lec 02 intersection design

8/17/2019 Lec 02 Intersection Design

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Faculty of Engineering - 3rd

Year 17

Traffic Eng. Intersection Design 15 –16

Dr. Firas H. Asad

In this lecture;

---------------------1- General

2- Types of At-Grade Intersections

2.1 T - Intersections

2.2 Four-Leg Intersections

2.3 Multileg Intersections

2.4 Traffic Circles

3- Design Principles

4- Channelization

Intersection Design تصميم لتقاطع ات

The information included in this lecture is largely taken from Traffic and Highway

Engineering (Garber and Hoel, 2009) and A Policy on Geometric Design of Highways

and Streets (AASHTO, 2011) .

1- General

AnU

intersectionU

is an area, shared by two or more roads, whose main function is to

provide for the change of route directions. Intersections vary in complexityد

�التعقي

ة

�درج

from a simple intersection, which has only two roads crossing at a right angle, to a

more complex intersection, at which three or more roads cross within the same

area. Drivers therefore have to make a decision at an intersection concerning which

of the alternative routes they wish to take (Garber and Hoel, 2009).

Generally, intersections can be categorized into three essential groups:

A- Grade-separated without ramps (overpass or underpass)

B- Grade-separated with ramps (interchanges), and

C- At-grade intersections.

Grade-separated intersections consist of structures that provide for traffic to cross

at different vertical levels without interruption; hence, the potential conflicts


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between intersecting streams of traffic are eliminated. Therefore, possibility of

collisions تصادم

is higher at at-grade intersections.


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2- Types of at-grade intersections

The basic types of at-grade intersections are T or three-leg intersections which

consist of three approaches; four-leg or cross intersections, which consist of four

approaches; multileg intersections, which consist of five or more approaches and

traffic circles.

2-1 T Intersections

Various types of T intersections ranging from the simplest one to a channelized one

with divisional islands and turning roadways. Channelization involves the provision

of facilities such as pavement markings and traffic islands to regulate and direct

conflicting traffic streams into specific travel paths (for more details, see Garber and

Hoel, 2009). Figures below show examples of T-intersections.


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Figure (d) shows a channelized T intersection in which the two-lane through road

has been converted into a divided highway through the intersection. The

channelized T intersection also provides both a left-turn storage lane for left-turningvehicles from the through road to the minor road and a right-turn lane on the east

approach. This type of intersection is suitable for locations where volumes are high

such as high left-turn volumes from the through road and high right-turn volumes

onto the minor road. An intersection of this type probably will be signalized.

2-2 Four-Leg Intersections

According to Garber and Hoel (2009) and AASHTO (2011), four-leg intersection is

designed with different levels of channelization. The unchannelized intersection

shown in Figure (a) below is adopted mainly at locations where minor or local roads

cross . In these cases, the turning volumes are usually low and the roadsتتق� � اطع

intersect at an angle that is not greater than 30 degrees from the normal.


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Figure (d) shows a suitable design (Right-Turn Channelization, Divisional Islands, and

Left-Turn Lanes) for four-leg intersection carrying high through and turning volumes.

This type of intersection is usually signalized.

2-3 Multileg Intersections

Multileg intersections have five or more approaches; whenever possible this type of

intersection should be avoided. In order to remove some of the conflicting

movementsة

�المتالقي

او

ة

�المتقاطع

ير

�الس

ات

�اتجاه

from the major intersection and hence increasing

safety and efficiency, one or more of the legs should be realigned .عمل اعادة مسار لها

In Figure A above, the diagonal leg of the intersection is realigned to intersect the

upper road at a location some distance away from the main intersection. This results

in the formationتش�� كي of an additional T intersection but with the multileg

intersection now converted to a four-leg intersection (Garber and Hoel (2009) and

AASHTO (2011)). A similar realignment of a six-leg intersection is shown in Figure B.


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2.4 Traffic Circles دو ر لمرور

According to AASHTO (2011) and Garber and Hoel (2009), a traffic circle is a circular

intersection that provides a circular traffic pattern with significant reduction in thecrossing conflict points. The Federal Highway Administration FHWA describes three

types of traffic circles: rotaries, neighbourhood traffic circles, and roundabouts.

Rotaries have large diameters that are usually greater than 100m, so allowing

speeds exceeding 50 kph, with a minimum horizontal deflection of the path of the

through traffic )الطول(المرور العابر .

Roundabouts are circular intersections with specific design and traffic control

features that include:

– Yield controlطريقة سيطرة الخضو for all entering traffic.

– Geometric characteristics of the central island �المركزي

زرة

�الج

allow travel speeds of

less than 50 km/h.

– Splitter islands ج� � زرات الفص� � ل on each leg of the roundabout have multiple roles:

separate entering and exiting traffic, deflect and slow entering traffic, and provide a

pedestrian refuge )ملج(حماية للسابلة .

Figure: Typical modern roundabout.


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Roundabouts designed in this manner are often referred to as modern roundabouts

to be distinguished from older rotaries or signalized traffic circles. Figure in the

previous page depicts typical modern roundabout.


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Neighbourhood traffic circles الس� �� كني دوارات الم�� رور ف�� ي االحي�� ا are typically built at the

intersections of local streets for traffic calming and/or aesthetics جمالي� �� ة . The

intersection approaches may be yield-controlled, uncontrolled, or stop-controlled,and the intersection diameter is typically between 15 and 30 m. They do not

typically include raised channelization on the entering roadway to guide driver onto

the circulatory roadway.


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3- Design principles for at-grade intersections

Garber and Hoel (2009) reported that the AASHTO manual consider the

fundamental UobjectiveU in the design of at-grade intersections is to minimize the

severity ش� د of potential conflicts among different streams of traffic and between

pedestrians and turning vehicles. The design should consider the following:

3-1 Alignment of At-Grade Intersections

The best alignment for an at-grade intersection is when the intersecting roads meet

at right or nearly right angles. Because this alignment provide much less road area

for turning, lower exposure time تع�� روق�� ت to other vehicles, and less visibility

limitations problems compared with those at acute-angle intersections. Figure

below shows alternative methods for realigning roads intersecting at acute angles to

obtain a nearly right-angle intersection.

3-2 Profile of At-Grade Intersections

In designing the profile (vertical alignment) at the intersection, a combination of

grade lines راد �حنالا طو �طخ that lead to avoiding the difficulty of controlling vehicles at

intersections. Grades should not be greater than 3 percent. The stopping and

accelerating distances for passenger cars on grades of 3% or less are not much

different from those of cars on flat grades.

3-3 Curves at At-Grade Intersections

The angle of turn ةرادت �سالا, turning speed, and design vehicle are the main factors

governing the design of curves at at-grade intersections. When the turning speed is

15 mph or less, the curves for the pavement edges are designed to conform to at


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least the minimum turning path of the design vehicle ميمية

�التص

ة

�المركب

. When the turning

speed is expected to be greater than 15 mi/h, the design speed should be also

considered.Figure below shows the minimum designs necessary for a passenger car making a

90-degree right turn using a simple curve.

3-4 Channelization of At-Grade Intersections

AASHTO defines channelizationزرا

�ج

�المروي

ة

�الحرك

يم

�تنظ

as the separation of conflicting

traffic movements into definite paths of travel by traffic islands or pavement

markings to facilitate the safe and orderly movements of both vehicles and

pedestrians. A properly channelized intersection will result in increased capacity,

enhanced safety, and increased driver confidence. Traffic islands are classified into

three categories based on their functions: channelized, divisional, and refuge.


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3-5 Minimum Pavement Widths of Turning Roadways at At-Grade Intersections

In cases where vehicle speeds are expected to be greater than 15 mi/h, such as at

channelized intersections and where ramps تدار �

لالس

ة

�

المخصص

ق

�

الطري

زا

�

اج intersect with localroads, it is necessary to increase the pavement widths of the turning roadways.


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3-6 Sight Distance at Intersections

The high crash potential االحتمالي�� ة العالي�� ة للتص�� ادم at an intersection can be reduced by

providing sight distances that allow drivers to have an unobstructed view of theentire intersection at a distance great enough to permit control of the vehicle. At

signalized intersections, the unobstructed view may be limited to the area where

the signals are located, but for unsignalized intersections, it is necessary to provide

an adequate view of the crossroads or intersecting highways to reduce the potential

of collision with crossing vehicles.

This requires an unobstructed triangular area (sight triangle) that allows a clear view

for drivers on the minor and major roads to see an approaching vehicle on the

crossing road in time to avoid a potential conflict as shown in Figure below.


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There are two types of sight triangles, approach sight trianglesالرؤي� ة عن� د االقت� رامثلث� ا and

departure sight عند المغادرة triangles.

The approach sight triangle allows for the drivers on both the major roads andminor roads to see approaching intersecting vehicles in sufficient time to avoid a

potential collision by reducing the vehicle’s speed or by stopping. The decision point

on a minor road of an uncontrolled or yield control intersection is the location

where the minor road driver should start his/her braking or deceleration maneuver

to avoid a potential conflict with an approaching major road vehicle.

The departure-sight triangle allows for the driver of a stopped vehicle on the minor

road to enter or cross the major road without conflicting with an approaching

vehicle from either direction of the major road.

In addition to the speeds of the approaching vehicles, the lengths of the legs of the

sight triangle also depend on the type of control at the intersection. At-grade

intersections either have no control (Case A) or are controlled by one of the

following methods: stop control on the minor road (Case B), yield control on the

minor road (Case C), traffic signal control (Case D), or all-way stop control (Case E).

Consideration should also be given to the maneuver من� اور of left turns from the major

road (Case F).


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4- Channelization

An intersection that is not properly channelized may lower the safety and efficiency

of intersection. In contrast, overchannelization could cause confusion for the

motorist and decrease the operating level of an intersection.

4-1 General Characteristics of Traffic Islands

These islands can be formed by using raised curbs, pavement markings, or the

pavement edges (Garber and Hoel, 2009).

A- Curbed Traffic Islands

A curbed island is usually formed by the construction of a concrete curb that

delineatesيح� د the area of the island, as shown in Figure below. It should be noted,

however, that because of glare الم� نعكس عل� ى الع� ين , curbed islands may be difficult toالض� و

see at night which makes it necessary that intersections with curbed islands have

fixed-source lighting. Curbed islands are used mainly in urban highways where

approach speed is not excessively high and pedestrian volume is relatively high.

B- Traffic Islands Formed by Pavement Markings

Also called flushed island because it is flushed with the pavement, as shown in

Figure in the next page. Flushed islands are formed by pavement markings that


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delineate the area of the island. Markers include paint, thermoplastic striping, and

raised retroreflective markers. Flushed islands are preferred over curbed islands at

intersections where approach speeds are relatively high, pedestrian traffic is low,and signals or sign mountings are not located on the island.

C- Islands Formed by Pavement Edges

These islands are usually unpaved and are mainly used at rural intersections where

there is space for large intersection curves.

4-2 Functions of Traffic Islands

Traffic islands also can be classified into three categories based on their functions:

channelized, divisional, and refuge. Channelized islands are mainly used to control

and direct traffic. Divisional islands are mainly used to divide opposing or same-

directional traffic streams. Refuge islands are used primarily to provide refuge for

pedestrians. In most cases, however, traffic islands perform two or more of these

functions.

A- Channelized Islands

The objective of channelized islands is to eliminate confusion to motorists at

intersections with different traffic movements by guiding them into the correct lane

for their intended movement. This is achieved by converting excess space at the


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intersection into islands to help guiding motorist. A channelized island may take one

of many shapes (f, e, d of Figure b), depending on its specific purpose. For example,

a triangularly-shaped channelized island is often used to separate right-turningtraffic from through traffic (see Figure a) whereas a curved, central island is

frequently used to guide turning vehicles (see Figure b). In any case, the outlines of a

channelized island should be nearly parallel to the lines of traffic it is channeling.

Where the island is used to separate turning traffic from through traffic, the radii of

the curved sections must be equal to or greater than the minimum radius required

for the expected turning speed.


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B- Divisional Islands

Often used at intersections of undivided highways to alert drivers that they are

approaching an intersection and to control traffic at the intersection. They also canbe used effectively to control left turns at skewed intersections. Examples of

divisional islands are shown in Figure below.

C- Refuge Islands

Refuge islands, sometimes referred to as pedestrian islands, are used mainly at

urban intersections to serve as refuge areas for wheelchairs and pedestrians

crossing wide intersections. They also may be used for loading and unloading transit

(public transport) passengersات

�الباص

اب

�رك

. The Figure on the next page shows examples

of islands that provide refuge as well as function as channelized islands.


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lec 02 intersection design

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