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 Local News  -   Sunday, August 7, 2005

Roads slickest when rain starts

Weather Wise


Rudi Keifer

This summer has brought plenty of rain to North Georgia.

In almost all storm situations, the primary threat to human life and health is traffic accidents.

Winter storms and the accompanying ice on roadways are an obvious case in point. Greater Atlanta and the Georgia mountains, though, are much more likely to have wet roads than frozen ones.

Most drivers have heard of hydroplaning. This condition occurs when a film of water forms underneath the car tires, making the rubber lose contact with the road. As a result, the tires skid, and control over steering and braking is lost.

But the trouble is in the detail. It is the beginning of a rainstorm that carries the greatest traffic hazards.

When the precipitation starts, you often see motorcyclists headed for the nearest coffee shop right away. They are not so much afraid of getting wet as they are of the treacherous road surface.

During dry conditions, the asphalt accumulates oil and grease from vehicles that travel over it. You can see this layer as a dark streak in the middle of the lane.

Most of this slippery stuff gets washed away by the rain over time. But at the beginning of the rainstorm, water slips underneath it because oil is lighter than water, and the two won't mix.

The two-layer combo of rainwater with a coating of oil on top can cause very treacherous driving conditions, and most motorcyclists like to avoid the first 10 minutes of a shower for that reason.

Experienced car drivers also use extra caution during the onset of precipitation. The hazard presented by oil plus rainwater is even greater on newly paved road surfaces. They look smooth and safe, but large amounts of residual oil remain in the fresh asphalt.

Rain falling on the new roadway will lift some of the oil to the top and make it much slicker than a comparative surface that's a few years old. So the first few minutes of a summer thunderstorm can be very hazardous in a newly paved area.

Another little-known aspect of wet roads is the banking of curves.

Sometimes you find turns in mountain roads leaning "inward," meaning the road leans to the right in a right turn, and vice versa. The car will feel very stable going through that turn.

But in the Georgia mountains, the roads often lean "outward" from the turn, and drivers taking the curve too fast drift across the centerline. The reason for the seemingly wrong inclination of the roadway is to allow for better drainage.

A turn banked inward will concentrate the flow of water like a funnel, which can result in a sudden hazard where the water crosses the road.

Tilting the roadway in the opposite direction makes the drainage spread out, thus lessening the risk of hydroplaning. But it also increases the tendency of the car to get carried out of the lane.

As always, extra attention to road conditions is in order during our summer rainstorms.

Rudi Kiefer is senior Web administrator and professor of physical science at Brenau University in Gainesville. He can be contacted at rkiefer@lib.brenau.edu.

Originally published Sunday, August 7, 2005

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