Installing stop signs or traffic signals where they are not needed can cause significant disruption of traffic flow and increase intersection delay for drivers. The delay increases travel time, annoys drivers, and the additional starts and stops cause increased fuel consumption and production of carbon monoxide, nitrous oxide, particulate matter, and other pollutants.
Justifying a signal installation requires considerable data collection and analysis, including:
- Traffic volume by approach and movement for the 16 highest hours in a day
- Pedestrian counts in crosswalks
- Intersection approach speed distributions
- Collision diagrams for recent crashes
- Condition diagram for the intersection
The MUTCD lists 11 warrants for the placement of traffic signals, summarized below (please refer to the MUTCD for details). If none of these warrants are met, a traffic signal should not be placed, and, the fulfillment of a warrant(s) does not in itself justify the installation of a signal.
- Minimum vehicular volume. The volume of intersecting traffic must be above a certain value.
- Interruption of continuous traffic. The traffic volume on a major street is so significant that the traffic on the minor street cannot safely merge, enter, or cross the major street.
- Minimum pedestrian volume. The volume of pedestrians crossing a major street exceeds a certain value.
- School crossing. At an established school crossing, a signal can be placed if it is shown that there are not enough gaps in the traffic for the children to safely cross.
- Progressive movement. To maintain the proper grouping of vehicles and to effectively regulate the group speed.
- Accident experience. When less restrictive remedies and enforcement has failed to decrease the accident rate below levels expected with signalization.
- Systems warrant. A common intersection that serves a principle network for through traffic flow.
- Combination of warrants. If warrants 1 and 2 are each satisfied by 80 percent of the stated values, a signal placement could be justified.
- Four-hour vehicular volume. The traffic volumes on the major and minor streets exceed a certain value for any four hours on an average day.
- Peak hour delay. The minor street traffic suffers major delay in entering or crossing the major street for only one hour of an average weekday.
- Peak hour vehicular volume. The traffic volumes on the major and minor streets exceed a certain value for only one hour of the day.
Installing a traffic signal at a low-volume intersection can significantly increase crashes and delays. The increase in delay and stops results in higher fuel consumption, increased travel time, and higher point source pollution. The length of delay is directly related to a number of factors. Cycle length is one factor that is influenced by traffic volumes and the need to safely accommodate pedestrians, the pedestrian crossing time could significantly increase the cycle lengths. Although traffic signals can reduce the total number of collisions at an intersection, research has shown that certain types of crashes (e.g., rear-end collisions) may actually increase after a signal is installed. For this reason, the type and number of crashes at an intersection are considered before the installation of a signal.
Traffic signals can represent a positive public investment when justified, but they are costly. A modern signal can cost $80,000 to $100,000 to install. In addition, there is the cost of electricity to operate a signalized intersection 24 hours a day (which can average about $1,400 per year).
It is important to carefully consider whether a traffic control device is needed. The costs and benefits must be carefully evaluated, and a careful analysis and engineering study must be completed.