Traffic signals are electrically operated traffic control devices which alternately direct traffic to stop and to proceed. This discussion tells what factors enter into traffic engineers' decisions to install traffic signals. Because there is a common belief that signals are the answer to all traffic problems at intersections, this is offered in the interest of developing broader public understanding about what signals will do - and what they won't do.
ADVANTAGES OF SIGNALS
Signals offer the maximum degree of control at intersections - they relay messages of both what to do and what not to do. The primary function of any traffic signal is to assign right-of-way to conflicting movements of traffic at an intersection, and it does this by permitting conflicting streams of traffic to share the same intersection by means of time separation.
By alternately assigning right-of-way to various traffic movements, signals provide for the orderly movement of conflicting flows. They may interrupt extremely heavy flows to permit the crossing of minor movements which could not otherwise move safely through the intersection.
When properly timed, the traffic signal increases the traffic handling capacity of an intersection, and when installed under conditions which justify its use, it is a valuable device for improving the safety and efficiency of both pedestrian and vehicular traffic. In particular, signals may reduce certain types of accidents, most notably the angle (broadside) collision.
DISADVANTAGES OF SIGNALS
While many people realize that traffic signals can reduce the number of angle collisions at an intersection, few realize that signals can also cause an increase in other types of accidents (it has been well documented that other types of accidents, notably rear-end collisions, usually increase when a signal is installed).
Normally, traffic engineers are willing to trade off an increase in rear-end collisions for a decrease in the more severe angle accidents; however, when there is no angle accident problem at an intersection, there is nothing to trade off and the installation of traffic signals can actually cause a deterioration in the overall safety at the intersection. This situation sometimes prompts the remark, "You mean you won't do anything until somebody gets killed!" What is not fully understood is that traffic signals are not a "cure-all" and that the primary goal of all traffic engineers is to attain the safest and most efficient traffic flow feasible.
In addition to an increase in accident frequency, unjustified traffic signals can also cause excessive delay, disobedience of signals, and diversion of traffic to inadequate alternate routes.
Traffic signals are much more costly than is commonly realized, even though they represent a sound public investment when justified. A modern signal can cost taxpayers between $80,000 and $100,000 to install - depending on the complexity of the intersection and the characteristics of the traffic using it. On top of this, there is a perpetual cost which is almost never considered - the cost of the electrical power consumed in operating a signalized intersection 24 hours a day. This now averages about $1,400 per year.
Because of the widespread belief that traffic signals offer the solution to all intersection traffic control and accident problems, a number of signals have been installed nationwide where no legitimate operational warrant exists. Traffic records clearly show the attitudes and misunderstandings which sometimes lead to unjustified installations should be resisted. It is important that the selection and use of this traffic control device be preceded by a thorough study of traffic and roadway conditions and that the determination of the type of control and method of operation be based on the study data.
Traffic signals should be used only where lesser forms of control have proven ineffective, since signals almost always create more "overall intersection delay." In fact, minor movements may experience excessive delay, particularly if the signal is improperly timed. As a result, many drivers switch to less desirable alternate routes or to residential streets to avoid the added delay.
This document is based on a booklet called "Are Traffic Signals Really a Cure-All?", published by the Arizona Department of Transportation.