Every vehicle rests on four palm-sized patches of rubber. The tyre's tread depth, pressure and temperature decide how quickly the car brakes, how stably it steers and whether water is channelled away in rain. Wear happens slowly — you may not notice it until grip suddenly fails in a corner or under hard braking. For the same reason you rarely see a bad tyre — you feel it first when you need it most.
Under Trafikförordningen 4 kap the minimum tread depth is 1.6 mm for summer tyres and 3 mm for winter tyres when winter conditions apply. Winter tyres are mandatory between 1 December and 31 March in winter conditions. Studded tyres may be used between 1 October and 15 April. As the driver you are responsible for ensuring the tyres meet these requirements — the check happens every time you start the car, not only at the periodic inspection.
You are driving 100 km/h on a motorway in heavy rain. Suddenly the steering feels light and the car does not follow your inputs — aquaplaning. Three options: brake hard, keep your speed, or release the throttle and keep the wheel straight. The right decision is the last one. Hard braking or steering on a film of water makes things worse. Worn tyres worsen the problem because the tread is what channels water away — at 1.6 mm you still have margin, but not much.
A common slip on the test is believing tyres only matter when they are visibly worn. In practice a few tenths less tread or a few tenths bar lower pressure already makes braking distance noticeably longer. On dry asphalt you will not feel the difference in everyday driving — it shows up at the first emergency brake or the first wet bend. By then the margin is already gone.