers on two/lane

roadways to a greater range of oncoming headlamp glare. 3onsider a right/hand curve as

illustrated in 4igure ?. #s the first driver begins to turn the vehicle into the curve, the

headlamps on this vehicle must also begin to turn. ecause headlamps produce higher

light levels toward the passenger side of the vehicle,traffic coming from the opposite

direction will be e%posed to higher light levels than would normally be e%perienced if the

same vehicles approached one another on a straight road. The impact of higher light

levels from oncoming traffic can be made even worse if the driver needs to look at the

center line that is closer to the headlamps of the oncoming vehicle in order to safely

navigate the curve. (igher levels will increase discomfort glare and result in increased



21/45


disability glarebecause the contrast/reducing veil of brightness in the eyes will be even

brighter.

F#$%&' 5.2( T-' '@@'76 )@ / 7%&9' )* )*7)#*$ 6&/@@#7 $/&' ')+%&' :A9#;#6 G-)+-

5.4 LESS RESTRICTED ROADWAY ACCESS

Cany two/lane highways can have abutting roadways or driveways that enter

traffic directly, whereas freeways with multiple lanes are more likely to have e%it and

entrance ramps. revious research, including a study sponsored by 1(T)#, has

demonstrated that discomfort from headlamp glare increases when the visual task is more

difficult, such as on curvy roads. The potential for ha'ards along a two/lane highway is

greater than along a divided, multiple/lane highway because vehicles can enter a two/lane

highway from a variety of locations and directions. Two/lane highways are also usually

not restricted in terms of bicycle travel or pedestrian use, unlike many divided multi/lane

highways. The greater ha'ard potential at any unknown location increases driving

difficulty, making glare more uncomfortable and driving possibly less safe than on

divided highways



22/45


5.5 F','& R)/0,/ M/*$+

Culti/lane highways tend to be well marked with indications for the traffic lanes

on each side of the road as well as a clearly marked median.Two lane highways,

depending upon the amount of traffic they carry and their width, may not have median

lines or edge lines to indicate the shoulder 6if a shoulder is present7= some very low

volume roads andKor very narrow two lane highways have no markings at all.

The purpose of roadway markings and delineation systems is to facilitate efficient

and safe traffic movement along a roadway. erhaps no one would disagree with the

observation that roadway markings make nighttime driving a much easier task. #s with

the roadway access issue, roadway markings in and of themselves have no direct impacton the amount of light that a driver is e%posed to by oncoming headlamps. (owever,

roadway markings can make the driving task easier by providing highly visibleJ9

guidance to a driver about the width of the road and the probable location6s7 of oncoming

vehicles, pedestrians or bicyclists. They can also alert drivers to the presence of curves or

intersections before drivers might otherwise detect them on unmarked roadways.

5.! CLOSER PROIMITY OF PEDESTRIANS

2n two/lane highways, pedestrians can be closer to vehicle traffic than on other

kinds of roads in densely populated areas, sidewalks are common, but they are less

commonly found along two/lane highways in rural areas. #lthough pedestrians might be

less common, they are not restricted from using the roadway. "f roadway lane markings

are not present, as is the case on many two/lane highways, there may be no clear

boundary separating the area of the roadway used by drivers from that possibly used by

pedestrians.

edestrians themselves are not always easily seen. Cany types of clothing, such

as dark coats and denim pants, have very low reflectanceJH and pedestrians wearing them

do not provide much contrast against the roadway pavement or against the dark ground



23/45


surfaces surrounding the road. "tems such as reflectori'ed tags or clothing can

dramatically increase a driver


24/45


!.1 LINK GLARE AND VISIBILITY TO CRASH RISK

*nlike the rather clear links between factors such as drug or alcohol use and crash

risk, the effects of glare on crash risk cannot be easily -uantified. olice reports do

contain references of nighttime crashes caused by poor visibility or glare. (owever,

because glare is dependent upon specific geometric factors, such as the location of a

headlamp in one


25/45


behind a vehicle. "f a driver is not vigilantly scanning important locations, particularly in

mirrors, then this behavior could constitute an increase in crash risk. "t should be noted

that while many interior rear/view mirrors can be dimmed to mitigate the impact of glare,

e%terior side mirrors cannot be adjusted as readily.

"ncreased discomfort could also speed up fatigue while driving or lead to riskier

behaviors such as erratic roadway lane positioning. The relationship between increased

discomfort and these types of behaviors, as well as the possible impact of those behaviors

on crash risk, is not well established, but the objective of ongoing 1(T)#/sponsored

research is to provide more definitive answers to these -uestions.

2bviously, headlamps cannot be eliminated for nighttime driving and will alwayscause some level of glare for most drivers. The dramatic level of recent complaints about

new headlamp technologies,however, brings into -uestion whether glare is unnecessarily

becoming a larger issue than it has been previously. #lthough complaints are often

focused on the color of the new headlamp technologies, this is not the only possible

contributing factor to glare. #s described earlier, new headlamp technologies and new

vehicle designs may combine with headlamp color to increase the level of glare and

thereby e%acerbate driver complaints. (" headlamps generate more light and they can

be located higher with respect to the road on an ever/growing percentage of light trucks

and sport utility vehicles on the road, resulting in higher light levels for oncoming drivers.

$ven though the alignment of these headlamps must meet )ociety of #utomotive

$ngineers standards>F when manufactured, they may very well become improperly aimed

after the vehicle has been driven many miles, after damage to the vehicle, or simply with

changes in vehicle loads. The combination of new headlamp color, higher light output,

higher mounting heights, and improper aim may, in fact, increase the prevalence of glare

on #merican roads, thereby reducing visibility and comfort, and increasing crash risks by

making ha'ards less visible.

CHAPTER "



26/45


FACTORS CONTRIBUTE TO GLARE AND ITS RISKS

receding sections of this report discussed what glare is, how it affects vision and

behavior, and why it is more problematic in certain locations 6e.g., two/lane roadways7

and for certain individuals 6e.g., older drivers7. This section discusses the potential impact

of various factors related to vehicles and headlamps on glare and crash risk

".1 C-/&/76'+6#7+ )@ 6-' V'-#7' /*0 I6+ L/+ A@@'76 G/&'

".1.1 H'/0/ M)%*6#*$ H'#$-6

#lthough the standards for re-uired headlamp beam distributions are consistent

for passenger cars, light trucks and sport utility vehicles, these standards are defined with

respect to the headlamp without regard for the eventual mounting height in these types of

vehicles. 61ote! istributions for headlamps mounted on heavy trucks, for e%ample, must

adhere to different standards.7 "n general, increasing headlamp mounting height results in

improved visibility, as measured using visibility distance. This is because the region of

ma%imum intensity tends to be increased in the vertical direction= resulting in potentially

higher illuminances down the roadway 6refer to 4igure >7. 1(T)#/sponsored research

involving computer simulations of visibility mounted at different heightsHH demonstrated

that headlamps with greater mounting heights tended to result in longer visibility

distances down the roadway. (owever, there was large variability in this trend, so that

some individual lamps with lower mounting heights can provide greater visibility

distances than some others with higher mounting heights.

This potential visibility improvement with high/mounted headlamps must be

balanced against the generally higher potential to produce glare to oncoming drivers.

5esearch sponsored by 1(T)#has also shown that increasing headlamp height results in

higher light levels at the eyes of oncoming drivers, both in meeting situations and when

headlamps are seen in rearview mirrors. #gain, there is substantial variability among

different headlamps so that this general trend may not be applicable to individual



27/45


headlamps. "n other words, it is not necessarily true that one headlamp set with a higher

mounting height than another necessarily produces more glare. (igher light levels can

increase disability glare, and increase the probability that a driver will not be able to see

objects along the roadway. The higher levels can also increase the sensation of discomfort

e%perienced by drivers e%posed to those levels, and if e%perienced in rearview mirrors,

could make people avoid looking behind them while driving. roposals to reduce

headlamp mounting height have been made, although none to date have been enacted.

".1.2 L/ A#

Cost research into the effects of forward lighting on driver visibility and glare to

oncoming drivers assumes that all headlamps are properly aimed, according to thestandards set forth by the )ociety of #utomotive $ngineers 6)#$7 for the aiming of

headlamps. These standards re-uire headlamps to be aimed within :.BJ degrees 6up or

down7 of proper aim. "n the *.)., headlamp aim is not a 4ederal re-uirement, but some

states have implemented the same industry standards mentioned earlier in this report.

"n recent years, fewer states have re-uired regular checking of headlamp aim as part of

the vehicle inspection process. 5esearch commissioned by 1(T)# in the early >EE:s

showed that improper aiming of headlamps was nearly as common in a state that

addressed headlamp aim in vehicle inspection 6&irginia7 as in a neighboring state that did

not have such re-uirements 6Caryland7.#s described earlier, in a more recent 1(T)#/

sponsored study conducted in 1ew @ork )tate, the majority of vehicles had at least one

headlamp that was improperly aimed either upward or downward. 4urther, while not

related to improperly aimed headlamps, changes in roadway grade and elevation along

hills and slopes can result in very different light levels ahead of the vehicle and toward

oncoming drivers because in these situations the entire vehicle pitch changes relative to

the ground.

(eadlamps aimed downward result in lower light levels ahead of a vehicle,

effectively moving the location of the headlamp beam distribution with the highest



28/45


intensity closer to the vehicle when headlamps are aimed upward, glare increases for

oncoming drivers, as well as for drivers ahead of the vehicle looking in their rear view

mirrors. 1(T)#/sponsored research shows that headlamps aimed upward by even one

degree will create enough glare for oncoming drivers to reduce the distances at which

objects along the roadway can be seen.

(eadlamps aimed upward will also result in increased dosages of light e%posure

6intensity M duration7 when two vehicles meet while driving in opposite directions. These

increases can be large for new headlamp types. 4or e%ample, in a study sponsored by

1(T)#,the distributions of several halogen and (" headlamps were compared using

simulation software. +hen both types of headlamps were aimed properly, the halogen

headlamps studied tended, on average, to produce similar glare dosages as the ("

headlamps studied. +hen the headlamps were improperly aimed upward by >.9 degrees,

the halogen headlamps could produce a dosage more than ten times that from properly

aimed halogen and (" headlamps, and when aimed similarly upward, (" headlamps

could result in even higher dosages than halogen headlamps. )ince the time to recover

visibility of objects is related to the glare dosage, improperly 6and upwardly7 aimed

headlamps can cause objects to remain invisible to oncoming drivers for longer durations

than properly aimed headlamps.

"n addition to headlamps, there is some concern with glare associated with the aim

of au%iliary lamps. These lamps are not regulated, and only some have the ability to be

aimed accurately.

".1.3 H'/0/ B'/ D#+6%6#)*

#s noted in preceding sections of this report, vehicle headlamps must conform tospecific re-uirements for minimum light levels in regions of the beam that are most likely

to contribute to forward visibility, and ma%imum light levels in regions that could result

in glare to oncoming drivers, or glare in rear view mirrors. The re-uirements are the same

regardless of the light source used in the headlamps= halogen headlamps and ("



29/45


headlamps, for e%ample, must conform to the same set of minimum and ma%imum light

levels at the same locations within the beam distribution. )ome locations within the

re-uired beam distribution are not restricted, either by minimum or ma%imum light

levels, such as the area that contains light directed downward and toward the left 6from

the drivers perspective7."n other words, current headlamp standards allow a very wide

range of luminous intensities within certain locations in the headlamp beam.

3learly, if headlamps are improperly aimed or when variations in roadway

geometries and in locations of potential ha'ards and oncoming vehicles are present, the

fi%ed beam distribution from headlamps may not always provide ade-uate visibility, or

prevent glare.

(" headlamps typically produce twice as much light as halogen headlamps. >>

irecting the light from a halogen headlamp into the regions with minimum re-uirements

for light, while keeping below the ma%imum allowable light levels, tend to result in a

smaller amount of 8leftover8 illumination that could be directed into less restricted

regions. 4or many (" headlamps, there is a higher amount of 8leftover8 illumination,

with this illumination often being directed to peripheral locations within the beam and to

other locations without restrictions on ma%imum light levels. Thus, many (" headlamps

are more likely to produce high light levels to oncoming drivers when they are aimed

upward than halogen headlamps aimed upward by the same amount.

resently, all standards for lighting on vehicles assume headlamps that are

permanently fi%ed in terms of their distributions. roposals for lighting on vehicles in the

near future will likely include adaptive lighting that changes light levels and beam

distributions in real time to respond to oncoming drivers, roadway geometries, and

weather.

".1.4 H'/0/ C))%&



30/45


# fre-uently mentioned characteristic of headlamps by members of the driving

public responding to 1(T)#E is

the 8blue8 color of (" headlamps compared to halogen headlamps. "ndeed, 1(T)#/

sponsored research and other studies have confirmed that people e%perience more

discomfort when e%posed to the bluer (" headlamps than when they are e%posed to the

yellower halogen headlamps for the same measured light level at the eye. (owever, this

increased discomfort from the bluer lights has not been demonstrated to translate into

greater reductions in visibility for objects viewed on a%is., nor off/a%is,AF reinforcing the

theory that discomfort and disability are separate phenomena.

".1.5 H'/0/ O6#7/ S+6'

rior to the >EH:s, vehicle headlamps consisted only of sealed beam filament

lamps. 4or these lamps, the entire headlamp included the light source with a reflector and

lenses to produce the re-uired beam distribution. )ince that time, headlamps with

dedicated reflectors and lenses have become available, so that only a capsule/like bulb is

replaced when a headlamp fails.

Core recently, projector style headlamps have become more common. "n these

units, an optical mechanism not unlike a slide projector produces the re-uired headlamp

beam distribution. The lighted area of projector headlamps can be -uite small, in

comparison with reflector headlamps. )ome evidence e%ists that this smaller si'e can

result in greater discomfort when headlamps are viewed from relatively close distances.

(owever, when viewed from more than appro%imately >:: feet, the si'e of a headlamp

has little impact on discomfort, according to 1(T)#/commissioned research. 1o

research has identified any impact of oncoming headlamp si'e on visibility= the same

1(T)# study showed that headlamps of different si'es, even differing in area by a factor

of nearly ten, but producing the same light level at oncoming drivers< eye, resulted in

e-ual visual performance for individuals e%posed to those headlamps.

".1.! H'/0/ /*0 W#*0+-#'0 C'/*#*'++ /*0 C)*0#6#)*



31/45


articularly during and following conditions of poor weather, headlamps and

vehicle windshields and mirrors can become soiled and even damaged by dirt, salt or

sand. ust as material inside the eye scatters light, dirt or spray build/up on windshields,

mirrors or the outer surfaces of headlamps will also scatter light. "n the case of dirty or

scratched windshields or mirrors, the contrast of objects along the roadway will be

reduced by scattered lightpotentially making some objects invisible that otherwise would

have been visible. +hen the outer surfaces of headlamps are dirty or damaged, the light

distribution can be altered, possibly enough so that the resulting visibility for the driver

using those headlamps is reduced or so that the light levels produced in the direction of

oncoming drivers is increased. (owever, the si'e of the reductions in light level for

forward visibility and of the increases in light levels for oncoming drivers were much

smaller than those found to be caused by improper headlamp aim.



32/45


CHAPTER

COUNTER MEASURES TO CONTROL GLARE

.1 TO REDUCE ILLUMINATION REACHING THE DRIVERS EYE

.1.1 P)/'0 L#$-6#*$

The technical development of a polari'ed headlight system consisting of

polari'ing filters on headlamps and a viewer filter on the drivers eyes was begun by

$dwin 0and in the >EF:s. #lthough polari'ed lighting on automobiles is still not

commercially available, e%tensive research was funded by the 4ederal (ighway

#dministration 64(+#7 in the >EJ:s. "n reporting their research on polari'ed head

lighting, (emion, (ull, 3adena, and ial 6>EB>7 describe the basic physics of polari'ed

light. 2rdinary light, including light from an automobile headlight, consists of

electromagnetic waves that vibrate in all directions perpendicular to the direction of the

beam. +hen ordinary light passes through a polari'ing filter, all of the light waves are

absorbed e%cept for those vibrating in a single plane and the light becomes 6linearly7

polari'ed. The polari'ing a%is of a filter is the orientation of the electric or magnetic field

of the light waves that are able to pass through the filter 6for e%ample, F9 degrees with

respect to vertical7.

T-' )/'0 -'/0#$-6 ++6'N # polari'ed headlight system can improve visibility

for drivers by decreasing the amount of light from oncoming headlights. 4or this system

to work, both vehicles must be e-uipped with the proper hardware. "n the following

description of the system, the beneficiary will be referred to as the driver and the

opposing vehicle will be the source of the glare.



33/45


# polari'er is placed over the opposing vehicle headlights with its polari'ing a%is

at F9 degrees from vertical.

#nother polari'ing filter is placed in front of the drivers eyes with its a%is

parallel to that of the polari'er in front of the headlights. This polari'ing filter is

sometimes referred to as an analy'er.

oth cars should have a polari'er and both drivers should have an analy'er. Two

types of analy'ers were developed. 2ne type, referred to as a visor, was a strip of

sheet polari'er that was hung from the sun visor in such a way that its bottom

edge intersected the line of sight from the driver to oncoming vehicles. +hen

there was no visible opposing vehicle, the driver could look under the visor.

#nother type of analy'er took the form of either full/ or half/glass spectacles. The

half/glass type operated like clip on sunglasses and could be used in the same way

as the visor. That is, drivers could look through the analy'er when a vehicle was

approaching and through the bottom, unpolari'ed portion when the analy'er was

not needed. The half/glass can be fitted over prescription glasses or over a pair of

neutral glasses with no optical function 6see 4igure H below7. The full/glass type,

for use by passengers, is functionally identical to sunglasses.

"f two cars are e-uipped in e%actly the same manner, then the orientations of the

analy'ers and polari'ers will be the same when the two cars are facing in the same

direction. (owever, when the two cars approach each other facing in opposite directions,

then the orientation of the analy'er and polari'er for one car will be perpendicular to

those in the other car. The polari'ing system reduces glare in the following way!

0ight from the opposing headlamps of an approaching car will pass through the

polari'er in front of that cars headlamps. #s e%plained above, the orientation of

the polari'er in front of the opposing cars headlamps is perpendicular to theorientation of the analy'er in front of the drivers eyes, since the cars are facing in

opposite directions.

Therefore, the light coming from the opposing headlamps will, upon passing

through the polari'er, become polari'ed in a direction perpendicular to the

polari'ing plane of the driver


34/45


)ince the drivers analy'er is oriented perpendicular to the plane of the polari'ed

light coming from the opposing cars headlamps, it will not allow this light to

pass through and reach the driverEBJ7. 1ight/driving glasses for

glare reduction are structurally and functionally very similar to sunglasses and, in fact,

some models are merely a remarketing of what was originally a pair of sunglasses. 0ike

sunglasses, these glare/blocking glasses work by filtering light prior to its reaching the

drivers eye. Cany models use color filters to eliminate short/wavelength or blue light

and so have a yellow tint. )ome are full/si'ed, some fit over prescription glasses, and

others are smaller, filtering only a thin strip of light in the upper portion of the eye. #

uni-ue model developed in the >E9:s 6ryan >EJA7, called 1ite/)ite, used a green,

ring/shaped filter that was affi%ed to prescription lenses so that the observer looked

though the clear center. The filter was purported to ...cause a shadow effect to fall across

the pupil to eliminate the oncoming glare of headlights.... #nother lens, described by

#drian 6>EBE7, had a clear circular center large enough to transmit light with a glare angle

of less than 9 degrees surrounded by a shaded area that gradually increased in

transmission according to the glare angle s-uared, so that the wearer could view glare

from oncoming traffic through the shaded area. "n most cases, the shading was not

uniform over the ring, but only covered the sector in which glaring headlamps usually

occur. #nother type of night/driving glasses, shown in 4igure H, has been introduced by

Glare blockers. Their product is an ultraviolet filtering polari'ing plastic that attaches to

the users own eyeglasses, covering the top half of the lenses, and is adjustable for proper



35/45


fit. The design is similar to the half/glass analy'er used in the studies of polari'ed

headlights reviewed above.

+ith a half/glass analy'er, the user can turn filtering on and off with a slight tilt of

the head, much like using bifocals. The product provides glare control on demand, and,

when glare control is not needed, allows the user to normally look through his own

glasses The basic idea behind all glare/reducing night driving glasses is that a reduction

in the amount of light at the eye will reduce glare. This reduction could be accomplished

by using simple neutral density 6n.d.7 filters, which filter all wavelengths e-ually and

cause minimal color distortion. +hy, then, are most night/driving glasses designed to

selectively filter out the shorter electromagnetic wavelengths, in the blue and near/

ultraviolet spectral rangeO elow, the theory behind this selective filtering will be

evaluated both from an empirical and a physiologicalKperceptual perspective= first, the

claims made by the distributors of these devices will be e%plored.

.1.3 G/&' S7&''*+

2n divided highways without independent alignment or large medians, glare

screens placed in the median of the roadway could be a cost/effective way to reduce glare

from opposing traffic. )creens can improve safety in temporary work 'ones, where lanes

can be narrow and only concrete barrier separates opposing traffic. *nder a review of the

characteristics of headlamps and vehicles that contribute to headlamp glare shows that

beam patterns, aim, and mounting height, and the interaction between these

characteristics, can play a large role in the light levels reaching the eyes of oncoming

drivers 6especially on unlighted, two lane highways7. This in turn makes some objects

invisible to drivers, particularly older these circumstances, blocking glare from oncoming

headlights will dramatically increase the visibility of the concrete barrier and the

delineation of the lane ahead.

The 1ational 3ooperative (ighway 5esearch rogram 5eport JJ 613(5/JJ

>EBE7, a report of the Transportation 5esearch oard, includes descriptions of the



36/45


different types of glare screens and the advantages and disadvantages of each. 13(5/

JJ defines a glare screen as . . . a device placed between opposing streams of traffic to

shield drivers eyes from the headlights of oncoming vehicles.. # glare screen can be any

type of object of a certain width and placed at a certain spacing that will prevent glare

from reaching drivers eyes. The object may be opa-ue or have intermittent openings that

allow a view of the opposing lanes while at the same time screening out light at angles

less than A: degrees from the drivers eye. To be effective, glare screens should!

5educe a large portion of the glare

e simple to install

e resistant to vandalism and vehicle damage

e repaired -uickly and safely

5e-uire minimal cleaning and painting #ccumulate a minimal amount of litter and snow

e wind resistant

(ave a reasonable installation and maintenance cost

(ave a good appearance

#llow for emergency access to opposing lanes

There are three types of glare screens, labeled Type ", Type "", and Type """. # Type "

glare screen is a continuous screen that blocks light from all angles. # Type "" screen is a

continuous screen of an open material that is opa-ue to light coming from angles of 'ero

to A: degrees from the drivers eye and becomes increasingly transparent for angles

beyond A: degrees. # Type """ screen is made up of individual elements that will block

light coming from angles of 'ero to A: degrees JFdrivers. "mportantly, these

characteristics have changed in parallel during recent years, potentially compounding the

negative effects of glare more so than any factor alone. 5esults from a recently completed

series of e%periments sponsored by the 4ederal (ighway #dministration largely confirm

these findings. from the drivers eye while providing clear visibility beyond A: degrees..

$%amples of Type " glare screens include earth mounds and concrete barriers. $arth

mounds are used in areas with wide medians and e%cess cut material. *sually in these

areas grades and cross/sections can be modified to leave or build this e%cess earth in the

median to block glare. 3oncrete glare screens are usually provided by simply e%tending



37/45


the height of a standard concrete barrier enough to block light from opposing vehicles.

Type "" glare screens include e%panded metal mesh, knit polyester fabric, and fencing.

$%panded metal mesh is the most widely used type of screen. "t is usually

manufactured from steel or aluminum sheets. arallel slits are cut into these sheets and

then the sheets are stretched until the slits form a diamond pattern. The metal between the

slits twists at an angle to form screen. Lnit polyester fabric also works as a glare screen

by diffusing light rather than blocking it. 3hain/link fences are effective Type "" glare

screens when the patterns of the wires have a spacing less than one inch.

The most common kind of Type """ glare screen is manufactured by placing

individually supported paddles at intervals so that they will block light from opposingheadlights. #n e%ample of these paddles is the Glare Guard, elliptically shaped A9/inch

blades made out of thermoplastic polyolefin 6T27 that attach via anchor bolts to a

concrete barrier.

Cedian lantation / The species to be planted in median would be of low or

medium height with ornamental value to enhance the visual e%perience of the road

corridor. "t will also act as a screen to prevent glare from the incoming vehicles. The

species recommended for median are mainly ougainvillea and Laner. ougainvillea is

considered as the most suitable species as it has a great aesthetic value and it is found in

various colours and shades. "t can also withstand e%treme temperature and climatic

conditions and also has low re-uirement of water. These species have been proposed

considering the climatic conditions, re-uirement of water and future management.

(owever, other species suitable and suggested by 4orest epartmentK 52 can also be

used.

.2 COUNTERMEASURES THAT INCREASE THE GLARE ANGLE



38/45


.2.1 I*7&'/+'0 M'0#/* W#06-

M'0#/* ,#06-

The #merican #ssociation of )tate (ighway and Transportation 2fficials

6##)(T27 defines the median as that portion of a divided highway separating the

traveled way for traffic in opposing directions 6##)(T2 >EEF7. ##)(T2 also defines

the principal functions of medians! to separate opposing traffic, provide a recovery area

for out/of/control vehicles, provide a stopping area in case of emergencies, allow space

for speed changes and storage of left/turning and */turning vehicles, minimi'e headlight

glare, and provide width for future lanes 6##)(T2, >EEF7. Cedians can be depressed,

raised, or flush with the traveled way surface, and they should be highly visible duringboth day and night. The width of the median is defined as the distance between the

through lane edges, including the left shoulders. )oaring traffic volumes after +orld +ar

"" created a need for the immediate design of many highway facilities. "n response to the

urgent need, designers often relied on rules of thumb in their approach to median design,

some of which have proven to be incorrect. #ccording to (utchinsonet al. 6>EJ?7, these

e%periences have greatly stimulated research on median performance and factors of

influence in median design. #mong the factors of influence are benefits such as safety,

comfort, and convenience. #t the time of the report, the e%tent to which medians could

provide these benefits was more a matter of opinion than of record, and so it was

difficult to use safety or comfort to justify any cost increases involved in varying the

median width. This left the choice of basic median width an administrative decision, to be

backed by engineering judgment. Today, it is known that reduced fre-uency of crossover

accidents and PtheQ reduction of headlight glare are safety features associated with a wide

median 6##)(T2 >EEF7. Therefore, ##)(T2 recommends that, in general, a median

should be as wide as practical. Cedians usually range from a minimum of >.A m to AF

m or more. The ##)(T2 report states that medians that are >A m wide or wider provide

the driver with a desired ease and freedom of operation. . . because there is a . . . sense

of physical and psychological separation from opposing traffic 6##)(T2 >EEF7.

"ncreasing the median width decreases the effects of glare from opposing headlights by



39/45


increasing lateral separation between vehicles. #t any distance between vehicles on the

road increasing lateral separation will increase the angle between the drivers line of sight

and the headlights of an opposing vehicle. "n addition, increasing lateral separation

effectively shifts the beam pattern of the opposing vehicle, resulting in lower intensities

of light directed at oncoming drivers. The combined effect is a reduction of both

disability and discomfort glare.

.2.2 I*0''*0'*6 A#$*'*6

"ndependently aligned roadways are those with hori'ontal and vertical alignments

developed independently to suit location and design re-uirements 6eet and 1eu'il,

>EBA7. "ndependent alignment places each roadway at a different elevation, with avariable amount of separation between the different directions of travel, in contrast to a

narrow/median design, where each roadway is at or near the same elevation and a

constant distance apart. +ith independent alignment, the driver on one roadway might

not even see an approaching vehicle on the other roadwayNwhich, of course, eliminates

glare entirely. )ince independent alignment is used when there is a large median width, it

provides all of the advantages of a wide median and more. eet and 1eu'il 6>EBA7

compared narrow median design with independent alignment and listed several

advantages for independent alignment, which are shown below.

A09/*6/$'+

"n addition to decreasing the effects of discomfort and disability glare, wide

medians provide numerous other benefits that increase the return on investment!

A77#0'*6 R'0%76#)*( +ide medians have the potential to decrease both the fre-uency

and severity of accidents.

S/@'6 Z)*'( +ide medians provide greater stopping or recovery abilities and space for

vehicles that run off of the left edge of the pavement.



40/45


D'& C)@)&6( sychological discomfort and stress from driving are greatly reduced

when a wide median is available.

V#+#8##6( "ncreasing the median width may also improve visibility. 0ess light from

opposing headlights will fall on the pavement, thereby decreasing the background

brightness for some objects on the road. This will increase the contrast between

pedestrians and other objects which are seen in positive contrast 6that is, object brighter

than pavement7. #dditional advantages of independent alignment over narrow/median

designs include!

F','& '/&6-,)&+ &'%#&'0( "ndependent alignment permits the designer to avoid areas

of difficult topographic, soil or drainage conditions.

M)&' E*9#&)*'*6/ F'*0( +ith independent alignment there is less likelihood of

slope failures and erosion, groundwater problems, and bedrock e%cavation. #lso, the

highway may be located and designed in a manner that produces a minimum of visual

and environmental disruption

.3 COUNTERMEASURES THAT INDIRECTLY MINIMIZE THE EFFECTS OF

GLARE

.3.1 U6&/9#)'6 H'/0#$-6+

*ltraviolet 6*&7 head lighting has the potential to reduce glare on the highway by

reducing the need for visible light. #lthough not itself visible, *& radiation 6*&57 can

improve nighttime visibility of pedestrians, lane lines, signs, and other objects on the

road. $ither replacing high/beam headlamps with *& headlamps or modifying the low/

beam photo metrics in a *&/enhanced lamp 6perhaps using a $uropean beam pattern7

could eliminate much of the glare e%perienced by drivers of oncoming vehicles and still

maintain visibility. $ven without any reduction in glare from visible light, disability glare

effects may be reduced by the improved visibility. *ltraviolet radiation cannot be seen by

most human observers, but, when it is absorbed by certain materials, *& is converted to



41/45


longer wavelength visible light. This phenomenon, called fluorescence, makes objects

more easily seen and is the reason that *& headlamps have the potential to improve

highway safety. *nlike standard automobile alignment can be adjusted to optimi'e

visibility without concern for glare. *&5 ranges from appro%imately F nm to F:: nm

wavelength, but *& headlamps operate in the relatively long/wavelength * band,

which ranges from ?A: nm to F:: nm.

A09/*6/$'+

* offers the potential to offset the negative effects of disability glare by

improving the visibility of objects in the presence of glare.

"mprovements in visibility raise the possibility that high beams could be

eliminated and the sharp/cutoff low beam pattern used in $urope might become

acceptable.

The greatest improvement in visibility is e%pected in places without street

lighting, which are currently the places with the greatest problems from glare.

)ince *& light is not normally visible, its backscatter under adverse weather

conditions is not visible either. Therefore, glare from backscatter could potentially

be reduced and visibility improved in such conditions

.3.2 F#'0 R)/0,/ L#$-6#*$

4i%ed roadway lighting is installed for many reasons, including reduction in

nighttime accidents 6primarily on major arterial roads and freeways7, pedestrian safety,

crime reduction, and area ambience 6on urban streets and areas with significant retail

activity at night7. "n addition to these specific purposes, a secondary benefit of fi%ed

roadway lighting is the mitigation of the effects of headlight glare. uring daylight,

headlight glare is generally not a problem because of high visual adaptation in bright

ambient light. #t night, a drivers adaptation level is much lower and headlight glare

becomes a problem. #daptation level is determined by the entire visual environment,

which can include the surrounding ambient lighting in visually comple% areas, the steady

stream of oncoming headlights, or the luminance of the pavement in areas without much



42/45


traffic or ambient lighting. 4i%ed roadway lighting is a countermeasure against headlight

glare whenever pavement luminance is a significant factor in determining the drivers

adaptation level.

A09/*6/$'+

The benefits of fi%ed roadway lighting as a countermeasure for headlight glare are

twofold.

4irst, fi%ed lighting increases the visibility of objects and pedestrians on the road, making

it possible to reduce the headlight illumination re-uired.

)econd, fi%ed roadway lighting raises the adaptation level of the driver, thus reducing the

effects of headlight glare.

.3.3 I&)9'0 9#+#8##6 ,#6- '++ -'/0/ #%#*/6#)*(

Given that motorists use high beams infre-uently in meeting situations, if at all

6(are mand (emion >EJH7, there is little to be gained from fi%ed lighting in terms of

reducing high beam usage. "f fi%ed lighting were to result in a reduction of headlight

illumination it would be due to the use of adaptive headlighting or the elimination of low

beam headlamps in some situations. 4or e%ample, some $uropean cities once encouraged

motorists to switch to parking lights within the city limits, where fi%ed overhead lighting

is prevalent and good. +hile this practice has been largely discontinued because of the

difficulty it created for pedestrians to see an approaching vehicle in advance, daytime

running lights might prove to be an ade-uate substitute. "n either case, implementing such

a policy in the *.). would re-uire significant improvements in the fi%ed/lighting

infrastructure and a clear definition of what constitutes good lighting.

R'0%76#)* )@ 0#+7)@)&6 $/&'( #de-uate pavement luminance from fi%ed lighting

would mitigate the effects of headlight glare in environments that are otherwise relatively

dark and where there is not a constant stream of traffic.



43/45


R'0%76#)* #* *#$-66#' /77#0'*6+(1umerous studies show that fi%ed lighting lowers the

nighttime accident rate. +ith nighttime accident rates often four times the daytime rate,

fi%ed lighting systems can pay for themselves in accident reduction alone.

CHAPTER

CONCLUSION



44/45


$uropean standards for vehicle headlampsEB have different aiming re-uirements

for headlamps with different mounting heights, but it should be noted that $uropean

headlamps adhere to different re-uirements for light distribution than those in the *.).

4actors such as headlamp color, particularly the bluish appearance of (" headlamps,

and the increasing prevalence of projector headlamps, can further compound the

sensation of annoyance and discomfort e%perienced by many #merican drivers.#dded to

these factors is the fact that the #merican driving public is aging and is more sensitive to

the deleterious and discomforting effects of headlamp glare than the younger driving

public of earlier decades. These factors in combination appear to e%plain much of the

passion of the #merican driving public regarding headlamp glare.

To some e%tent, headlamp glare is unavoidable. (eadlamps must facilitate

visibility at great distances, something that can only be accomplished by bright lights

having the potential to produce high light levels in the forward scene. resent standards

for headlamps,in principle, work to strike the proper balance between visibility and glare,

but these standards are silent with respect to headlamp aim, and do not differentiate

among a wide range of headlamp mounting heights.

1ew headlamp technologies 6e.g., light emitting diodes7, even newer than ("

and projector headlamps, are beginning to be introduced on vehicles in the *nited )tates.

(eadlamps with distributions that change in response to driving conditions 6e.g., poor

weather, curves in roadways, the presence of pedestrians, or the presence of oncoming

vehicles7 are likely to be included on vehicles of the future. EE,>:: #lready, 8steerable8

headlamps that turn with the steering wheel into curves are standard e-uipment on some

vehicles sold in this country. (eadlamp functions presently classified under the term

8advanced forward/lighting systems 6#4)78 are not addressed by current headlamp

re-uirements.

"ndeed, new headlamp systems may aid in reducing the tradeoff between visibility and

glare by allowing systems to respond dynamically to the driving situation. #t the same

time, these dynamic headlamp systems will present new challenges for regulation of



45/45


vehicle lighting. y conducting research into driver needs, 1(T)# is working to gain the

knowledge necessary to develop reasonable re-uirements for vehicle headlamps that will

be fle%ible, and ultimately beneficial for the #merican driving public. #mong the

research activities underway are!

#nalyses to compare the relative effects of headlamp mounting height, headlamp

light source, headlamp aim and headlamp beam characteristics on forward

visibility and on glare to oncoming drivers along two/lane roadways. These will

help identify priorities for effective glare countermeasures.

#ssessment of headlamp aim on a representative sample of vehicles. This activity

will help identify the e%tent and causes of improper headlamp aim.

# field e%periment to study visual performance of older drivers following

e%posure to light creating glare. This activity will help clarify the impact of age

and light level on recovery from glare under realistic conditions.

#n on/road study of the effects of different levels of glare on driving behaviors

6such as visual distraction7. This activity will identify light levels that start to

affect driving performance.8

evelopment of a prototype safety/based adaptive headlamp system based on

drivers needs regarding visibility and glare reduction. The prototype will be used

to develop an evaluation protocol to define performance re-uirements for adaptive

lighting. #daptive headlamps may afford new options for resolving the tradeoff

between glare and visibility by automatically reducing glare when other drivers

are nearby and increasing intensity under other conditions.

#ll of these activities are presently underway and will be documented in a subse-uent

report to 3ongress containing the findings and recommendations for 1(T)# regarding

priorities for mitigating the impacts of headlamp glare on *.). highways while

maintaining or increasing seeing distances.

kiran.docx

Documents

Transcript of kiran.docx