American Scientist

To the Editors:

I enjoyed Henry Petroski’s Engineering column on traffic lights, “Traffic Signals, Dilemma Zones, and Red-Light Cameras” (May–June). Some time ago I was asked by a New York City driver (via the Brooklyn College physics department) to determine whether the yellow lights were unreasonably short on a city street where he had gotten a red-light ticket. Indeed, New Yorkers often complain that yellow-light times are set too short and even accuse the city of doing this deliberately to produce revenue. After determining the formula for Y, the minimum yellow-light time, I realized that its value depends critically on two quite questionable parameters: the braking time and the reaction time. In particular, the latter—the time from perceiving the yellow signal to pressing the brake—depends on the driver’s age. Although one second is reasonable for a young driver, it would be at least double that for a driver in his 60s, and this difference does imply that yellow-light times (set nominally at three seconds in New York City) are in fact less than the minimum.

Michael I. Sobel
Professor Emeritus of Physics
Brooklyn College
Brooklyn, NY

To the Editors:

Henry Petroski’s informative Engineering column about traffic signals reminded me of two techniques that I no longer see in the United States. One is a change from green to the intermediate half green and half yellow before the wholly yellow light, which was more common in the United States a half century ago. This sequence does not require more lights and provides more time to prepare for the red one without causing a “loss of drivers’ respect for the yellow light.” The second technique I used to see is a change from red to the intermediate half red and half yellow before the green light, a sequence often seen in Europe. This series alerts drivers to be ready to go when the light is green, so traffic flows as soon as the light is green, permitting more cars to pass through the intersection.

M. Craig Pinsker
Glen Allen, VA