Intersection Features.

7/30/2019 Intersection Features.

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[Inventory of Intersection ] [CE 6502]

[M.M.A. Kader Chowdhury] Page 1

AN INVENTORY FOR INTERSECTION DESIGN

ESSENTIAL PHYSICAL ELEMENT:-

These essential physical elements can be described in four ways:-

A. Traffic point of view.

B. Pedestrians point of view.

C. Important road signs.

D. Important road markings.

I.Traffic point of view.

a) Number of Legs.

The number of legs should be kept minimum.

If possible, some minor roads may be connected with each other before joining a major

road.

The alignment between the roads should be vertical or close to 90.

b) Area Assigned.

The area assigned for construction of an intersection should be reasonable large so thatfuture modification.

The use of corner plots in an intersection should be controlled or restricted so that theydont compromise the desired output of the intersection.

In an intersection, the alignment of the structures at the corner plot should be diagonal(V-shaped) in order to provide adequate sight distance clear to ensure safety.

Rights-of-way on abutting residential or commercial properties should be controlled.

c) Channelizing anddivisional islands It may be added to an intersection to help delineate the area in which vehicles canoperate, and to separate conflicting movements.

Islands can also provide for pedestrian refuge.

Geometric layout like channelizing and staggering should be selected so that hazardousmovements like cutting, diverging, merging, etc by drivers are eliminated.

Multiple maneuvers should be converted to elemental maneuvers.


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The area of conflict should be eliminated by reducing excessive pavement areas.

The approach and departure paths and the actual conflicting area should be clearlydefined. If physical islands cant be constructed, ghost islands may be drawn.

The drivers should be confronted with only one decision at a time. Preference should begiven to the major movements.

d) Spacing of Intersections

The spacing of the intersections should be maintained basing on the roadway

classification. A guideline is appended below:

Number Type of street Suggested minimum

spacing

1 Expressway 1000 meters

2 Arterial streets 500 meters.

3 Sub-arterial streets 300 meters4 Collector streets 150 meters

5 Local streets Free access.

e) Pedestrian and NMV facilities

Adequate priority must be given on pedestrians and bicyclists.

f) Street Lighting

Sufficient street lighting should be provided.

g) Geometric Proportioning

Proportioning of geometric elements should be done by maintaining a balance between

the capacity of an approach and that of an exit. Additional spaces may be removed if

required.


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h) Variation in Junction design

Topromote in built safety, junctions at all residential areas should preferably be of T orStaggered type.

To enhance capacity, junctions in commercial areas should be of grid iron pattern.

Roundabouts are more preferable instead of traffic signals for multi-leg intersections.

Existing roundabouts should be removed when they are controlled by traffic signals.

i) Provisions for Right/Left Turning

Exclusive right turning and U turn lanes should be provided. Not all the vehicles should

be allowed to reach the intersection.

The left turning vehicles should be channelized to a predetermined lane so that

directional conflict is minimized.

Construction of such channels requires corner widening at the intersection in the form of

a three centered compound curve (ratios of the radii R1:R2:R3 = 2.5:1:5.5).

j) Layout of the intersection

The layout should follow the natural vehicle paths.

Smoothness, in contrast to abrupt and sharp corners, should guide minor streams into

stopping or slowing down positions.

k) Traffic control devices Assign right of way to both motorized and non-motorized traffic and include traffic

signals.

Pavement markings, STOP signs, YIELD signs, pedestrian signal heads and otherdevices should be placed sufficiently.


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l) Nose treatment

Nose is found at the beginning of a channel or island. If the noses are not treated well, these road features may be deleterious and act as a

major cause of accidents.

There are some means ofnose treatment like markings around the nose, tigers tail,retro reflective colors, mild longitudinal slope with marking etc.

m) Box junctions They are normally used on large busy junctions such as traffic light controlled cross

roads, T-Junctions and even roundabouts.

These have crisscross yellow lines painted on the road.

A driver should control his speed on approach to box junctions, slow down and stopbefore the box junction.

He should not to enter a box junction unless his exit road is clear.

He should move off, after looking carefully toensure it is safe.

Method of Using a Box Junction


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How to use a Box Junction Box junctions make it possible to keep the flow of traffic moving and therefore keep the

junction clear, by preventing traffic from stopping in the path of crossing traffic. A driver has to enter the box junction if he wishes to turn right and his path is blocked by

oncoming traffic.

It is OK for him to wait on the yellow crisscross providing his exit road is clear.

When the way is clear move away smartly and do not cause an obstruction.

One MUST NOT enter the box until the exit road or lane is clear.Because there is a line of traffic already in the road on the right, there is no room to join thisqueue without blocking the flow of traffic shown by the yellow cars.

II. Pedestrians point of view

a) Picket Rail

Picket rails should be provided at all the corners of an intersection and in the vicinity of

an intersection to control, regulate and streamline pedestrian movements who often have a

tendency to violate existing traffic rules.

Figure: - Picket rail

b) Pedestrians Landing Treatment

Likely pedestrians crossing locations should correspond to the placement of sidewalks alongapproaching streets and pedestrian curb cut ramps need to ensure accessibility to crossing

locations.

The landings of footpaths, pedestrian refuges should be flushed with the pavement surface.

In busy intersections, if slanted landing is used for the handicaps, the texture of the landing is

made different from the surface of the sidewalk.


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It will also facilitate other pedestrians moving without stumbling at the edges. Besides, the

level/height of footpaths at driveways may be kept same to that of footpaths giving priority to

the pedestrians.

It will make walking easier and will also act as a speed hump (base size 6 to 14) across

the drive way providing additional safety to the road users.

Figure: Landing treatment.

c) Bollard Treatment

Bollards are used to prevent the potential of damage arising from vehicle derailment.

Where the sidewalk and road are made at same level or the landing is used forpedestrian to reach crossing at road level, bollard treatment is warranted.

Bollards also prevent entry of motorcycles and other motorized vehicles on to thefootpath.

Figure: - Bollard

d) Impact Treatment/Shock Absorbing Bollards Impact treatment is also associated with nose of an island or channel.

There are mainly two types of treatment to minimize the severity of impact between the

vehicle and the obstacle. These are termed as (i) collapsible treatment and (ii) energy

absorbing treatment


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If the collapsible is used, the impact is lessened by the breakdown of the constituents of the

collapsible. Energy is absorbed by the change of shape of the obstacle (e.g. signal post) in

some cases.

Figure:-Shock absorbing Bollard

e) Crossing Facilities

Zebra Crossing/ Ladder Crossing are a kind of priority controlled pedestrian crossing facility.

At STOP controlled, YIELD controlled, and uncontrolled intersections, pedestrians ability to

cross the street and the delay experienced is influenced by the yielding behavior of motor

vehicles. Signalized pedestrian crossings may have separate signals for pedestrians.

They may also be equipped with countdown timers.

In Bangladesh, the situation prevailing all around the intersections and links may be termed as

uncontrolled crossing.

In UK, Zebra/ladder crossings are accompanied by a yellow flashing light so that vehicle

drivers can understand the presence of a pedestrian crossing from a distance and control their

vehicles accordingly.

Figure: - Crossing Facilities.


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f) Time Separated Pedestrian Crossing Facilities

These types of crossing facilities are provided when the volume of pedestrians is toohigh.

These sorts of crossings are normally marked with dashed lines.

In some developed countries where the number of pedestrians and vehicles are both toohigh, a separate type of signal having an All Red Period has been successfully used.

This type of signaling allows diagonal crossing as well.

Figure: - Crossing facilities.

g) Determination of Walking Speed

Under normal conditions, pedestrian walking speeds on sidewalks and crosswalks range

from 2.5 feet per second to 6 feet per second.

Elderly pedestrians and young children will generally be in the slower portion of this range.

A walking speed of 3.5 to 4 feet per second for crosswalk signal timing is widely accepted

as a guideline for walking speed in crosswalks.

h)Call button (Pelican Crossing) Use should be limited to only those locations with traffic-actuated signals (i.e., where the

signal does not cycle in the absence of minor street traffic).

Where call buttons are used, a notification sign should be provided. Pedestrian call buttonactuation should provide a timely response, particularly at isolated signals (i.e., not in a

progression sequence), at mid-block crossings, and during low-traffic periods (night, forexample).

i) Pedestrian Island/ Refuge Island

Pedestrian islands are mainly used to give temporary shelter to pedestrians from vehicular

traffic and in some extent for placing sign post and signal post adjacent to the road.

Raised islands should preferably be at least 8 m2 (80 sq. ft) in area. If an island has to be

provided for stop signs, traffic signals and pedestrian crossings, it should be at least 6 m (20


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ft) long with a minimum width of 1.2 m (4 ft) and preferably 1.8 m (6 ft) at the point where the

signal pedestal is erected

The approach end of an island should be offset a minimum of 1 m (3 ft) from the edge of the

adjacent traffic lane and should be preceded by appropriate pavement markings. Stoppingsight distance measured from 1.15 m (3.75 ft) eye height to zero on the pavement should be

provided at the approach end of all islands.

j) Roughness Treatment/Speed Reducer

In order to promote pedestrian safety, sometimes speed of vehicles is compromised.

Especially, before a pedestrian crossing or schools or some residential areas, some

portion of the road is made rough textured so that drivers reduce speed to avoid

inconvenience in riding.

Figure: Roughness Treatment

k) Raised Pedestrian Crossing/ Raised Intersection

Pedestrian crossings may be raised in the form of a speed hump (6 ft to 14 ft base size).

Again if the accident rate of any intersection is too high, the entire intersection may be raisedproviding roughness treatment all over.


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Sometimes, in residential areas, the mid-section of the junction is kept rough to deter thedrivers from speeding as people from the adjacent locality may come out to the street formerrymaking or other purpose.

l) Staggered Pedestrian Crossing (at Grade) Its considered to be the best type of crossing in terms of safety point of view. It canprovide embedded safety against the tendency of pedestrians to rush and cross the streetseven then its not safe at a particular moment.

Figure: Raised Pedestrian Crossings

Some Special Points Emphasized by AASHTO (2001)

As per AASHTO (2001), five basic elements should be considered in intersection design

a. Human factors

Driving habits

Ability of drivers to make decisions

Driver expectancy

Decision and reaction time Conformance to natural paths of movement

Pedestrian use and habits

Bicycle traffic use and habitsb. Traffic considerations

Design and actual capacities

Design-hour turning movements

Size and operating characteristics of vehicles

Variety of movements (diverging, merging, weaving and crossing)

Vehicle speeds

Transit involvement

Crash experience

Bicycle movements Pedestrian movements

c. Physical elements

Character and use of abutting property

Vehicle alignments at the intersection

Sight distance

Angle of the intersection

Conflict area


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Speed-change lanes

Geometric design features

Traffic control devices

Lighting equipment

Safety features

Bicycle traffic Environmental factors

Cross walksd. Economic factors

Cost of improvements

Effects of controlling or limiting rights-of-way on abutting residential orcommercial properties where channelization restricts or prohibits vehicularmovements

Energy consumptione. Functional intersection area

There are a number of design standards for urban intersections based on roadway features at

intersection.


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III. IMPORTANT ROAD SIGNS

No Parking No Stopping Keep Left Keep Right

.

Keep Left/Right Pedestrian Crossing Ahead Pedestrian Crossing Here

Round about Crossing Ahead Signal Ahead

.

Give Way/Yield No Overtaking Parking


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IV. IMPORTANT ROAD MARKINGS AT INTERSECTIONS

Direction Markings Pedestrians Crossings Pedestrian Crossing

Area: No Stopping/

Parking

Ghost Island Disabled Persons Parking Bus Lane

Chevron Markings No Stopping No Parking


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References

1. Dr . Shamsul Hoque, CE 6502, Geometric Design of Highways, Class notes.2. Dr.L.R.Kadiyali, Traffic Engineering and Transport Planning.3. TRRL Guide Book. Intersection design.4. AASHTO (2001).

Intersection Features.

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