Winter tyres are compulsory between the 1st December and 31st March. They must have a tread depth of 3mm and can be studded. You are also allowed (but not obliged) to use studded tyres from the 1st October and until the 30th April – use your common sense based on weather conditions.
Every year the first accounts of ice on the roads catch people off guard. Unsuspecting divers on the road first thing in the morning go sliding off into ditches, or other cars, as they are unprepared for the icy conditions. The roads become chaotic and that will start the onset of owners changing to winter tyres.
It is also worth knowing that if you have studded winter tyres and drive in Europe you might be in for a shock. Many European contries do not allow studded tyres and there are stories of drivers being handed a pair of pliers and told to pull out the studs before they enter. Who knows if it is just a tall tale, but it is worth finding out before you head down to the alps for your skiing holiday.