Intersection Design Chapter 7 Dr. TALEB AL-ROUSAN

Intersection Design

Chapter 7

Dr. TALEB AL-ROUSAN

Introduction An intersection: is an area, shared by two or more

roads, whose main function is to provide for the change of route direction.

Intersection vary in complexity: Simple intersection: tow roads crossing at right angles. More complex: three or more roads cross.

Drivers have to make decision on intersections concerning which of the routes they wish to take.

Due to the above effort, which is not required at non-intersections, Intersections tend to have high potential for crashes.

The overall traffic flow on any highway depends largely on the performance of the intersections as they operate at lower capacity than through sections of the road.

Classes of Intersections1. Grade-separated without ramps.2. Grade-separated with ramps (Interchanges).3. At-grade intersections. Grade-separated intersections usually consist of

structures that provide for traffic to cross at different levels (vertical distance) without interruption.

Potential for crashes at grade-separated intersections is reduced because many conflicts between intersecting steams are reduced.

At-grade intersections do not provide for the flow of traffic at different levels, therefore conflicts conflict between intersecting streams exist.

Types of At-Grade Intersections

1. T or three-leg intersections: consist of 3 approaches

2. Four-leg or cross intersections: consist of 4 approaches.

3. Multi-leg intersections: consist of 5 or more approaches.

Channelization involves the provision of facilities such as pavement markings and traffic islands to regulate and direct conflicting traffic streams into specific travel paths

T- IntersectionsFigure 7.4a:

Simplest form of T intersection.

Suitable for minor or local roads.

Suitable when minor roads intersect important highways with angle less than 30 from the normal.

Suitable for use in rural two-lanes highway that carry light traffic.

T- Intersections Cont. Figure 7.4b:

At location of higher speeds and turning volumes, which increase potential of rear-end collisions, an additional surfacing or flaring area is provided.

Flare is provided to separate right-turning vehicles from through vehicles approaching from the east

T- Intersections Cont. Figure 7.4c:

In cases where left-turn volume from the through road onto the minor road is high but doesn’t require a separate left-turn lane, an auxiliary lane may be provided.

T- Intersections Cont. Figure 7.4d:

Channelized T intersection: one with divisional islands and turning roadways.

Two-lane highway has been converted into a divided highway through the intersection.

Provides both a left-turn storage lane and right-turn lane for turning traffic.

Suitable for locations where volumes are high.

An intersection of this type will probably be signalized.

Four-Leg Intersections

Figure 7.5a: Used mainly at

locations where minor and local roads cross or when minor road crosses a major one.

Turning volumes are usually low.

Roads intersect at angles less than 30.


Figure 7.5b: When turning

movements are frequent, right turning roadways are provided.

Common in Urban areas where pedestrians are present.


Figure 7.5c: Suitable for two-lane highway that is not

a minor crossroad and that carries moderate volumes at high speeds or operates near capacity.


Figure 7.5d: Suitable when high through and turning volumes. This type of intersection is usually signalized.

Multi-leg Intersections Have five or more approaches. See Figure 7.6. In order to remove some of the conflicting movements from

the major intersection to increase safety and operation, one or more of the legs are realigned.

For 5-leg intersection, realigning one road will create an additional T intersection, and convert the multi-leg into a four-leg intersection (Fig. 7.6 a)..

For 6-leg intersection, realigning two roads will create an convert the multi-leg into a two four-leg intersections (Fig. 7.6 b).

Two factors to consider when realigning roads: The diagonal road should be realigned to the minor road. The distance between the intersections should be such that they

can operate independently.

Multi-leg Intersections/ Traffic Circles

Is a circular intersection that provide a circular traffic pattern with significant reduction in in the crossing conflict points.

FHWA describes three types of traffic circles:

1. Rotaries.2. Neighborhood Traffic Circles.3. Roundabouts


Rotaries: have large diameters > 300 ft which allow speeds

exceeding 30 mi/h with min. horizontal deflection of the path of the through

traffic. Neighborhood Traffic Circles:

have diameters smaller than rotaries which allow much lower speeds.

Used at intersections of local streets as a mean of traffic calming or as an aesthetic device.

Consist of pavement markings not raised islands. May use stop control or no control at the

approaches.


Roundabouts: have specific defining characteristics that separate them from other circular intersections

Yield control at each approach. Separation of conflicting traffic movements by

pavement markings or raised islands Geometric characteristics of the central island

that typically allow traffic speeds for less than 30 mi/h.

Parking usually not allowed within the circulating roadway.

Figures 7.7 a & b show the features and dimensions of roundabouts respectively.

Categories of Roundabouts Roundabouts can be categorized into six

classes based on the size and the environment in which they are located.

1. Mini-roundabouts.2. Urban compact roundabouts.3. Urban single-lane roundabouts.4. Urban-double-lane roundabouts.5. Rural single-lane roundabouts.6. Rural double-lane roundabouts

See Table 7.1 for the characteristics of roundabout categories.

Design principles For At-Grade Intersections

Objectives: To minimize the severity of potential conflicts among different

streams of traffic and between pedestrians and turning vehicles.

To provide smooth flow of traffic across the intersection. Design should incorporate the operating characteristics of

both vehicles and pedestrians using the intersection. For example: corner radius of an intersection should not be

less than either: Turning radius of the design vehicle Radius required for the design velocity

Design should ensure adequate pavement widths of turning roadways and approach sight distance.

At-grade intersections should not located at or just beyond: Sharp crest vertical curves. Sharp horizontal curves.

Design principles For At-Grade Intersections Cont.

Design involves: Design of alignment. Design of suitable channeling system Determination of minimum required

widths of turning roadways at speeds higher than 15mi/h.

Assurance of adequate sight distance for type of control used at intersections.


Alignment (Horizontal): Best alignment when intersecting roadways

meet at right or nearly right angles. Less road area is required for turning at

intersections. Lower exposure time for vehicles crossing the

main traffic flow. Visibility limitations specially for trucks are not

as serious as those at acute angles. See Figure 7.8 for alternative methods of

realigning skewed intersections.


Profile (Vertical alignment): Combination of grade lines should be provided to

facilitate the driver’s control of the vehicle. For example:

large changes in grades should be avoided (grades should not be > 3% since stopping and accelerating distances on such grades are not much different from those on flat sections).

In any case it is not advisable to use grades higher than 6% at intersections.

When it is necessary to adjust grade lines of the approaches at an intersection, it is preferable to: Keep or continue with the grade of the major road

across the intersection and Adjust the grade of the minor road at a suitable

distance from the intersection


Curves: Design is affected by Angle of turn. Turning speed:

For speeds < 15 mi/h curves are designed to conform to minimum turning path of the design vehicle.

For speeds < 15 mi/h, three types of design are used (Simple curve (R > 25 ft), simple curve (R >20 ft) with taper (1:10), and 3-centered compound curve) See Figure 7.9, and 7.10.

For speeds > 15 mi/h, design speed should also be considered.

Design vehicle. Traffic volume. See Tables 7.2, 7.3, and 7.4 for min. turn radii


Channelization: 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.

Traffic Island: is a defined area between traffic lanes that is used to regulate the movement of vehicles or to serve as a pedestrian refuge.

A properly channelized intersection will result in increased capacity, enhance safety, and increased driver confidence.

Over channelization should be avoided as it may create confusion to motorists which may result in a lower operating level.


Channelization is normally used to achieve:1. Direct paths of vehicles.2. Control merging, diverging, or crossing angle of

vehicles.3. Decrease vehicle wander and area of conflict by

reducing amount of paved area.4. Provide pedestrian refuge5. Give priority to predominant movements.6. Provide clear indication of proper path.7. Provide separate storage lanes for turning vehicles. 8. Provide space for traffic control devices.9. Control prohibited turns.10. Restricts speed of vehicles.11. Separate traffic movements at signalized

intersections with multiple phase signals.


Factors affecting design of Channelization are:

Availability of ROW. Terrain Type of design vehicle. Expected vehicular and pedestrian

volumes. Approach speeds. Bus stop requirements. Location and type of traffic control

devices.

General Characteristics Of Traffic Islands

Curbed Traffic Islands: used in urban highways with high pedestrian volumes

and low speeds. Because of glare, they bare difficult to see at night, thus

intersections should be lighted. Pavement markings (Flushed islands):

Markers include: paint, thermoplastic stripping, and raised retroflective markers.

Preferred over curbed island at intersections where approach speed is high, pedestrian traffic is low, and where signals or sign mountings are not located on the island.

Islands formed by pavement edges: Usually unpaved Used at rural intersections where there is space for large

intersection curves.

Functions of Traffic Islands Channelized Islands:

Used to control and direct traffic. See Figure 7.12.

Divisional Islands: Used to divide opposing or same-direction

traffic streams. See Figure 7.13.

Refuge Islands: Used to provide refuge for pedestrians.

Minimum Size of Islands Islands should be large enough to command the

necessary attention by drivers. AASHTO recommends island area of:

50 sq ft for urban intersections. 75 sq ft for rural intersections. 100 sq ft is preferable for both.

AASHTO recommends island min side lengths: 12 ft ( 15 preferred) for triangular island (small). 15 to <100 ft for medium islands. 100 ft or more for large islands

See figures 7.15 and 7.16.


Min. Pavement Widths of Turning Roadways : When vehicle speeds > 15 mi/h on channelized

intersections and where ramps intersect with local roads, it is necessary to increase the pavement widths of the turning roadways.

Three classifications of pavement widths are used: Case I: One-lane, one-way operation with no provision for

passing a stalled vehicle. Case II: One-lane, one-way operation with provision for

passing a stalled vehicle. Case III: two-lane operation, either one-way or two-way.

See Tables 7.5 for design widths of pavements for turning roadways.

See Example 7.1.

Sight Distance At Intersections High crash potential at an intersection can be

reduced by providing sight distances that allow drivers to have an un obstructed view of the entire intersection at a distance great enough to permit control of the vehicle.

At signalized intersection, the unobstructed view may be limited to the area where signals are located.

For unsignalized intersections, it is necessary to provide an adequate view of the cross roads.

The sight distance required depend on the type of control at the intersection (no control, yield control, stop control, signal control).

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Intersection Design Chapter 7 Dr. TALEB AL-ROUSAN