Traffic signal 32&35:DCE:FET:IIUI

DEPARTMENT OF CIVIL ENGINEERING FACULTY OF ENGINEERING AND TECHNOLOGY

TRAFFIC SIGNALSMUHAMMAD YASEEN 32-FET/BSCE/F14MUHAMMAD JUNAID 35-FET/BSCE/F14

TRAFFIC SIGNALS:

Traffic signals are controlled devices which could alternatively direct the traffic to stop and proceed at intersection using red and green traffic light signals automatically.

Purpose of Traffic Signals

Traffic signals manage the right of way at signalized intersections to

provide for the safe and efficient movement of vehicles and pedestrians.

Traffic engineers and technicians develop and implement signal timing

at each intersection to distribute green time amongst the competing

traffic flows to provide for efficient operations.

NEED OF TRAFFIC SIGNALS

•Orderly flow of traffic.

•An opportunity for pedestrians or

vehicles to cross an intersection.

•Help reduce the number of conflicts.

•Reduce the frequency of accidents.

•Improve safety.

Signal Aspects

GREENAMBER

The indication given by a signal is known as the signal aspect.

Traffic Signals : Advantages

• Provisions for side-street vehicles to enter the traffic stream.

• Provisions for the progressive flow of traffic in a signal-system corridor.

•Provisions for pedestrians to cross the street safely.

•Potential reduction of accidents, conflicts ensuring safety.

Traffic Signals : Disadvantages

• Excessive delay;

• Increased traffic congestion, air pollution and gasoline consumption;

•Disobedience of signals;

•Increased frequency of crashes, especially rear-end collisions.

Parts of a Traffic Signal• Traffic signals have four main parts:

Display Controller

Detector Support

Signal display

The signal display is what the driver sees. A simple signal display consists of three colors: red, yellow, and green.• A typical signal display is more than 3 feet tall and weighs up to 50

pounds.

Signal ControllerThe traffic signal controller is the “brain” at an intersection that tells each signal display when to change colors. Years ago, traffic signals were controlled by a simple electric mechanism that made the display change every so many seconds, like a clock. These simple mechanisms were not able to adjust when traffic changed during the day.Later in the pre-computer era, electromechanical controllers were developed that could be adjusted for the time of day. These controllers were much like the switch timers in the local hardware store that have green and red pins to set lights to go on and off on a predetermined schedule.

CONTINUE…………

Now, computerized traffic signal controllers get real-time information about

the presence of vehicles from sensors in the roadway and on poles.

Controllers adjust the amount of green time for different movements based

on sensor information. The most sophisticated controllers and software

programs allow for the coordination of signals along a roadway, so that

vehicles can travel through a succession of intersections without having to

stop.

DETECTORS Detectors are used to identify the presence of a vehicle, so that the controller

knows how to adjust the amount of green time to provide. There are many types of

detectors. One type is known as the loop detector. The square black lines you may

see in the roadway as you approach an intersection are the visible part of the loop

detector, which is a thin wire buried in the roadway. When a vehicle drives over the

loop, it sends a message to the signal controller. The signal controller's software

evaluates the messages from all the loops at the intersection and changes the

signal timing appropriately. Small TV cameras can also be used to detect vehicles.

DETECTORS

Types of Signal Operation

Traffic signals can operate on a pretimed basis or may be partially or

fully actuated by arriving vehicles sensed by detectors. In networks, or

on arterials, signals may be coordinated through computer control.

TYPE

1PRE-TIMEDTY

PE 2SEMI-

ACTUATED TYPE

3FULLY ACTUATED

Types of Signal Operation

PRE TIMED OPERATION

A signal whose timing(cycle length,green time etc) is fixed over

specified time period and does not change in response to changes in

traffic flow at the intersection.

No vehicle detection is necessary at this mode of operation.

Pretimed Operation:

In pretimed operation, the cycle length, phase sequence, and timing of

each interval are constant. Each cycle of the signal follows the same

predetermined plan. “Multi-dial” controllers will allow different

pretimed settings to be established. An internal clock is used to activate

the appropriate timing. in such cases, it is typical to have at least an AM

peak, a PM peak, and an off-peak signal timing.

SEMI-ACTUATED OPERATION

A signal whose timing(cycle length ,green time etc) is affected when

vehicle are detected(by video,inductance loop detecters,embeded-

pavement etc) on some but not all approaches is called semi-

actuated signal.

This mode of operation is found where a low volume road intersect a

high volume road.

Semi-actuated Operation

In semi-actuated operation, detectors are placed on the minor approach(es)

to the intersection; there are no detectors on the major street. The light is

green for the major street at all times except when a “call” or actuation is

noted on one of the minor approaches. Then, subject to limitations such as a

minimum major-street green, the green is transferred to the minor street.

CONTINUE………

The green returns to the major street when the maximum minor

street green is reached or when the detector senses that there is no

further demand on the minor street. Semi-actuated operation is often

used where the primary reason for signalization is ‘‘interruption of

continuous traffic,”

Fully Actuated

• A signal whose timing(cycle length,green time etc) is completely

influenced by the traffic volumes,when detected,on all of the

approaches.

Fully actuated signals are most commonly used at intersections of

two major streets where substantial variation exist in all approach

traffic volumes over the course of a day.

Full Actuated Operation

In full actuated operation, every lane of every approach must be

monitored by a detector. Green time is allocated in accordance with

information from detectors and programmed “rules” established in the

controller for capturing and retaining the green. In full actuated

operation, the cycle length, sequence of phases, and green time split

may vary from cycle to cycle.

SPACING BETWEEN TRAFFIC SIGNALS

REFERENCE;http://www.ctre.iastate.edu/research/access/toolkit/4.pdf

DIMENSIONS

Referencehttp://www.oksolar.com/images/signal_dimensions_large.jpg

HEIGHT

Reference;http://lincoln.ne.gov/city/pworks/engine/dconst/gpp/gppdisc/sigshop/images/height.gif

Pedestrian Signals

Reference;http://ichef1.bbci.co.uk/news/660/media/images/82931000/jpg/_82931048_trafficlights_reuters.jpg

CONTINUE………….

BEACONS

• Beacons are generally used to identify a hazard or call attention to a critical control device, such as a speed and continuously in accordance with the programmed signal –timing.

Lane Use Control Signals

Reference http://safety.fhwa.dot.gov/older_users/handbook/images/fig_24lg.jpg

REFERENCES:TRAFFIC ENGINEERING BY WILLIAM R. MCSHANE

http://safety.fhwa.dot.gov/older_users/handbook/images/fig_24lg.jpg

http://ichef1.bbci.co.uk/news/660/media/images/82931000/jpg/_82931048_trafficlights_reuters.jpg

http://www.ctre.iastate.edu/research/access/toolkit/4.pdf

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