Ken Schram interview from last week:
In the interview a "proper" yellow time was mentioned. What was not pointed out is that the yellow times are based on the heavily flawed Institute of Transportation Engineer's “Kell and Fullerton” formula. Some of the issues with this calculation include the following:
1. The formula has no safety factor. None. Nada. Zero. Zip. Human safety usually requires a safety factor of 5.
2. The “perception+reaction” time is based on ideal laboratory conditions, not 90-percentile in the real world, and lacks the required “decision making” component.
3. The length of the vehicle is ignored unless the intersection has an “all-red”. Also, how they measure a vehicle is not clear. You use overall length for overall length, not the wheelbase.
4. The intersection width is ignored unless the intersection has an “all-red”. Even then the width is mis-calculated.
5. The deceleration rate of 10 is too aggressive.
6. Vehicle breaking capability is ignored.
7. Street material and surface condition is ignored.
8. Weather conditions are ignored.
9. Approach speed value favors short yellows, not necessarily real-world speeds and safe yellows.
The SAE and auto industry have published studies on real-world driver breaking. Based on their findings, the ITE formula is way off base. Simply fixing #2 would add 1.5 seconds to the overall calculation, causing citations and intersection danger to plummet.
The camera companies spend millions every year to keep this formula the touchstone calculation in universal use. They and cities are horrified at the prospect of making any of the nine fixes as it's clear revenue would plunge.
The ITE has shown no interest in fixing the flaws in the calculation to favor safe and efficient traffic flow instead of the current situation of maximizing cash flow.
Only when judges are educated to appreciate the depth of the flaws in the ITE calculation and begin throwing out all tickets based on it will there be a change to more safe and sane green and yellow light times.