I. What Traffic Signals Can Do
Traffic signals are appropriately used to help solve several specific kinds of problems at an intersection, including problems of assignment of right-of-way, some specific signal coordination problems, and occasionally, driver discomfort.
Assignment of Right-of-Way:
By far the primary use of traffic signals is to help motorists take turns at an intersection, which engineers call assignment of right-of-way. Traffic Signals are used when other control methods, such as stop signs, do not work. A signal is sometimes the best solution when cars stack up on a side street, while waiting for a gap in main street traffic. Many times, the main street delay caused by the installation of a signal is not high compared to the delay reduction on side streets that have high traffic volumes.
Traffic signals are also helpful at times when a high volume of accidents occur at an intersection caused by incorrect assignment of right-of-way by drivers. These accidents typically involve two or more cars, each from the main street and side street, colliding at right angles. The cause of these accidents can also be contributed to inadequate visibility or sight distance at an intersection. As a result, some accidents can not remedied with traffic signals. For that reason, traffic engineers carefully consider the types of accidents that occur at an intersection when choosing the most appropriate traffic control device.
Signals can sometimes help keep the traffic regulated to a specific speed when signals are coordinated. Coordinated signals attempt to provide green lights for as many through drivers as possible, minimizing the interruption of main street traffic. When a large section of road is not equipped with signals, then groups of cars tend to spread out and the coordination does not work as well as it could. Signals are occasionally used in these circumstances to help groups of cars at a constant speed.
Traffic signals also provide a more comfortable entry for motorists on side streets, although this advantage has to be balanced against the discomfort signals can create for main street drivers. Drivers can feel pressured to enter an intersection quickly along roadways with a high volume of traffic. The installation of a traffic signal can alleviate that pressure by controlling the assignment of right-of-way.
II. What Traffic Signals Cannot Do
The advantages of signal operation are balanced by real disadvantages for the motoring public. When used inappropriately, traffic signals do not solve the problems described above; indeed, they can actually create the same problem.
Right-of-Way Assignment and Efficiency:
As we have seen, traffic signals are used to assign right-of-way when other methods do not work. A basic principle of traffic engineering is to provide the least amount of control necessary to provide a safe and efficient operation, so that drivers will not face undue restriction. When less restrictive methods do work, they generally work better than traffic signals, especially when considering the overall performance of the intersection. A car entering a busy street is able to move into a smaller gap than would be created by a traffic signal.
Right-of-Way Assignment and Safety:
We have also seen that traffic signals can reduce the likelihood of accidents related to the assignment of right-of-way. Signals are not good at addressing problems associated with interruption of the traffic stream. Accidents in this category usually involve two cars moving in the same direction in read-end or sideswipe collisions. Signal coordination can help here, but it cannot completely fix the problem because it cannot completely remove the interruption.
Finally, traffic signals are not good at moving traffic when they are closely spaced along the roadway. Generally, traffic signals on major streets should not be spaced more closely than one-half of a mile. Closer spacing creates problems with signal coordination, sometimes making smooth traffic flow on a main street impossible.
Motorist Discomfort and Safety:
Accidents can occur at all intersections which carry traffic. The accident rate at most intersections is less than one accident for each million entering vehicles. Accidents occur when at least one driver makes a poor judgment which causes the paths of two or more cars to intersect at the same time. Drivers are more likely to make poor judgments when a safety problem, such as inadequate sight distance, is present but not obvious. Driver discomfort is therefore usually treated as a separate issue from safety, and is compared with the discomfort experienced by main street drivers who will face the interruption caused by the signal.