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When the Rain Starts, Driving Technique Changes

Is it time to find out whether our seemingly endless summer has dulled our collective memory of how to drive safely in the rain? Will we automatically reduce our speed? Will we intensify our focus on driving defensively?

Understanding why wet roads are more slippery helps the discussion. Friction—the resistance to the relative motion of two surfaces in contact with one another—can have various causes and physicists have found the problem of determining which one plays the primary role in helping tires stick to the road surprisingly difficult. The latest solution from a team of Italian and German physicists proposes that the loss of friction on wet roads arises because water fills in the tiny irregularities on the road surface.

On dry roads, the tire rubber pushes down into the tiny pits on the road’s surface. This deformation of the rubber combined with vibrations caused by the irregularities of the road surface then heat up the rubber. This heat takes away some of the tire’s kinetic energy, or energy of motion, thereby producing the friction necessary for enabling a driver to control the car.

On wet roads, however, the pressure of the tire rubber traps the water in the small pits in the road. The trapped water prevents the rubber from expanding into the pits thereby smoothing out the surface by reducing or eliminating the irregularities. In effect, wet roads prevent the tire rubber from reaching out and touching the bottom of the puddle. This, in turn, means less loss of kinetic energy—i.e. less friction between the tire and the road surface. Hence it becomes more difficult for the driver to control the car.

The calculations based on this theory showed a 20% to 30% loss of friction for tires braking on wet roads without skidding. This description notes that, “this research applies to the limited loss of friction that drivers experience when there is not too much water on the road and the vehicle is not traveling too fast (less that 60 kmh or about 35 mph.)

The theory addresses as well our general observation that at higher speeds or when there is more water on the road, the more dramatic loss of friction known as hydroplaning can occur. When a vehicle is hydroplaning the tires aren’t in contact with the road surface, merely the skin of water on top, meaning the driver’s ability to control the vehicle is essentially minimized.

So, here are some points to keep in mind at all times when driving in the wet:

• Wet roads can double your stopping distance, so reduce your
speed by as much as a half.
• Slow down gradually.
• Avoid aggressive braking or steering.
• Increase your following distance.
• Be especially careful driving through puddles.
• When pulling onto a road, allow extra space for oncoming
• Watch oncoming traffic for spraying water from puddles across
the median onto the windshields of cars in your lane.
• If you find yourself hydroplaning, don’t brake to slow down.
Release the accelerator to allow the wheels to turn freely and
your momentum to gradually decrease.

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