The following chart shows the increases in stopping distance of worn tyres. The second arrow (100%) shows the stopping distance required to bring a vehicle from 60 mph to rest with a new tyre with tread depth assumed to be 8mm.
The first arrow shows the stopping distance required in dry conditions. As you can see, as the tread depth reduces so the required stopping distance increases. This increases to a point where a tyre with just 1.6mm tread remaining takes an extra 60% of stopping distance to bring the car to a halt.
Although the legal limit for tyres is, in general terms, 1.6mm, most motoring organisations recommend changing tyres at 2mm. Many manufacturers recommend a minimum tread depth of 3mm.
Obviously there is also an adverse effect on (wet) vehicle handling as tread depth reduces. The chart shows sopping distances in a straight line only, though.
The adverse effects on wet performance are due to the loss of the tyres’ ability to remove water from the road surface therefore inreasing the danger of the vehicle aquaplaning.
Many manufacturers now introduce special compounds to improve wet grip. These compounds usually contain a higher content of silica.