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It may surprise you to learn that adding more stop signs or traffic signals along a roadway does not necessarily slow drivers down or increase safety. In fact, the overuse of signs and signals can lead drivers to ignore or not properly obey them.
Studies have shown that when stop signs are placed at intersections where they don’t appear to be needed, motorists become careless about stopping.
Installing traffic signals where they are not needed can create traffic congestion, add travel time, and frustrate drivers, who may start driving impatiently.
To make travel efficient and safe, and to help ensure the proper observance of stop signs and traffic signals, they are usually installed only where they are absolutely necessary. Other solutions, for example, a yield sign may also provide enough safety without affecting traffic flow.
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 starts and stops cause increased fuel consumption and production of carbon monoxide, nitrous oxide, particulate matter, and other pollutants.
Two-way stop signs assign the right-of-way at an intersection. The warrants for the installation two-way stop signs in the Manual for Uniform Traffic Control Devices (MUTCD) are listed below. Because a stop sign causes substantial inconvenience to motorists, it should be used only where warranted. It may be warranted where the following conditions exist:
The amount of delay created by the stop sign depends on both major and minor street flows. The gaps in the major flow traffic stream must be adequate to allow the stopped traffic to move through, or turn right or left through the intersection. The term "critical gap" is often used to describe the median gap accepted by drivers for specific turning maneuvers and roadway characteristics. According to the 1997 Highway Capacity Manual, typical critical gaps are 6.2 to 6.9 seconds for right turns from a minor roadway, and 7.1 to 7.5 seconds for left turns from a minor roadway. Left-turning movements take longer, and left-turning drivers must cross more traffic streams. Additional delay for minor street vehicles is also determined by the vehicle arrival rate. The arrival rate of vehicles on the minor street is related to how long drivers will wait in the queue to get to the stop line.
The delay times at stopped approaches can become excessive if either major or minor flow is high. The advantage of a two-way stop is that the major flows do not have to stop and they incur almost no delay at the intersection (i.e., the majority of the traffic does not have to stop).
Four-way stop control is often controversial as it can often confuse motorists and can cause more average delay than other types of control. The multiway stop sign should only be used where the volume on all approaches to the intersection is approximately equal and the traffic volumes are relatively low. However, the four-way stop sign can be useful in situations where two-way stop control has not solved the safety problems, but signalization is not yet warranted.
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:
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.
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.