Stopping distance is the sum of two parts: reaction distance (what the car covers while you react) and braking distance (what the car covers while the brakes act). A human reacts in about one second. At 50 km/h that is 14 metres, at 90 km/h 25 metres — before the brakes even begin to work. Braking distance depends on speed, tyre grip and road surface. Together these two parts decide whether you can stop in time.
Under Trafikförordningen 3 kap 14 § the vehicle's speed must be adapted to what road safety requires, and you must be able to stop within the distance you see clear. That means you yourself weigh in wet roads, darkness, gradient and the vehicle's load before choosing speed. The posted limit is a ceiling — not a guarantee that this speed is safe right now.
You are driving 80 km/h on a wet country road and see the car ahead brake hard for an animal. In one second of reaction you cover 22 metres before your own brake pedal starts to work. On wet asphalt the braking distance roughly doubles compared with dry road. Three options: keep a short gap, keep a normal gap, or adapt both speed and gap to the rain. The right decision is the last one. On a wet surface the three-second rule is too short — count on four or five seconds.
Many believe doubling speed also doubles the risk — you only need double the distance. Braking distance grows with the square of speed. If you double the speed, braking distance becomes roughly four times as long. At 30 km/h on dry asphalt the braking distance is about 4–5 metres. At 60 km/h it becomes around 18 metres. That is the difference between stopping before the crossing and not.