Road Hazards during Winter Weather Conditions

By: Mark Bloomer

Snow and ice frequently results in slippery road conditions during the wintertime. Road surface slipperiness seems to be affected by a variety of factors, mainly related to changes in precipitation type during a winter storm event and changes in temperature before, during and after a winter storm. The most dangerous road conditions seem to occur when water freezes on road surfaces. This can either be in the form of freezing rain or in the form of frozen precipitation that melts and refreezes. Conditions that deteriorate rapidly or change quickly from one place to the next can be especially hazardous since they may catch motorists my surprise. The following are perhaps two of the most extreme conditions that occurred in Northern Maine within the past five years. Each event resulted in hundreds of accidents.

Mid December 2000 brought an early winter cold spell. For several days average highs were only around 10 degrees and average lows were around 10 below. Road surfaces became very chilled, and the low December sun angles did little to warm the surfaces. At the end of the cold spell a storm moved in and brought an inch of fresh snow. The snow changed over to rain resulting in slushy road conditions. A layer of ice formed beneath the slush as the rain and melting snow refroze on very cold surfaces. The slipperiness of slush over freshly frozen ice was extreme resulting in many cars sliding off the road. One of the factors that added to the extreme slipperiness was the fact that this was an early season precipitation event and there was little or no preexisting sand and salt on the roads.

Another extreme event occurred in late February of 2003. Light snow was falling and temperatures were near freezing. Because temperatures were near freezing, a lot of the snow was melting on road surfaces. An extremely strong cold front moved through early in the morning. Temperatures dropped from around 32 F down to around 0 F in just a couple of hours while light snow continued to fall. The water on the roadways froze and light snow quickly covered the fresh glaze of ice. Blowing snow and a light hazy arctic snowfall reduced visibilities considerably, and hundreds of accidents occurred on the roadways that morning.

Both of these extreme conditions occurred when water on the roadways froze into a layer of ice. In each case the ice was hidden from view, either by a top layer of slush as in the first case or by a top layer of snow as in the second case. Being aware of the temperature extremes in each case may have alerted travelers to the potential for the presence of an icy layer. In the first case, extreme cold preceded the event and in the second case, extreme cold moved in during the event.

Drifting snow can also be especially hazardous for travelers. Snow blowing across the road can rapidly reduce visibility while adding snow to the roadways. The combination of these two frequently results in rear-ending type collisions in areas of drifting. Drifted snow is more densely packed than fallen snow, and snow drifts that are deeper than the height of a vehicle above the road can cause a vehicle to ride up onto the snow and become stranded as its wheels are lifted off the road surface. One of the most dangerous hazards resulting from drifting snow commonly occurs in the late winter and very early spring. The higher sun angle later in the season causes increased mixing in the atmosphere resulting in stronger wind gusts. Likewise, the higher sun angle begins to heat the roadway which causes snow blown onto the roads to melt upon contact with road surfaces. Areas where snow has been drifting over the roads and melting turn into dangerous patches of ice as the cool dry wind refreezes the melted snow. These icy patches can catch motorists by surprise since they can form in otherwise clear and dry road conditions.

In addition to freezing rain and refreezing snow, sleet is typically more hazardous than plain snow. The density of sleet can cause tires to ride up over the ice pellets and the ice pellets can behave like miniature ball bearings causing the wheels to loose traction.

Road safety during the winter months involves being aware of conditions including how the weather in recent days has affected roads and how current weather changes are impacting road surfaces. Freezing and refreezing conditions are the most hazardous and can occur during a variety of situations. It is advisable for both winter travelers and public works personnel to keep in close touch with the weather and its changes during the winter months. The National Weather Service is here to help keep everyone informed of the weather and its changes, and we wish everyone a safe and comfortable winter season.

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