Unit 13 ( BASIC CHARACTERISTICS OF A ROAD SYSTEM ( Part 3 ) )

8/6/2019 Unit 13 ( BASIC CHARACTERISTICS OF A ROAD SYSTEM ( Part 3 ) )

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BASIC CHARACTERISTIC OF ROADHighway Engineering C3010 / UNIT 13

BASIC CHARACTERISTICS OF A ROAD

SYSTEM

( Part 3 )

OBJECTIVES

General Objective

To understand the basic characteristics of a road system.

Specific Objectives

At the end of the unit you should be able to :-

• describe the characteristics of speed.

• state the types of speed.

• identify the method to measure the speed.

UNIT 13




13.0 INTRODUCTION

Speed of travel is a simple and widely used that measures of the quality of traffic flow. Basically, speed is the total distance traversed divided by the time of

travel. Speed is commonly expressed in miles per hour or feet per second. Its

reciprocal, travel time, is usually expressed in units of minute per mile.

13.1 SPEED

Speed is the rate of travel usually expressed in kilometers per hour (km/h)

and is generally qualified according to three main types:-

a.) Spot speed

b.) Running speed

c.) Journey speed

13.1.1 Spot speed

Spot speed is the instantaneous speed of a vehicle at any specified point.

INPUT

THE CHARACTERISTICS OF SPEED




13.2 SPEED MEASUREMENT METHOD

13.2.1 REGISTRATION NUMBER AND VANTAGE POINT METHODS

The system requires observers to be stationed along the route, one

at each section point. Using synchronized watches the observers record

the time and registration number for each passing vehicle. Subsequent

matching of numbers carried out either manually or by computer, identifies

the journey times of through vehicles and allows non-stop and stopping

vehicles to be estimated. The method does not reveal the cause, location

or provide running speeds which are often needed.

Where the length of the route or section under consideration is

short, such as a street in the central area of a city, it is often possible to

station an observer where there is a reasonably unobstructed view of the

entire length. Vehicles are selected at random and their course trace

along the road by nothing the time of entering the section, the duration

and nature of any delay and the time of leaving. In this way, all the

necessary data is obtained to evaluate journey and running speeds anddelays. Close circuit television may also be used for this purpose over a

network of streets covered by cameras. Alternatively, time-lapse

photography can be used to obtain the data. Very extensive information

on traffic flows; congestion point, speeds, delays and parking can be

derived from aerial photography over extensive areas. Time-lapse

photography and aerial method can be co-ordinated to give more detailed

information.




13.2.2 MOVING OBSERVATION METHOD.

The median and mean speeds are only equal if the speed

distribution is symmetric which tends to be the case in free-flowing

conditions. The large number of runs required makes the method costly

and difficult to apply for limited access roads, particularly rural motorways.

A partial moving vehicle method has been used successfully where only

section journey times are recorded while classified vehicle counts are

continuously made at the mid-points of each run. The data is more

accurate and manpower is effectively employed. Other modified methods

have included the mounting of a rime-lapse camera in a good, forward

field of vision, to take photographs at intervals of 1 s. while good results

were obtained, care had to be exercised in avoiding tailing high vehicles;

the outside lane gave a better observational position. A fuller record of

delays is available for analysis and the influence of parking and

pedestrians can be included in the assessments. While crew running time

reduced, subsequent analysis is lengthy, although an important advantage

is the ability to vary the section points in the aggregation of a route. Similar possibilities are available for using video recorders for subsequent replay

of television film.

A method of measuring speeds on motorways has been evolved by

Duncan and is called the two-speed method. The test car is driven several

times over a length of road at each of two constant speeds with an

observer separately recording the total number of cars overtaking (“overs”)

and undertaken (“unders”). For each of the pair of speeds selected the

value of the ratios under / overs are plotted and joined by a straight line.

From the analysis diagram estimates of mean speed, the standard

deviation of speeds and percentiles for the speed distribution.




13.2.3 TIME-LAPSE PHOTOGRAPHY, VIDEO AND PEN RECORDERS

This method employs a camera to record the distance moved by avehicle in a selected short period of time. Exposures are made at a

constant time interval and the interval and the distance traveled between

exposures is measured by projecting the film. The distance divided by the

time interval between exposures gives a speed measurement. Cameras

usually 16 mm triggered to take single shots at pre-selected frequencies

or use suitable repeat-cycle timers called intervalometers. The projectors

are operated to advance the film frame by frame and are equipped with

resettable frame counters. Films are either on to prepared grid screens or

back projected through a mirror box into a plastic or frosted screen.

Photographic methods are saturation flow, turning movements, vehicle

spacing and lane use, congestion and delay, and may also be used to

study a wide range of vehicle characteristics and pedestrian behavior.

The more recent introductions of suitable portable video cameras,

with less costly playback studio equipment, extend the instrumentation

range available for traffic studies and film is re-usable. Static and mobile

mounting positions for cameras can be adopted.

The main advantages of cinematography are that observations do

not disrupt the pattern of activities; accurate analysis of complex varying

characteristics can be undertaken albeit that many playback run of a film

will be required; difficult observational techniques and crew training areobviated; records can be permanent and permit previously undetected but

influential items to be analyzed, and both the space and time of events are

recorded. A very considerable disadvantage is the time-consuming nature

of the subsequent analyses, but using oscilloscopes with light pens or

coupling to a teleprinter, coordinates can be abstracted an movements




processed directly by a computer. Further difficulties may be

experienced in finding a vantage point with in adequate field or

view ; weather conditions and camera failures may also occur.

Impulse received from detectors can be fed to multi-pen apparatus,

which event-mark a moving mark, a moving chart. Suitably arranged

detectors can evaluate lane volume, lane speed, and headways or push

button switches used to record events. Chart speed is usually variable.

13.2.4 RADAR SPEED METERS.

The apparatus transmit high-frequency electromagnetic waves in a

narrow beam towards a selected vehicle, and the reflected waves, altered

in length depending on the vehicle’s speed, are return to a receiving unit

calibrated to directly record the spot speed.

Another type develop more recently uses a system of narrowreflecting strips producing an image of the vehicle as a succession of

separated vertical events focused on a photodiode. The output voltage

from the cell varies as a frequency directly related to the speed of the

target vehicle. Measurements are made at right-angels to the flow and the

meter is passive in operation, not transmitting light or radio energy.

Recently develop optical meters enable the speed of other vehicles

to be measured directly from a moving vehicle by timing its passage for a

length of travel. The target vehicle can be moving in any direction relative

to the observer provided that it remains visible during the timing operation.




13.2.5 ENOSCOPE.

To overcome this parallax effect, use can be made of an

Enoscope. This instrument, also known as a Mirror-box, is an L-

shaped box, open at both ends, which contains mirrors set at a 45-

degree angle. One of these boxes is located at each end of the test

length, and the observer takes up a position approximately midway

between. As he looks into the appropriate Enoscope his line of sight

is bent so as to be perpendicular to the direction of travel. Thus he

can start and stop the stop-watch the instant the vehicle passes by

the appropriate box, and so more accurate measurements can be

obtained. Night-time measurements can also be taken by placing

small lights at the reference points directly opposite the mirror-boxes.

As vehicles flash by they break the beam, thus again indicating the

beginning or ending of timing.

13.3 SPOT SPEED DATA ANALYSIS

Usually, travel speeds are measured continuously at various points.

The system detectors measure the speed of each vehicle that passes over

the detection zone. This data is recorded and processed to analyze the

travel speeds and volumes. The daily average volume of vehicles for each

24-hour period is presented for each month in terms of average, median,

and 85th-percentile speed.




i. Average speed

Average speed is the most commonly used speed statistic and

does the total number of vehicles divide the sum of all individual

speed measurements.

ii. Median speed.

Median speed is the speed under which 50 percent of vehicles

travel.

iii. 85th percentile speed

85th percentile speed is sometimes referred to as the critical speed

as it is commonly used as a guide in establishing reasonable speed

limits. This represents the speed under which 85 percent of the

vehicles are traveling.

The last three columns of the table reflect the percentage of

vehicles exceeding 65 miles per hour, 70 mph and 75 mph.




Wardrop has shown that time and space-mean speeds are

connected by the relationship

S S T V

V V 2

σ +=

Where σs is the standard deviation of V s

13.3.2 ANALYSIS OF SPEED STUDIES

Because in any speed study a considerable number of speeds are

observed, statistical techniques are used to analyze the data obtained.

Depending upon the accuracy of the data, the use to which the derived

results are to be put and number of observations obtained, a suitable

class interval is chosen.

13.3.2.1 Statistic

Table 1.1 shows speed observations obtained on a major

traffic route. Individual speeds have been grouped into 4 km/ h

classes given in column 1 an interval which reduces the data into

an easily managed number of classes yet does not hide the basic

form of the speed distribution. In the selection of class intervals

thought should be given to the dial readings when observation of




the speed is made. Most speeds will be recorded to the nearest dial

reading and these form convenient mid-class marks.

( Table 1 )

1 2 3 4 5 6 7 8

Speed

class

(km/h )

Frequenc

y

Percentage

frequency

Cummulati

ve

frequency

Percentage

cumulative

frequency

Deviation (2)x (6) (2)x( 6)²

44 -47.9

48 – 51.9

52 – 55.9

56 – 59.9

60 – 63.9

64 – 67.9

68 – 71.9

72 – 75.9

76 – 79.9

80 – 83.9

84 – 87.9

88 – 91.9

92 – 95.9

96 – 99.9

100 –

103.9

104 –

107.9

108 –

111.9

112 –

115.9116 –

119.9

120 –

123.9

1

2

2

4

11

24

40

48

63

40

34

29

25

13

5

3

1

2

2

1

0.286

0.571

0.571

1.143

3.143

6.875

11.429

13.714

18.000

11.429

9.714

8.286

7.143

3.714

1.429

0.857

0.286

0.571

0.571

0.286

1

3

5

9

20

44

84

132

195

235

269

298

323

336

341

344

345

347

349

350

0.286

0.857

1.429

2.571

24.000

37.714

55.714

67.143

76.857

85.143

76.857

85.143

92.286

96.000

97.429

98.286

98.571

99.143

99.143

100.000

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

-9

-16

-14

-24

-55

-69

-120

-96

-63

0

34

58

75

52

25

18

7

16

18

10

81

128

98

144

275

384

360

192

63

0

34

116

225

208

125

108

49

128

162

100

∑ 350 ∑-180 ∑ 2980




The mean speed is given by

mid- class mark of selected class + class interval ∑ ( column 7 )

∑ ( column 2 )

82 – 4.180 = 79.9 km/h

350

The standard deviation is given by

Class interval ∑ (frequency (deviation)²) - ∑ ( frequency x deviation ) ²

∑ (column 2 ) ∑ (column 2 )

The value of ∑ (frequency x deviation) has already been

calculated in column 6 and it now necessary to calculate the

frequency ( deviation ) ² for each speed class. These values are

given in column 8.

4 2980 _ -180 ² = 11.6 km / h

350 350




Percentage of vehicles traveling at a speed equal to , or less than.

( ) (( ) )dvV V xV

u

22

2/exp2/1 σ π σ

σ

−−= ∫

Where

V is the mean speed ,

σ is the standard deviation of speeds.

A demonstration of the fit of the observed cumulative speed

distribution to a cumulative normal distribution may be obtained by

plotting the probits of the percentage of vehicles traveling at or less

than a certain speed, against the speed upper class limit. Value of

probits may be obtained from the suggested reading or can be

obtained from figure table 1 . The use of this technique converts a

cumulative normal curve into a straight line whose equation is

Probit of percentage of vehicles traveling at ;

Speed <(

( ( )∑−+

=2

15

devision frequency

V V V

Using the derived values of σ and V this givens

Probit of percentage of vehicles travelling at

a speed < V = 5 + 0.0862 ( V – 79.9 )

= 0.0862 V – 1.6887




(Table 2)

1 2 3 4 5 6 7 8

Upper

speed

class limit

( km / h)

Column 1

minus

mean

speed

Column 2

divided by

standard

deviation

Normal

area

Probability Theoretical

frequency

Observed

frequency

((6) – (7) )² /

(6)

44 35.9 -3.10 -0.499

48 31.9 -2.75 -0.497 0.002 0.7 1

52 27.9 -2.40 -0.492 0.005 1.8 2 2.27

56 23.9 -2.06 -0.480 0.012 4.2 2

60 19.9 -1.72 -0.457 0.023 8.1 4

64 15.9 -1.37 -0.415 0.042 14.7 11 0.93

68 11.9 -1.025 -0.349 0.066 23.1 24 0.04

72 -7.9 -0.680 -0.252 0.097 33.9 40 1.1076 -3.9 -0.336 -0.132 0.119 41.9 48 0.89

80 +0.1 0.009 0.004 0.137 48.0 63 4.69

84 +4.1 0.354 0.138 0.134 46.9 40 1.02

88 +8.1 0.70 0.258 0.120 42.0 34 1.52

92 +12.1 1.04 0.351 0.093 32.6 29 0.40

96 +16.1 1.39 0.418 0.067 23.4 25 0.11

100 +20.1 1.74 0.459 0.041 14.3 13 0.12

104 +24.1 2.08 0.481 0.022 7.7 5

108 +28.1 2.42 0.492 0.011 3.8 3

112 +32.1 2.76 0.497 0.005 1.8 1 0.01

116 +36.1 3.11 0.499 0.002 0.7 2

120 +40.1 3.46 0.500 0.001 0.4 2

124 +44.1 3.81 0.500 0.000 0 1∑ 13.10

13.3.2.2 Graphic - Histograms and Frequency Curve

The information revealed by the frequency distribution table

in best presented graphically by histograms and frequency curves

as in Fig 8.14.1. This histogram is plotted directly from columns 1




the 4 of the table and the frequency curve is found by rounding off

the histogram in such a way that the area under the curve is equal

to the area of the histogram. The modal speed is the speed

occurring most frequently and is the peak of the frequency curve.

The curve is also useful for determining the pace of the vehicles

where the pace is the speed range, for some nominal increment of

speed (usually 20 km/h ) which contains the most vehicles. In the

example the mode is approximately 68 km/h and the 20 km/h pace

is 60-80 km/h.

Fig 13.3.1 Histogram And Frequency curve spot speeds on three-lane rural trunk road.




13.3.2.3 Cumulative Frequency Curve

The cumulative frequency curve or ogive is used for

determining the number of vehicles traveling above or below a

given speed. It is plotted from columns 5 and 1 of the table and the

shape of the curve is seen in Fig. 8.14.2. The median speed,

another measure of central tendency, is that speed below which

50% of the vehicles are moving, in this case, 70 km/h. Percentile

speeds ( i.e. that speed below which a specified percentage of

vehicles are traveling ) are also readily indicated. The percentile

speeds of particular interest are the 98th percentile which can be

used in the consideration of speed limit imposition or overtaking

distances, and the 15th percentile shows the slower vehicles whose

speed may be causing interference within the traffic stream. These

percentile speeds are respectively 112 km/h, 90 km/h and 58km/h

for this road.




13.5 CALCULATION OF DENSITY

The relationship between traffic speed, volume, and density is shown by

the fundamental equation

k = su

q

Where

q = average volume of flow ( vehicle/hr )

k = average density or concentration ( vehicle/mile )

su = space-mean speed ( mph ).

Fig. 14.3: Example Speed Histogram

Fig. 13.3: Example Speed Histogram




TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT

INPUT

1. Speed is the rate of travel usually expressed in kilometers per hour (km/h)

and is generally classified into to three main types. What are the three main

types of speed?

a. ____________ b. ____________

c. ____________

2. What is the definition of density?

ACTIVITY 10

Question




1. The three main types of speed are:

a. Spot speed

b. Running speed

c. Journey speed

2. Traffic density, also referred to as traffic concentration, is defined as the

average number of vehicle occupying a unit length of roadway at a given

instant; it is generally expressed in units of vehicles per mile. Traffic

density bears a functional relationship to speed and volume.

FEEDBACK ON

Answer




1. Describe two measurement methods that are used to determine the speedof vehicle.

2. How to analyze the speed data?

Question




1. MOVING OBSERVATION METHOD.

1. Moving Observation Method

The median and mean speeds are only equal if the speed

distribution is symmetric which tends to be the case in free-flowing

conditions. The large number of runs required makes the method

costly and difficult to apply for limited access roads, particularly

rural motorways. A partial moving vehicle method has been used

successfully where only section journey times are recorded while

classified vehicle counts are continuously made at the mid-points of

each run. The data is more accurate and manpower is effectively

employed. Other modified methods have included the mounting of

a rime-lapse camera in a good, forward field of vision, to take

photographs at intervals of 1 s. while good results were obtained,

care had to be exercised in avoiding tailing high vehicles; the

outside lane gave a better observational position. A fuller record of

delays is available for analysis and the influence of parking and

pedestrians can be included in the assessments. While crew

running time reduced, subsequent analysis is lengthy, although an

important advantage is the ability to vary the section points in the

aggregation of a route. Similar possibilities are available for using

video recorders for subsequent replay of television film.

Answer




Enoscope.

To overcome this parallax effect, use can be made of an

Enoscope. This instrument, also known as a Mirror-box, is an

L-shaped box, open at both ends, which contains mirrors set

at a 45-degree angle. One of these boxes is located at each

end of the test length, and the observer takes up a position

approximately midway between. As he looks into the

appropriate Enoscope his line of sight is bent so as to be

perpendicular to the direction of travel. Thus he can start and

stop the stop-watch the instant the vehicle passes by the

appropriate box, and so more accurate measurements can be

obtained. Night-time measurements can also be taken by

placing small lights at the reference points directly opposite

the mirror-boxes. As vehicles flash by they break the beam,

thus again indicating the beginning or ending of timing.

2. a. Statistic.

b. Graphic - Histograms and Frequency Curve

Unit 13 ( BASIC CHARACTERISTICS OF A ROAD SYSTEM ( Part 3 ) )

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Transcript of Unit 13 ( BASIC CHARACTERISTICS OF A ROAD SYSTEM ( Part 3 ) )