Road slipperiness can contribute to
car accidents. In 1997, over 53,000 accidents were caused by slippery roads in the
United Kingdom out of an estimated 4,000,000 accidents (or approximately 1.3 per cent) . 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%. As such, road agencies have a number of approaches to decreasing road slipperiness. Most roads are designed with a
convex camber to provide sufficient
drainage, thereby allowing surface water to drain out of the road. Trouble sections include entrances and exits of banked outercurves, where the
cross slope is close to zero. Unless these sections have a longitudinal grade of at least 0.4–0.5%, the
drainage gradient (resultant to crossfall and longitudinal grade) will be lower than 0.5% so water will not run off the road surface.
Storm drains may be installed 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. Road slipperiness can be prevented or delayed by proper pavement design. The
aggregate used in the pavement should be selected with care, as certain aggregates such as
dolomite may
polish, or wear smooth under the action of tires. With asphalt pavements and surface treatments, using too much asphalt or asphalt emulsion can cause
bleeding or
flushing, a condition where excess asphalt rises to the top and fills in the road texture. Both problems increase slipperiness, especially when the pavement is wet. Once lost,
pavement texture can be restored with retexturing procedures such as
diamond grinding of pavement, surface treatments such as
chipsealing, waterblasting and resurfacing with
asphalt concrete. 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 provide 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. ==See also==