Check visibility conditions for travel.
The rule for driving in fog, heavy rain, snow or darkness is that you must be able to stop within the distance you can see to be clear. Stopping distance has two parts: the thinking distance you cover at unchanged speed while you react, which grows in proportion to speed, and the braking distance, which grows with the square of speed. Doubling your speed therefore roughly quadruples the braking component, which is why visibility limits bite so hard. Surface grip sets the deceleration available — around 7 m/s2 on dry asphalt, 5 on wet, 2.5 in snow and barely 1.2 on ice — so the same 200 metres of visibility permits a very different speed depending on what is under the tyres.
Stop-within-sight speed limit
Stopping distance = speed x reaction time + speed^2 / (2 x deceleration); the safe speed is the largest speed whose stopping distance equals the visibility
This is a simplified physics model for planning and education only. Real stopping distance depends on tyres, brakes, load, camber, gradient and how hard you actually brake, and the figures here take no account of legal speed limits. Always drive to the conditions and obey posted limits.
Because braking distance rises with the square of speed. Cutting sight distance from 200 m to 100 m only drops the safe speed by roughly a third, not a half — and conversely, small speed increases eat visibility margin very quickly.
Meteorologically, fog is visibility below 1,000 m; below 200 m is dense fog and below 50 m is where most jurisdictions expect you to consider stopping in a safe place. Front fog lights help below about 100 m; high beams make it worse by scattering light back at you.
Yes if you are tired, on a long drive, or scanning for hazards rather than expecting one. The 1.5 s default assumes an alert driver already looking at the road; 2.5 s is a fairer figure late at night.