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Good Question: What Causes Traffic Jams?

MINNEAPOLIS (WCCO) -- As Minnesotans enter into the throes of road construction season, they encounter unexpected traffic back-ups. So, what are the top causes of traffic jams? Good Question.

According to MnDOT, traffic jams and or congestion can be attributed to any number of reasons: construction, incidents, weather (sun, rain, snow) or community events. Road construction also plays a major factor as many drivers divert to adjacent roads or commute at alternative times of day.

About half of traffic is what's considered recurring. That's too many people wanting to do the same thing at the same time (i.e., rush hour). The other half is non-recurring and generally more unexpected.

According to a 2007 study from ECMT, the top three causes of non-recurring traffic are traffic incidents, weather, then work zones.

"Many parts of our road system are at capacity, they were built to handle a certain number of cars 25 years ago," says Steve Wilson, a traffic engineer with SRF Consulting. "If something happens or a car breaks down or people start and stop again, now we're over capacity and once you go over capacity, it gets very chaotic."

Road construction can cause delays, but Wilson says those tend to be shorter-term as drivers find alternative routes.

"You can't get around the fact that if you used to have three lanes and now you only have two lanes, there's going to be congestion," he says.

Driver behavior does play a big role. A broken down car can slow down an entire lane. A distracted driver who stops short can create a ripple effect that takes time to smooth back out.  Accidents – even ones on different roadways – can back up traffic for miles.

Wilson likes to use I-394 as an example of traffic jams drivers might not expect. On the weekdays, the traffic is slow, but moves steadily. On the weekends, there can be congestion with fewer cars because the drivers aren't as familiar with the road.

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