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Identify the Errors in the Yellow Signal Light Equation

They are the reason why drivers run red lights. This equation computes the duration of yellow lights and all-red clearance intervals. Can you identify the errors? Those who have taken an introductory physics class find the first error within 20 seconds. There are over 5 mistakes. Post a comment when you see an error.

Note that t = 1.0s and a = 10 ft/s2 are for the 50th percentile passenger sedan driver driving on dry pavement.

This equation appears on FHWA website and in DOT traffic signal manuals throughout the world:

For an explanation and history of this equation, watch the webinar hosted by the National Society of Professional Engineers (Professional Engineer in North Carolina chapter):

The Yellow Change Interval -- Physics in Opposition

Analyses of red-light camera data empircally demonstrates that these errors cause over 90% of red-light running.

Imagine the legal liability.

The equation is unlawful under engineering practice law.


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