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Traffic Control Signals 

Why are Traffic Signals needed?

When traffic volume increases beyond the capacity of an intersection, it may be necessary to install a traffic signal. The criteria which determines the need for traffic signals includes: the total vehicular and pedestrian volumes, delays to side street motorists and pedestrians, and collision history at the intersection.

Are Traffic Signals the answer to traffic problems?

The function of a traffic signal is to assign right-of-way between two or more flows of traffic at an intersection. A properly timed traffic signal can significantly improve the flow of traffic and can improve safety for both pedestrians and vehicles.

Traffic signals are not a cure-all for traffic problems, nor will they necessarily prevent collisions. Unjustified traffic signals, or those not properly timed, can cause excessive delays, a disregard for the traffic laws, diversion of traffic through residential neighbourhoods, as well as an increase in collisions.

A traffic signal is a control device. While many people realize that traffic signals can reduce the number of right-angle or broadside collisions at intersections, few people realize that traffic signals can also cause a significant increase in rear-end collisions.


Special Signal Functions

Left Turn Arrow Phasing, or Flashing Advanced Green

It is sometimes necessary to install protective left turn phasing at locations with high volumes of left turning vehicles, where there is excessive delay, or where turning movement collisions are common.

Flashing Red

The rules of the Highway Traffic Act require motorists to stop before entering a signalized intersection when they see a flashing red signal.  Motorists must stop at the white stop line or crosswalk, on the near side of the intersection.  A flashing red signal should be treated as a stop sign, and entrance into the intersection should only be attempted when the way is clear.

Flashing Amber

When traffic signals flash amber, it means that the other approach has a flashing red display, and motorists should proceed through the intersection with caution.

Signals out

When power is out at a traffic signal and all lights are dark, the intersection should be treated as if it were an all-way stop.  Enter the intersection in accordance with the rules of an all-way stop.

Traffic Signal Pre-emption

Fire trucks and trains are sometimes given priority at traffic signals.  When they approach an intersection, the signals automatically transfer control to a special signal operation called pre-emption.  In pre-emption, the traffic controller safely provides a green signal for the approaching emergency vehicle, or prevents vehicles from crossing railway tracks.


Signal Timing

Traffic signals are timed to alternate the right-of-way between two or more approaches, and signals may be programmed to operate with "fixed" timing.  Fixed timing is defined as an intersection without pushbuttons for pedestrians and/or detectors for vehicles.  At many intersections, vehicle and pedestrian detectors are used to assign the right-of-way, and signal timing at an intersection will vary, based on changing traffic demands.  The majority of vehicle detectors are sensors imbedded in the pavement, and when they sense the metal in vehicles, the traffic controller can then respond to their presence.  Pedestrian detectors are in the form of pushbuttons which must be pushed to obtain a "walk" signal and sufficient crossing time.

Coordination of Traffic Signals

The coordination of traffic signals provides the greatest benefit to motorists since it reduces the interruption in the flow of traffic.  Coordination along a street is based on the following criteria:  signal spacing, the volume and speed of traffic, and the traffic signal timing.  A well managed, coordinated traffic control system saves fuel, reduces travel time, and reduces air-born pollutants.

The goal of coordinating traffic signals is to get the greatest number of vehicles through the signal network with the fewest stops and delays, in a safe and efficient manner.  Where optimum conditions cannot be achieved, the intersection approaches with the busiest traffic movement are given priortiy.

  
For more information on this or any other traffic related issues, please contact the following:
 
Traffic & Transportation Engineer
Contact: Enrique Figueredo Kamm, P.Eng

Cornwall