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Welcome to the age of the Anthropocene, where human activity has significant impact on the Earth's ecosystems. As a consequence, our weather, one of the major causes of traffic congestion, is changing. Climate projections indicate areas such as Toronto can expect nearly double the current heavy rainfalls by 2050.

"In terms of climate change we also see that there's likely to be less snowfall in Southern Ontario, for example," says. Dr. Jean Andrey, dean of the faculty of environment at the University of Waterloo. "We're also likely to get ... potentially more freezing-rain days. All of these things affect our commutes."

In Toronto, where the slightest hint of precipitation seems to cause mass driver panic on already jammed roadways, climate change is one more factor to consider in traffic projections.

"Any type of different weather pattern will change things significantly here in the city, whether it's snow, whether it's rain," says Rob Valentine, a veteran radio traffic reporter for Toronto's 680NEWS. "Rain has a tendency to slow things down just as a whole [due to] general driver caution more than anything else."

Traffic delays aren't just hard on nerves and personal timetables, they're bad for business in terms of lost productivity. Why is precipitation so paralytic, and what can we do about it?

Studies, such as one based on data from Toronto's Gardiner Expressway and conducted by Andrey and colleague Daniel Unrau, examine driver response to rainfall.

"When it rains, even though people are reducing their speed by what seems like a very small amount, around 5 km/h, that's enough to reduce the ability of the roadway to clear the same number of vehicles," Andrey says. "It's the same thing that happens with an accident – braking propagates down, but with precipitation it's over a longer stretch spatially."

In precipitation, tire-pavement friction and visibility are reduced. Would a differential speed limit -- with faster driving permitted in fair conditions, slower speeds required during inclement weather -- help regulate traffic flow? Rain and especially snow compound slowdowns caused by collisions, Valentine says, simply because it takes crews longer to reach the scene.

"Anything to reduce collisions will reduce congestion," Andrey says. "During rain or snow traffic collisions increase by roughly 50 per cent – more during snow and a little less during rain. Certainly things that we could do to reduce incidences are important and differential speed limits might be one of those."

The Transportation Association of Canada commissioned Andrey and a team to look at traffic safety implications in a changing climate. There's promise, for instance, in new technologies such as "smart" highways which would enable shorter spaces between vehicles.

"The overall message from that was the focus in the future must increasingly be on rain, because medium to heavy rainfall is an issue that's going to grow in importance," Andrey says. "A second takeaway is that heavy rains are a problem not just for vehicle-to-vehicle collisions, but also for vehicle-to-pedestrian collisions. That will be where we need to pay increasing attention ... in part because we're creating cities which by design are intended to be more walker-friendly."

So the good news is, they're looking for solutions to traffic problems caused by rain and snow. The bad news is, traffic is terrible in good weather too.

"For us, it's worse in conditions when it's sunny." Valentine says. "Really nice days seem to have the worst impact on drivers because they just lose their minds — they think oh, it's a beautiful day, time to enjoy yourself, you have the windows rolled down, you're enjoying the music and you're not concentrating as much as you should be. So there's a level of distraction, in that regard."

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