Fog, spray and a lorry at 22:15 in November
It is 22:15 in November. You are on a rural main road at 80 km/h. The fog is patchy: in the dips visibility falls to about 50 m, on the open stretches it is about 200 m. The road is wet, the outside temperature display shows 2 degrees, and a light drizzle is falling. Ahead of you a lorry is travelling at about 60 km/h with its rear fog light on; spray from its wheels makes it hard to judge the gap and it hides the road markings. Three hundred metres back you passed a sign warning of game crossing for the next 3 km. Your car has ABS and winter tyres with 5 mm of tread. The car behind you sits about 15 m back with main beam on, and its light is dazzling you in the mirrors.
1. Choose a speed for the foggy stretches and show the calculation, assuming 1 second reaction time and a wet-road friction coefficient of about 0.5.
Read the worked answer
The governing rule is: you must be able to stop within the distance you can see.
Stopping distance = reaction distance (v x t) + braking distance (v squared / (2 x friction x 9.81)).
At 80 km/h (22.2 m/s):
- reaction: 22.2 m
- braking: 22.2 squared = 493; 493 / (2 x 0.5 x 9.81) = 493 / 9.81 = about 50 m
- total about 72 m, against 50 m of visibility. 80 km/h is too fast.
At 60 km/h (16.7 m/s):
- reaction: 16.7 m
- braking: 278 / 9.81 = about 28 m
- total about 45 m, inside 50 m but with only 5 m to spare, before allowing for game, a bend, the lorry's spray or a worse surface.
At 50 km/h (13.9 m/s): 14 m + 20 m = about 34 m, which leaves a defensible margin.
So: about 50 km/h in the dips, at most 60 km/h on the open stretches where you genuinely can see 200 m, and easing off early when the fog thickens. Keep at least 2 seconds to the lorry (about 28 m at 50 km/h), and more, because its spray hides the road and it can brake for something you cannot see.
2. Which lights do you use, and how do you deal with the lorry's rear fog light and the main beam behind you?
Read the worked answer
Your car: dipped beam, day and night, in fog. Main beam is counter-productive because the droplets reflect the light straight back and you see less. Rear fog light: on only while visibility is below about 50 m, off again on the open stretches, where it dazzles the driver behind, masks your brake lights and is itself an offence.
The lorry's rear fog light is being used correctly for the fog. It is very bright, so do not fix your gaze on it; look at the right-hand edge line and keep the lorry's outline in peripheral vision. Leave enough gap that a sudden braking manoeuvre by the lorry still leaves you a margin.
The main beam behind you: you cannot switch it off. Flip the interior mirror to the anti-dazzle position, do not brake to educate the driver, and do not speed up beyond your visibility. A slightly larger gap to the lorry also lets you brake gently and progressively, which reduces the chance of being hit from behind. If the driver stays on main beam and tailgates, let them pass at the first genuinely safe place; do not wave them past into a fog bank.
3. The lorry driver waves you past on a straight stretch. Do you overtake?
Read the worked answer
No. A wave is an invitation, not a guarantee, and the lorry driver's view of what is behind the lorry or emerging from the next fog bank is never complete.
The numbers are against it. Overtaking an 18 m lorry doing 60 km/h with a 20 km/h speed difference takes roughly (18 m plus safety gaps of about 10 m each) divided by 5.6 m/s, so around 7 seconds, and covers well over 130 m of road, while the dip visibility is 50 m and the open stretch is 200 m at best. Most of the manoeuvre would be driven blind.
Add the hazards already present: wet road, spray that hides the centre line, fog that thickens without warning, and a 3 km game-crossing zone where a deer can step out at any moment. Each one alone is a reason to wait; together they are decisive.
Correct behaviour: stay back with 2 seconds or more, be patient, and overtake only where you can see the whole manoeuvre through: a long, clearly visible straight, no oncoming traffic, no fog bank ahead and no game warning. On this road tonight, that condition is not met.