Yellow Change Intervals for Straight and Turning Movements
Perception Time (s)
t = Z - y
t - ts
Ends (feet from
Yellow Duration (s)
Thru Yellow (s)
(a + Gg)
Zone End (s)
For the mathemathical proofs and the physics
behind this spreadsheet, see
Short Yellows and Turns:
Where you see red is a condition where ITE forces a driver to run a red light.
When the light turns yellow, ITE forces drivers between the start and end of the dilemma zone
to run the red light.
Sometimes the end of the dilemma zone is farther from the intersection than the start.
That means there is a region on the road where the driver can choose to stop or go and either is good.
Grade = rise/run * 100.0.
Grade is a percent.
Positive grades are inclines.
Negative grades are declines.
The yellow duration is always the peception time + half the time it takes for a car to brake.
Note that you can never stop your car within the time the light is yellow.
This spreadsheet uses ITE's equation for yellow light durations.
The equation embeds Newton's Laws of Motion.
Therefore by the laws of the universe, no yellow light duration can be set less than these values.
If less, then the yellow duration is illegal by the Canute Rule.
ITE's equation does not handle grades greater or less than 10 percent.
It does not work in hilly places like San Francisco.
This is because ITE's equation invokes small angle approximation formulae.
ITE's equation does not handle slippery roads.
ITE's equation does not handle coefficients of friction.
Even with properly set yellow durations, ITE's equation always guarantees a steady flow of cars running red lights.
For details on ITE's
Derivation of the Yellow Light Interval Equation:
This spreadsheet does not consider the obsolete unsafe practice of including the all-red clearance interval with the yellow duration.
The numbers above are just yellow times.
add more time if the yellow included the all-red clearance interval.