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Rudi Kiefer: Oil, water make for slippery mix on roads
Rudi Kiefer
Everybody knows that oil and water don’t mix. But did you know that there’s a greater slip hazard in traffic during the warm season than in winter?

We don’t often get snow or ice on North Georgia’s roadways. Most of the time, winter storms come with rain and arrive on a regular basis.
This means that the highways get washed off periodically.

Standing water creates its own problems, but there’s little oil accumulated on the tarmac. It’s different in summer, when rain storms are more sporadic. Sometimes Hall County has weeks of drought before a thunderstorm can wet things down again.

Here’s where the unexpected comes into play. Oil from motor vehicles has accumulated in cracks and pores of the asphalt. A sudden rain hits the surface.

Since oil and water don’t mix, and oil is lighter than water, a soapy film develops on the top.

I recall motorcycle instructors in Germany warning about the “Schmierfilm” building up. That’s as bad as the word sounds. Tires slip on the oil, and the oil film itself slides on the thin bed of water underneath. Many rear-end collisions are caused not by hydroplaning, but by “oil planing” during the early phase of a rain storm.

Precipitation occurring after a lengthy drought period, common during North Georgia summer, presents a great hazard because slippery conditions occur quickly. Young and inexperienced motorists can be caught unaware of this, not knowing that the onset of rain is riskier than a lengthy downpour.

On Interstates 985 and 85 in our area, traffic often moves at the speed limit, then slows suddenly when on-ramps merge into driving lanes. That’s where a lot of the crashes happen.

Two-wheeled motor vehicles are especially affected by this. Even large, wide wheels on a motorcycle or scooter have a contact area per tire that’s no larger than a credit card. Experienced motorcyclists increase their safety distance as soon as rain begins, and car drivers should allow for extra stopping distance behind a bike as well.

Where a road is newly paved, the nice black surface seems to invite fast driving. But fresh asphalt is still releasing oil. When a rain shower occurs, oil rising on the water makes the road even more slippery than it does on an older highway. In every case, the start of precipitation should be a warning sign to motorists.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at

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