Tread depth:

Worn tyre profiles cannot cope with wetness, slickness or slush. The shallower the tread depth, the longer the braking distance on wet roads. With shallow tread depth, loss of traction (aquaplaning) will occur even at slow speeds.
Tread wear indicators in the longitudinal grooves of the tyres are meant to warn the driver. If the tread is worn down to 1.6 mm, these indicators show in the form of cross webs. For the sake of safety, you shouldn't wait that long, and exchange the tyres earlier instead. With summer tyres, exchanging is recommended as soon as the tread is worn down to 2 mm. With wide base tyres, 3 mm should be considered as the minimum level. Winter tyres lose their adhesive force on snow-covered roads even at 4 mm tread depth. You should exchange the tyres as soon as they are worn down to this value.