The review of tyres, road surfaces and accidents, jointly funded by the AA Motoring Trust and Country Surveyors’ Society, reveals the importance of tread depth. It says that about one mile in six of the principal road network in the UK falls below the accepted safety threshold, and that in the wet, skidding accidents on these roads increases by almost 50%. Exacerbating the situation is the fact that 10% of car drivers have at least one illegal tyre.
The report, Get A Grip, claims that stopping distances on wet roads are doubled, but adds: “Grip on wet roads is markedly reduced when tread depth is less than 3mm – when there is a 2mm film of water on the road, even 2mm of tread may give no better stopping friction than a bald tyre.”
The risk of accident trebles on wet roads if tread depth is at the legal 1.6mm minimum and rises seven-fold when the tread depth is less than 0.5mm. In addition, different tread depths on front and rear tyres create problems.
The AA research backs the findings of a Motor Industry Research Association study commissioned by tyre maker Continental in October 2004 and is likely to add to calls for a 3mm tread depth legal limit. The tyre industry is already producing promotional literature explaining the benefits of deeper tread to motorists.
The AA believes the tyre industry needs to fund more research into the role of tyre characteristics in accidents, including the contribution to accidents of tread-depth differences between front and rear tyres.
It urges fitters to promote the message that when tyres are bought in pairs, they should be fitted to the back wheels, not the front, and says tyre makers need to develop affordable technologies to maintain consistent tyre pressure, such as reliable on-board tyre pressure monitoring.