Monografia

FOREWORD
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