Yellow Signal


The MUTCD addressed in these pages is the Manual on Uniform Traffic Control Devices of the Federal Highway Administration as adopted by the State of California.

Yellow signal duration, standards for determining such timing, automotive braking performance and driver braking and reaction time performance are discussed.

Available intersection traffic accident data involving red light running will be linked or summarized.

Further information will be added to this page and its sub-pages to expand these preliminary observations.

Intersections controlled by traffic lights timed consistently with the standards of the MUTCD have yellow light intervals that are based upon a driver reaction time of one second traveling at the speed limit, and a deceleration of 20 feet per second squared to a stop at the limit line of the intersection. For any speed limit there is an implicit distance required to achieve such a stop referred to as the “critical distance” on this site. The corresponding critical time is, of course, yellow light interval. There are other complications of intersection rules and local law but these basics are addressed here for the sake of simplicity.

The critical distance (and time) falls within what is commonly called a 'dilemma zone' in which drivers must make judgments, upon appearance of the yellow signal, about whether to stop or to continue through an intersection. Closer to the intersection it becomes clearer that it may be traversed safely and that a stop would be difficult or impossible to achieve. Further from the intersection it becomes clearer that a stop is possible and prudent. The dilemma zone between these clarities contracts or expands with good and bad driving practices. It is subject to many factors including road conditions, traffic, the mechanical capabilities of the vehicle, and all of the distractions of life.

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