German speed rules and explicit distance models
Speed selection combines the applicable legal maximum with the distance, grip and information actually available. A posted maximum or an ordinary road limit does not require driving at that speed. Current StVOsection 3 requires adaptation to conditions, personal ability and the vehicle and load. A slower vehicle's design limit also remains relevant even if other traffic may legally travel faster.
Vehicle and road categories
Ordinary built-up, rural, motorway and motor-road situations can have different provisions. Passenger cars, heavy goods vehicles, buses, trailers and motorcycles must not be treated as one identical category. Autobahn access depends on the actual design-speed requirement and other rules; a category label alone is not proof of permission for every moped or scooter.
When fog, snow or rain reduces visibility below 50 metres, the cited German rule sets a maximum of 50 km/h and still requires a lower speed when necessary. This is not permission to travel at 50 when the safe visible stopping distance is much shorter. On a sufficiently narrow carriageway where approaching vehicles could be endangered, the visible-distance requirement is more restrictive than simply stopping somewhere within the full view.
Interpret a calculation's assumptions
Common learner formulas estimate reaction distance as speed divided by ten and multiplied by three, and ordinary braking distance as speed divided by ten squared. They are teaching approximations, not exact laws guaranteeing a real vehicle's stop. Keep reaction and braking components separate. A formula for an emergency-braking example changes the specified braking component, not the reaction component automatically.
For an explicit model at 50 km/h, these formulas give 15 metres reaction distance and 25 metres braking distance, for 40 metres total. At 100 km/h they give 30 plus 100, or 130 metres. The braking part increases fourfold when speed doubles under that model. Surface, tires, gradient, load, braking system and actual response can change real performance.
Following and changing conditions
Keep a gap that allows an appropriate response when the vehicle ahead brakes. A truck or bus may have a separately defined motorway minimum, but that number does not replace a larger gap required by conditions. Read the actual vehicle, road and speed thresholds before applying it. Rain, glare, low sun, wind and unfamiliar roads can all reduce the usable margin. Reassess early rather than attempt to restore it through a last-second maneuver.