I've heard that yellow lights are timed at 3.2 seconds before turning red. Can you validate this? Also, why do they choose that interval?
— Mona C., via email
Mona, you're close. While yellow light intervals will vary (sometimes by the decisecond) from stoplight to stoplight, most yellow lights are set at about three seconds for left-turn arrows and about four seconds for all through lanes on streets with a speed limit of 35 mph or less.
These time intervals, which never seem quite long enough, are based on guidelines set by the Oregon Department of Transportation, the Institute of Transportation Engineers and the Manual of Uniform Traffic Control Devices.
The speed limit, the size of the intersection, the driver perception-reaction time, the deceleration rate and more factor into the equation.
"At 35 mph, you're moving 51 feet per second; and in three seconds, you can go 150 feet, which is well beyond the majority of the intersections in Medford," said Alex Georgevitch, Medford's transportation manager.
Some big, complex intersections — for example, the one at Riverside Avenue and Crater Lake Highway (aka "the Big X," as it is commonly referred to by engineers) — and intersections with the (in)famous red-light cameras have a slightly longer clearance time. The yellow light at "the Big X" also is followed by an all-red second to protect drivers from the gun-it-at-the-last-second types.
Georgevitch said questions about yellow lights are often raised after someone receives a ticket for running a red light.
Mona, although we hardly suspect that you are such a person, we want to remind you and other readers that the yellow light provides enough time to safely exit the intersection or come to a complete stop but is not a grace period for drivers running late.
Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501 or by email to firstname.lastname@example.org.