The test results are displayed in the graph below. The stopping distance is different for the two road surfaces due to their different water retention properties.from the graph of results, it can be seen that the stopping distances start to increase dramatically at tread depths of below 3mm.
At the legal minimum tread depth of 1.6mm, the stopping distance is increased by 36.8% on the hot rolled asphalt and 44.6% on the smooth concrete.
From the independent research, RoSPA recommends that tyres are changed once the tread reaches 3mm in depth.
Tyre pressure also affects a vehicle's safety by reducing its handling capability. If a tyre is over or under inflated, less rubber will be in contact with the road which leads to increased tread wear, and may increase the vehicles stopping distance. It is recommended that you check all of your tyre pressures regularly, as well as inspecting the tyre for damage.
More information about how to ensure your tyres are safe, you can visit RoSPA's Tyre Information Fact Sheet.