CE416 Lecture 9: Highway Intersection 1

Lecture 10
Highway Intersection

CE416 – Fall 2015 Prof. Mohamed Esekbi

Highway Intersection

 Definition:
An intersection is an area, shared by two or
more roads, whose main function is to
provide for the change of route directions.
 Intersections vary in complexity from:
 simple intersection: has only two roads crossing
at a right angle
 complex intersection: three or more roads cross
within the same area.

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CE416 Lecture 9: Highway Intersection 1

Highway Intersection

 Drivers therefore have to make a decision

at an intersection concerning which of the
alternative routes they wish to take.
 Intersections tend to have a high
potential for crashes.

Highway Intersection

 The overall traffic flow on any highway

depends to a great extent on the
performance of the intersections,
 since intersections usually operate at a
lower capacity than through sections of
the road.

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CE416 Lecture 9: Highway Intersection 1

Basic Forms of Intersections

T Y
Scissors

Cross Staggered

Staggered and
skewed multiway

Highway Intersection

 Intersections are classified into three

general categories:
 at-grade
 grade-separated without ramps,
 grade-separated with ramps (commonly
known as interchanges),

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CE416 Lecture 9: Highway Intersection 1

Examples of At-Grade Intersections in Urban Areas

Three-Leg Intersection

Examples of At-Grade Intersections in Urban Areas

Y-Intersection

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CE416 Lecture 9: Highway Intersection 1

Examples of At-Grade Intersections in Urban Areas

Four-Leg Intersection

Examples of Grade Separated Intersections

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CE416 Lecture 9: Highway Intersection 1

Examples of Grade Separated Interchanges

Trumpet

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CE416 Lecture 9: Highway Intersection 1

3-leg directional

Cloverleaf interchange

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CE416 Lecture 9: Highway Intersection 1

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;
 Multi-leg intersections, which consist of five
or more approaches.

T (3-leg)intersections
 The intersection shown in Figure (a) is suitable
for minor or local roads and may be used when
minor roads intersect important highways with
an intersection angle less than 30 degrees from
the normal. also suitable for use in rural two-lane
highways that carry light traffic

 At locations with higher speeds and

turning volumes, which increase the
potential of rear-end collisions between
through vehicles and turning vehicles,
usually an additional area of surfacing or
flaring is provided, as shown in Figure (b)

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CE416 Lecture 9: Highway Intersection 1

Examples of T Intersections
 Flare is provided to
separate right-turning
vehicles from through
vehicles approaching from
the east
 In cases where left-turn
volume from a through
road onto a minor road is
sufficiently high but does
not require a separate left-
turn lane, an auxiliary lane
may be provided, as
shown in Figure (c)

Examples of T Intersections
 Channelization involves
the provision of facilities
such as pavement
markings and traffic
islands to regulate and
direct conflicting traffic
streams into specific travel
paths.
Figure (d) shows a channelized T intersection in which the two-lane
through road has been converted into a divided highway through the
intersection.
intersection of this type probably will be signalized.

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CE416 Lecture 9: Highway Intersection 1

Examples of Four-Leg Intersections

Four-Leg Intersections
 unchannelized intersection
shown in Figure (a) used mainly
at locations where minor or local
roads cross.
 it also can be used where a
minor road crosses a major
highway.
 In these cases, the turning
volumes are usually low and the
roads intersect at an angle that
is not greater than 30 degrees
from the normal.

Examples of Four-Leg Intersections

When right-turning movements
are frequent, right-turning
roadways, such as those in Figure
(b), can be provided.
also common where pedestrians
are present.

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CE416 Lecture 9: Highway Intersection 1

Examples of Four-Leg Intersections

The layout shown in Figure (c) is suitable for:

a two lane highway that is
not a minor crossroad and that
carries moderate volumes at high speeds or
operates near capacity.

Examples of Four-Leg Intersections

Figure (d) shows a suitable design for four-lane
approaches:
carrying high through volumes and
high turning volumes.
This type of intersection is usually signalized.

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CE416 Lecture 9: Highway Intersection 1

Examples of Multileg Intersections

Multi-leg intersections have
five or more approaches.
Whenever possible, this
type of intersection should
be avoided.
In order to:
• remove some of the conflicting
movements
• increase safety and operation,
one or more of the legs are realigned.
Figure (a) , the diagonal leg of the
intersection is realigned

Examples of Multileg Intersections

This results in the formation of an

additional T intersection
but with the multileg intersection now
converted to a four-leg intersection.
two important factors to consider:
1. the diagonal road should be
realigned to the minor road
2. the distance between the
intersections

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CE416 Lecture 9: Highway Intersection 1

Examples of Multileg Intersections

realignment of a six-leg intersection

Figure (b), forming two four-leg
intersections.
realignment to be made to the
minor road.
forming two additional T
intersections and resulting in a total
of three intersections.

the distances between these intersections should be great enough

to allow for the independent operation of each intersection.

Traffic Circles (Roundabouts)

A traffic circle is a circular intersection that provides a circular traffic
pattern with significant reduction in the crossing conflict points.

There are three types of

traffic circles:
1. rotaries,
2. neighborhood
traffic circles
3. roundabouts.

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CE416 Lecture 9: Highway Intersection 1

Traffic Circles

Rotaries
have large diameters that are usually
greater than 100m, thereby allowing
speeds exceeding 45km/h, with a
minimum horizontal deflection of the path
of the through traffic.

Traffic Circles

Neighborhood traffic circles

have diameters that are much smaller than
rotaries and therefore allow much lower
speeds.
 Consequently, they are used mainly at the
intersections of local streets,
 traffic calming aesthetic device.
 they consist of pavement markings and do
not usually employ raised Islands.

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Traffic Circles
Roundabouts
have specific defining characteristics that separate them
from other circular intersections. These include:
 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 travel speeds of less than 30 mi/h
 Parking not usually allowed within the circulating
roadway.

Traffic Circles

Geometric Elements
and Example of
Roundabout

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Example of Roundabout

Traffic Circles
Roundabouts can be further categorized into
six classes based on the size and 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

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Characteristics of Roundabout Categories

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