December 18


Snow on Traffic Signals Lights

By Mike Spack

December 18, 2009

Snowy led Most of you have probably heard about snow sticking to LED traffic signal lights, blocking the view and causing crashes.  The Associated Press put together an article on the topic this week.  It even quotes a few people from Minnesota.

The problem discussed in the article is slushy snow that falls in a windy storm can fly onto and then stick to the signal lights.  Old fashioned light bulbs were warm and would melt away the snow.  Most traffic signals have upgraded to using LED lights which allow the snow to stay because they aren't warm.  

There was a big push for LEDs about ten years ago because of the economic benefits (LEDs last longer and use less power – they're about three times more economical than incandescent lightbulbs).  I was a member of the local traffic signal committee and we discussed LED lights at length, including their inability to melt snow.  Another big issue was whether or not we need a $2000 battery backup system at every signal to keep the lights working when the power goes out (see MnDOT's dark signal report). Our conclusion after going through crash data was that there are few crashes caused by a traffic signal light that can't be seen.  Most people are more attentive in snowy or power outage situations and come to a voluntary stop when they are uncertain.

I would prefer to see the media focus on inattentive driving.  I think this is a much bigger safety issue (although its harder to get good photos for a news story).  Here are the crash risk results from a 2006 study by the Virginia Tech Transportation Institute (these results don't fully address the impacts of texting):

  • Driving drowsy increases an individual’s near-crash or crash risk by four to six times.
  • Engaging in secondary tasks that require multiple steps or eye glances away from the forward roadway increases risk by two to three times.
  • Certain behaviors increased the risk of involvement in a near-crash or crash. Reaching for a moving objects increased risk nine times, looking at an external object 3.7 times,reading 3.4 times, applying makeup 3 times, and dialing a hand-held device 2.8 times.
  • Talking or listening to a hand-held device increased risk by 1.3 times, but this result wasnot statistically different than normal driving.
  • Looking away from the forward roadway for long glances at inopportune moments increase crash risk by two times that of an alert driver.
  • I read this on the AP too. Do you think that the new option for yellow reflective tape on the outside of the backplates will help?
    I do agree, though, that the amount of collisions caused dy distracted driving, DUI, speeding, and other causes drastically outnumber the amount of collisions caused by snow.

  • While accident data may seem to suggest that snowed out signals aren’t a major contributor to accidents, aren’t we supposed to take all reasonable steps to mitigate accident potential? Also don’t assume that because one signal approach is snowpacked, that the others aren’t clear. Traffic from one approach may make the “dark signal stop” and proceed when traffic on the cross street has just received a visible green indication. We need to look at visor designs and other low cost factors to help reduce this hazard. We get results, not make excuses. And just because low speed fender benders aren’t reported, it doesn’t mean that they aren’t a hassle to those involved in them.

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    Mike Spack

    My mission is to help traffic engineers, transportation planners, and other transportation professionals improve our world.

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