ROAD safety organisation RoadSafe is calling on motorists to 'tread carefully' this summer following independent research undertaken by MIRA Ltd (formerly the Motor Industry Research Association) which reveals that braking performance deteriorates in wet conditions when tyre tread depth drops below 3mm.

Although the legal tread depth limit is 1.6mm, a threshold set by motoring experts some 13 years ago, RoadSafe says there is now compelling evidence that motorists should check and change their tyres at 3mm.

Road accidents where wet conditions are a contributory factor are on the increase, the road safety partnership says.

RoadSafe's chairman Adrian Walsh says: "A disproportionately high number of road accidents occur in the wet. Let's not forget last summer was one of the wettest on record.

"It's a well known fact that in wet conditions braking distances substantially increase. A third of all road accidents already occur during the summer months, and with heavy downpours and flash floods becoming a permanent fixture of an English summer there is every possibility this figure could rise."

The trend is already upwards according to Town and Country Assistance, which states that road accidents where wet conditions are cited as a major cause have increased from one in 12 in 1998-99 to one in four in 2003-04.

MIRA's senior engineer Simon Aldworth, who carried out the research into tyre wet grip performance at the organisation's headquarters in Nuneaton, said: "There has been a long-standing belief within the motoring industry that wet weather tyre performance deteriorates more rapidly as the tread depth wears away.

"A comprehensive test of different vehicles and tyre tread depths now scientifically confirms these claims."

MIRA's research was carried out on four separate vehicles, a mid-range family hatchback, an MPV, an executive saloon and a high-performance saloon car, measuring stopping distances at 50mph in conditions that represented moderately heavy rain (0.5mm to 1.5mm water depth).

The research discovered that tyres with a 3mm tread had a 25 per cent better performance than those at 1.6mm. In terms of stopping distance this represents an extra eight metres (25ft) in the wet.

In one test, when a tyre with a tread depth of 8mm was compared to one with only 1.6mm, the stopping distance increased by 13 metres (42.25ft) - the equivalent to double the length of a typical living room.

"Noone is suggesting that 1.6mm tyre tread depth is not safe," says Adrian Walsh. "However, with 300,000 crashes every year and a climate which is now much more susceptible to heavy rain, there is a need to take action now.

"If motorists replaced their tyres sooner stopping distances would improve, accidents reduce and lives saved."