Road slipperiness

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Road slipperiness or skid resistance is the technical term for the cumulative effects of snow, ice, water, loose material and the road surface on the traction produced by the wheels of a vehicle.^[1] Road slipperiness can be measured either in terms of the friction between a freely-spinning wheel and the ground, or the braking distance of a braking vehicle, and is related to the coefficient of friction between the tyre and the road surface. Public works agencies spend a sizeable proportion of their budget measuring and reducing road slipperiness through drainage, snow removal and street sweeping in order to maximise road safety; even a small increase in slipperiness of a section of road can increase the accident rate of the section of road tenfold.^[2]

A special road safety problem is Split friction or μ (mu) - split; when the friction significantly differs between the left and the right wheelpath. The road may then not be perceived as hazardous when accelerating, cruising or even braking softly. But in a case of hard (emergency-)braking, the car will start to rotate over the wheelpath offering highest grip. Split friction may cause jack-knifing of articulated trucks, while trucks with towed trailers may experience trailer swing phenomena. Split friction may be caused by an improper road spot repair that results in high variance of texture and colour (thin ice on newly paved black spots thaws faster than ice on old greyish asphalt) across the road section.

[edit] Measurement

The two ways to measure road slipperiness are surface friction testing and stopping distance testing. Friction testing can use surface friction testers or portable friction testers, and involves allowing a freely moving object, usually a wheel, to move against the surface. By measuring the resistance experienced by the wheel, the friction between the ground and the wheel can be found.^[1]

Stopping distance testing involves performing an emergency brake in a test vehicle and measuring the distance required to come to a stop. This can be measured either from the length of the skid marks left by the vehicle, or by the "chalk-to-gun" method, where the brakes are connected to a small gun filled with chalk powder, which marks the point when the brakes were applied. This has the advantage of measuring the full stopping distance, while simply measuring the skid marks only measures the distance from the point where the wheels began to lock or slip.^[1]

[edit] Reduction

Road slipperiness is one of the largest causes of car accidents — in 1997, over 53,000 accidents were caused by slippery roads in the United Kingdom alone.^[3] A small change in road slipperiness can have a drastic effect on surface friction: decreasing the coefficient of friction from 0.45 to 0.35, equivalent to adding a dusting of wet snow, increased the accident rate by almost 1000%.^[2] As such, road agencies have a number of approaches to decreasing road slipperiness. Most roads are designed with a convex camber to allow surface water to drain out of the road, with storm drains at regular intervals and modern paving materials are designed to provide high friction in most conditions. Permeable paving allows water to soak through the paving material, reducing slipperiness in very adverse conditions.

Snow and ice removal also decreases road slipperiness; snowploughs and snow blowers can remove the snow from the road surface while gritters drop road salt and sand, which both melts the snow and ice from the road surface, and provides a rougher surface to grip onto. However, in dry conditions, sand and salt on the road surface can themselves increase road slipperiness and pose a danger to road traffic, and therefore, roads are cleared by street sweepers after roadworks and gritting to make sure that all the loose material is cleared from the road surface.

[edit] References