A traffic signal is an electrically powered traffic control device by which traffic is warned or directed to take some specific action. Under favorable conditions, traffic signals can be coordinated to ensure a continuous or nearly continuous flow of traffic at a defined speed along a given route. A thorough study is always made by a Traffic Engineer before a traffic signal is installed as they exert a profound influence on traffic flow and safety. Experience shows that right angle collisions may decrease after the installation of a signal, but the number of rear-end collisions may increase. If a traffic signal is installed when not warranted, it may also increase the overall delay, increase the adverse effects of collisions, and reduce intersection capacity.
In evaluating the need for traffic signals, the Traffic Engineer will compare the existing conditions against nationally accepted minimum guidelines or warrants. These warrants are briefly explained in the following:
Warrant 1 - Minimum Vehicular Volume
The number of vehicles on the street for each of any eight hours of any average day are compared with the national traffic signal warrant criteria to determine if there is a need for the signal to relieve the congestion.
Warrant 2 - Interruption of Continuous Traffic
The number of vehicles on the major street for the same eight hours are analyzed to determine if congestion on the major street causes cars on the minor street to wait excessively to enter or cross the major street.
Warrant 3 - Minimum Pedestrian Volume
The pedestrian volume crossing the major street is analyzed for the same eight hours to determine the frequency of vehicle and pedestrian conflicts.
Warrant 4 - School Crossings
The volume of vehicles and school children crossing are evaluated to determine if special controls are needed for the protection of schoolchildren.
Warrant 5 - Progressive Movement
This warrant is satisfied if the installation of a traffic signal will facilitate a continuous flow of traffic along the roadway.
Warrant 6 - Collision Experience
This warrant is satisfied if there are at least five reportable collisions in a twelve month period at the intersection that are of the type susceptible to correction by a traffic signal.
Warrant 7 - Systems Warrant
A traffic signal installation may be warranted to encourage concentration and organization of traffic flow on the arterial network.
Warrant 8 - Combination of Warrants
In exceptional cases, signals may be justified where no single warrant is satisfied, but where any two of Warrants 1, 2 or 3 are satisfied to the extent of 80 percent or more of the stated numerical values.
Warrant 9 - Four Hour Volume Warrant
This warrant is satisfied when, for each of any four hours of an average day, the vehicles per hour on the major street and the corresponding vehicles per hour on the higher volume minor street approach are above the Four Hour Volume Warrant values published in the Traffic Manual.
Warrant 10 - Peak Hour Delay Warrant
This warrant is intended for application where traffic conditions are such that for one hour of the day, the peak hour, the minor street traffic suffers undue delay in entering or crossing the major street.
Warrant 11 - Peak Hour Volume Warrant
This warrant is satisfied when the vehicles per hour on the major street and the corresponding vehicles per hour on the higher volume minor street approach are above the Peak Hour Volume Warrant Values which are published in the Traffic Manual. It is important to point out that the satisfaction of one signal warrant does not necessarily justify the installation of a traffic signal. Other factors that must be considered are delay, congestion, driver confusion and other evidence of the need for right-of-way assignment. Traffic studies have shown that excessive vehicle delay results in driver frustration and may cause disobedience to traffic signal indications which can lead to a higher accident rate. Additionally, an improperly installed or unwarranted traffic signal can create increased traffic volumes on adjacent routes, which may not have been designed to handle the increased flow of vehicles which are trying to bypass the unaccepted traffic signal.