Why Stoplights Hate Idaho Fog (And Why You Might Hate Stoplights)

Feb 11, 2014

Treasure Valley drivers made their way to work Monday morning in dense fog. This photo was taken on Amity in SE Boise.
Credit Emilie Ritter Saunders / Boise State Public Radio

Ada County highway officials say they received dozens of complaints Monday morning as heavy fog affected the operation of stop lights. 

Mike Boydstun, the Ada County Highway Department’s traffic operations engineer, estimates between 30 and 60 people called the department to report problems between 7 and 9 a.m.

Boydstun says sunder normal circumstances, a camera on a traffic light acts as an eye that ‘sees’ approaching cars.  The light then changes to move cars through an intersection as efficiently as possible.  When there’s fog, though, the camera’s vision can be blocked. That means drivers can sit at stop lights for an usually long amount of time before they finally get a green light. Busy intersections where lights cycle on a regular basis - and that don’t rely on the video detection system - aren’t affected by fog.

Boydstun says it’s a problem Ada County motorists have to deal with in winter more than any other time of year.

“The winter time and the fog does seem to cause the most issues for the detection,” Boydstun says. “Snow can be a bit of an issue as well because as snow is falling – if it’s a heavier snowfall – vehicles aren’t able to be seen as well.”

ACHD says when fog is hindering a light’s operation, it can be switched to an automatic cycle that doesn’t rely on the camera detection system.  A worst-case-scenario can arise, though, when the fog isn’t dense enough to trigger that switch, but is too dense for a light’s camera to detect approaching cars. If you’ve waited for several minutes at a light, Boydstun says this was the likely cause.

Boydstun adds that drivers who go to work very early in the morning are most likely to encounter a stoplight being affected by the fog. That’s because most lights in Ada County are set to switch to a pre-programmed cycle by 7:00 a.m. Some lights make that change as early as 6:30 a.m.

The Boise Police Department encourages drivers to prepare for such situations by leaving earlier. Lynn Hightower, a police spokesperson, says an officer may be empathetic to fog-induced frustrations, but adds those who drive through a red light risk getting a citation.

Boydstun says the proliferation of cell phones over the past decade means ACHD hears more complaints about slow-to-change lights.  But he says the calls are valuable in diagnosing problems and getting them fixed.

“We do get calls from the public,” he says. “And we take those calls very seriously because we want people to be able to get where they’re going.”

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