CE 401

Highway Engineering

Rakesh Kumar

Intersection Control and Design

Learning Objectives

• To distinguish different types of intersections

• To define design objectives and considerations

• To distinguish different types of control devices and their purposes

• To define and calculate fundamental elements of a traffic signal timing system(Chapter 8, p.287-303)

Intersections

• Why are they important?• Types:

– At-grade intersections– Interchanges– Grade separation

At-grade Intersections

• At-grade: same level

Interchanges

• utilize grade separation

Intersection Design

• Reduce conflicts between road users• Improve efficiency and safety• Consider

– Human factors– Traffic– Geometrics– Economics

Conflicts

Four-leg intersection Staggered-T intersection

Intersection Sight Triangle

Hidden Vehicle

Intersection Sight Triangle

Intersection Sight Triangle

Traffic Control Devices

• Devices used to regulate, warn, or guide traffic, placed on, over, or adjacent to a street, highway, pedestrian facility, or shared-use path by authority of a public agency having jurisdiction

• Promote safety and efficiency by providing for the orderly movement of all road users

Traffic Control Devices

• To be effective, a traffic control device should meet five basic requirements:– Fulfill a need– Command attention– Convey a clear, simple meaning– Command respect from road users– Give adequate time for proper response

• Communication is the key!

Types of Control Devices

• Manual on Uniform Traffic Control Devices, FHWA

http://mutcd.fhwa.dot.gov/HTM/2003r1/html-index.htm– Provide guidelines for

adopting any particular type of intersection control, in the form of warrants

Signs

• Convey information through their shape, color, message, and placement

Channelization

• Separate or regulate conflicting movements

• Define paths of travel • Use traffic islands or pavement

markings

Channelization

Rotaries and Roundabouts

• Channelized intersections with a central circle surrounded by a one-way roadway

Uncontrolled Intersections

• Sufficient stopping sight distance is crucial

Is sufficient stopping sight distance provided?

Traffic Signals

• Purposes– Improve overall safety– Decrease average travel time and

increase capacity through an intersection

– Equalize the quality of service for all or most traffic streams

• Separate individual movements in time rather than in space

A Bit of History…

• While other inventors are reported to have experimented with and even marketed their own three-position traffic signals, Garrett A. Morgan was the first to apply for and acquire a U.S. patent for such a device. The patent was granted on November 20, 1923. Morgan later had the technology patented in Great Britain and Canada as well.

• The Morgan traffic signal was a T-shaped pole unit that featured three positions: Stop, Go and an all-directional stop position. The third position halted traffic in all directions before it allowed travel to resume on either of the intersection's perpendicular roads. This feature not only made it safer for motorists to pass through intersections, but also allowed pedestrians to cross more safely.

Basic Timing Elements

• Traffic movements are grouped and allowed to move in phases

Two-phase signal

Three-phase signal

Four-phase signal

Basic Timing Elements

• Elements within a phase:– Green interval: the period of the phase during

which the green signal is illuminated. – Yellow/amber interval: the portion of the phase

during which the yellow light is illuminated. – All-red interval: the period during which the red

light is illuminated for all approaches – Intergreen interval: the interval between the end

of green for one phase and the beginning of green for another phase

Street A

Street B

All-red

Intergreen

Intergreen Period

• Allow each vehicle to stop prior to entering the intersection or to pass through without stopping

a

vvtS r 2

20

0 Stop Zone

Intergreen Period

• Allow each vehicle to stop prior to entering the intersection or to pass through without stopping

LWIvG 0Go Zone W L

Intergreen Period

To Eliminate dilemma zone:

G W L

S

Dilemma Zone

02

20

00

a

vvtLWIvGSD r

0

0

2 v

LW

a

vtI r

Practice Problem

Given the following information pertaining an intersection and its signal timing:

• Intergreen time of 6 sec• Intersection width of 60ft• Level grade with coefficient of friction of

0.5Does a dilemma zone exist for a 18ft-long vehicle approaching at 50mph? Assume a perception-reaction time of 1sec

Types of Signal Controllers

• Pretimed– Fixed interval lengths in fixed sequence

• Semi-actuated (traffic-adjusted)– Predefined timing schemes selected based on

traffic flow information

• Actuated– Varied length and/or sequence of signal

indications – React to arrivals of vehicles/pedestrians– Isolated or coordinated

Infrared