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Asphalt is slippery on dry and wet roads says TRL report

NEW ASPHALT roads are as slippery in dry conditions as they are in wet, and could need a runningin period with lower speed limits to safeguard unsuspecting motorists, it emerged this week.

Research by TRL sparked by two fatal accidents in Derbyshire last summer has found that stopping distances in dry conditions on newly laid stone mastic asphalt (SMA) are similar to those on wet roads.

The report by senior TRL scientist Peter Roe says the results could have far reaching consequences.

'The condition will not be unique to Derbyshire and could be relevant to many newly laid asphaltic surfacings. Further work is necessary to determine how long this condition can last in order to decide on an appropriate strategy, either to reduce the risk or to warn motorists that the problem exists, ' the report says.

The research follows police investigations in September into the two crashes. 'Our tests involved skidding a vehicle in the dry. The friction was quite significantly lower than we were expecting, ' Derbyshire Constabulary traffic support unit PC Jim Allen told NCE.

Although the road friction had no bearing on the accidents and the roads were properly built, police notified Derbyshire County Council engineers who commissioned the report.

TRL found that car wheels locked during a skid on a dry road can heat up a thick coating of binding bitumen or asphalt.

'Heat and shear stresses generated in a locked wheel dry skidding may result in a change in the viscosity of the bitumen, leading to a reduction in the apparent coefficient of friction between the tyre and the road, ' the report notes.

'It seems to be an early life problem which may be due to excess bitumen binding which you can find in SMA.' said Derbyshire County Council environmental services assistant director Mike Ashworth.

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