Traffic Signal Justification

An engineering study of traffic conditions, traffic volumes, pedestrian activity, intersection geometry, and the physical layout of the surrounding road grid network is 1st gathered. This data is then compared to national guidelines, or "signal warrants," for determining whether a traffic signal is justified. If 1 or more of the following 8 signal warrants is met, a signal may be justified; however, satisfaction of 1 warrant in and of itself is not necessarily sufficient justification.

Engineering judgment as to the site-specific characteristics shall dictate the need for a signal installation. Under no circumstances should a traffic control signal be installed unless it enhances the overall intersection safety and/or operation. If you would like to request a new traffic signal, contact Larry McGlothlin by email or at 919-560-4366, ext. 36435.

8-Hour Vehicular Volume
The need for a traffic signal shall be considered if 1 of 2 conditions is satisfied. The 1st condition considers if a certain number of cars use the main street and the side street for 8 hours within a 24-hour period. The number of cars needed varies based on the number of lanes. For example, if the main street and the side street each have 2 lanes, there have to be 600 vehicles an hour using the main and 200 an hour using the side street.

The 2nd condition considers the interruption of continuous traffic where the traffic volume on the major street is so heavy that traffic on a minor intersecting street suffers excessive delay.

4-Hour Vehicular Volume
This warrant considers if there is heavy volume on both the main and side street for any 4-hour period of a day. This is based on a sliding scale, where the more volume there is on the main street, the less volume is required on the side street to satisfy this condition. Other variables such as speed and number of travel lanes also factor into the curve.

Peak Hour
This warrant is intended for use at locations where traffic conditions are such that for a minimum of 1 hour out of the day, the minor-street traffic suffers undue delay when entering or crossing the major street. This is also based on a sliding scale where the volume relationship between the major and minor streets determines if this warrant is satisfied. Speed and number of lanes are variables that also factor into this equation.

Pedestrian Volume
The pedestrian volume is intended for applications where the traffic volume on a major street is so heavy that pedestrians are impeded from crossing the main street. Volume of 100 pedestrians an hour in any 4 hour period in the day or 190 pedestrians in a 1 hour period satisfies this requirement.

School Crossing
Consideration is given to installing a signal at locations where school children have to cross a major street. This warrant takes into consideration the number of gaps in traffic flow when children are crossing the roadway and where there are a minimum of 20 students during the highest crossing hour.

Coordinated Signal System
Sometimes progressive movement in a coordinated signal system can necessitate installing a signal where it may not otherwise be needed in order to maintain proper grouping of vehicles. This warrant is mainly based on the spacing of the adjacent traffic signals.

Crash Experience
This warrant is applicable where the frequency and severity of crashes are the principal reason for installing a signal. If trial alternatives and enforcement fails to reduce the crash frequency; then, 5 or more accidents, of types susceptible to correction by a signal, within a 12-month period and where the vehicles per hour are 80% of the volumes listed in the tables for warrant 1 satisfy this condition.

Roadway Network
Installing a traffic control signal at some intersections might be justified to encourage concentration and organization of traffic flow on a roadway network. This warrant considers various conditions associated with total entering volume from all approaches.