Winter driving: The science behind ice, salt and potholes


Armies of snowplows stand by whenever winter weather sweeps through, ready to do what they can to keep our streets and highways free from snow and ice. However, even with their best efforts, there are still some aspects of winter driving that we just can't escape.

Snow removal is definitely important in winter, as the roads would become completely impassable. However, ice is just as bad, if not worse, and can produce even more dangerous driving conditions. Fortunately, we have ways of dealing with both, as we discovered long ago that water will freeze at much lower temperatures if you just add salt to it. Water freezes at 0°C, of course, but if you add ordinary table salt, sodium chloride, the freezing point can drop down to -21°C (if you use enough salt). The key for salt to be effective, though, is to get it on the roads either before the storm hits, or at least at the beginning of it, and for the temperatures to remain high enough where the salt stands a chance at getting the roads down to bare asphalt.

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If the roads ice over and the temperatures stay that way due to a long bout of frigid weather, putting salt down isn't as effective. Areas of the country that experience this kind of deep, persistent cold still use salt, but they mostly mix it with sand, and sometimes with other types of grid or gravel. The salt does what it can to limit the ice, while the sand and other stuff give a better surface for tires to get traction on.

One particularly treacherous driving situation in winter, and we've all seen the signs warning of this, is due to bridges icing over before the rest of the road. Sometimes, even salting doesn't help prevent this. Roads take longer to accumulate ice and snow because they have only one surface exposed to the elements. Wind blowing across the road surface draws away heat, as do the snowflakes melting on the road, but the ground underneath can act as an insulator, keeping the road warmer for longer. For a bridge, with air flowing across both the upper and lower surfaces of it, the temperature will drop much more quickly, either matching or being very close to the temperature of the air. Therefore, if the air is cold enough to

All this freezing and thawing of the roads causes a particularly jarring and frustrating thing for most drivers — potholes. These can form at any time of year, of course, just from simple wear and tear on the roads. However, winter is especially good at producing them, typically in one of two ways. The first is if water seeping down into the asphalt freezes suddenly, it can tearing open cracks in the pavement as it expands to form ice. The other way is if the water gets down under the pavement. As it ice forms there, the expansion can heave up the asphalt, while at the same time push down the ground underneath the road. When the ice melts, the asphalt collapses into the cavity left behind. In either case, once the crack or the collapse forms, repeated tires driving over the hole will make it bigger and bigger until it's patched.

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Does the future offer us any solutions to these issues?

You bet! Rather than approaching this from the perspective of changing water's freezing temperature, a better way of dealing with them is to leave the water alone and just ensure that it never reaches its natural freezing point. Heated sidewalks are already a big hit in countries like Norway and Iceland (which heats its sidewalks with geothermal energy!), and they've been suggested for use in cities like Edmonton and Saskatoon too.

Taking it a step further, the people behind Solar Roadways already want to turn our roads and highways into power generators, by replacing our traditional asphalt and concrete highways with special layered tiles that would collect solar power. Add heating elements to those tiles in regions with more wintry weather and the roads stay clear of ice and snow, and the weatherproof materials would prevent potholes as well!

Just to leave you with something else cool, and to acknowledge the awesome job that snowplows do, here's a video of the epic snowplowing that takes place on Canadian highways (specifically along the 401 at Yonge St, from late December 2012):

(Photo courtesy: The Canadian Press)

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