Traffic Flows

Due to conflicting traffic flows, highway intersections are of great concern to traffic engineers. Intersections can be a major source of crashes and vehicle delays (as vehicles yield to avoid conflicts with other vehicles). Most highway intersections are not signalized due to low traffic volumes and adequate sight distances. However, at some point, traffic volumes, accident frequencies and their resulting injury severities, as well as other factors, reach a level that warrants the installation of a traffic signal.

The installation and operation of a traffic signal to control conflicting traffic and pedestrian flows (also referred to as movements) at an intersection has advantages and disadvantages. The advantages include a potential reduction in some types of crashes (particularly angle crashes), provisions for pedestrians to cross the street, provisions for side-street vehicles to enter the traffic stream, provisions for the progressive flow of traffic in a signal-system corridor, and possible improvements in capacity and reductions in delays. However, signals are by no means the perfect solution for delay or potential crash problems at an intersection. A poorly timed signal or one that is not justified can have a negative impact on the operation of the intersection by increasing vehicle delay, increasing the rate of vehicle crashes (particularly rear-end crashes), causing a disruption in traffic progression (adversely impacting the through movement of traffic), and encouraging the use of routes not intended for through traffic (such as routes through residential neighborhoods). Traffic signals are also costly to install, with some basic signal installations costing in excess of $100,000.