Carl F. Thelin

AMBER LIGHT MATH

FLASH - see the update link at the bottom of the page.

How long should a yellow signal be? That issue has filled up several columns of the local newspaper’s letters to the editor. One writer wanted the amber light to last longer. Another said red-light runners were good people who didn’t mean to blow through the signal. I assisted the columnist with this reply:

            I said, “Baloney - you can try to reduce the danger of running red lights but none will protect you from a determined, risk-taking scofflaw. Break it down this way:

  •     You need to know that the traffic signal is there. Don’t tailgate a high profile SUV or truck that blocks your view of the signal — the brake lights on that vehicle are no substitute for what’s going to happen with the traffic signals.

• You need to decide what to do. This might take anywhere from 0.5 to over 3 seconds. If you are all keyed up, with your right foot poised to press on the brake pedal, make that a half second to decide to push down on the pedal. If you are daydreaming, not paying attention, a longer time number applies. Let us use a number like 1 second, here.

  • You need to do something. It takes time for a driver to lift his foot from the accelerator and place it on the brake pedal. It also takes a small bit of time for the pedal to go down and for the hydraulic fluids to push the brake elements into action. For example, assuming you are quick, let’s use 0.5 second for the foot to push down when it is ready. That means it takes 1.5 seconds before stopping actually begins.

   • You expect to stop quickly. This time frame is dependent on how hard you brake — slam your foot down – assume 0.8 g braking - your passengers will complain, and you might even scare the daylights out of the driver behind you. You will slow from 45 to 0 mph in about 3 seconds in less than 100 feet. That is panic braking and not a nice way to drive.

            Braking from 45 mph with at 0.1 g means you have to keep an eye out ahead, plan to use about 700 feet and take about 20 seconds to stop. It is reasonable to double the deceleration rate; come to a stop in 350 feet in half that time. Put it all together and a driver traveling 45 mph will take 1.5 seconds to react and begin braking. At 45 mph, you travel 66 feet per second; you cover 100 feet just to begin slowing.  Add 350 feet for braking and the total distance is 450 feet. It takes about 12 seconds to stop comfortably. Now factor in that most signals are across the intersection another 50 feet away. You need to be able to see the signal change from green to yellow while almost 500 feet away. Do you see why you should not follow that semi-trailer too closely and why state law says yellow means slow to a stop, not punch it?

            Many traffic jurisdictions have added a second or two to the red signals after the amber light to allow the intersections to clear out before cross-traffic starts moving. This will help drivers “too close when the yellow light comes on so they figure they must go through.” Problem solved, right?  No! What we find is that the wise guys/smart alecks become aware that they can get away with more.

"FLASH"  I just downloaded a good report on updated amber light math from my (almost) home town Albuquerque newspaper . See it HERE.