Guidelines for Minimum Signal Sight Distance

The guidelines for minimum visibility distance in the 2003 Manual on Uniform Traffic Control Devices (MUTCD) are based on a research model developed in 1977, a portion of which was based on stopping sight distance concepts from 1965. Since then, roadway design criteria, vehicle performance characteristics, and driver characteristics have changed. These changes provide justification for reevaluating the guidelines for minimum signal sight distance. This paper identifies the basis for the current guidelines (which were not published) and develops updated guidelines for signal sight distance. As with the original guidelines, the updated guidelines are based on a theoretical model that addresses reaction time, braking distance, and queue length. The key changes incorporated into the updated model are calculating the stopping distance on the basis of deceleration rates, using a reaction time consistent with the AASHTO stopping sight distance, and eliminating the provision for a vehicle queue at the stop line, as the driver should be able to respond to the queue on the basis of the stopping sight distance provided by the road. The updated values for minimum signal sight distance are typically 20% to 25% less than those currently in the MUTCD. The lack of guidance in the MUTCD about placement of the “Signal Ahead” sign is also pointed out, and it is suggested that it be located on the basis of the signal sight distance and not according to warning sign placement guidelines.