Freezing fog, 14 standing passengers and a lorry 25 m ahead
It is 05:25 in January. You are driving a 12 m bus outside a built-up area where the limit for your vehicle is 70 km/h, with 22 passengers seated and 14 standing; the document permits 30 standing. Freezing fog limits visibility to about 50–60 m, the road is wet and the outside temperature is −1 °C. A lorry with a trailer is ahead, doing 45 km/h; you are at 55 km/h and about 25 m behind it, using its tail lights as a guide. Your headlights are on dipped beam; the rear fog lamp is off. In 400 m the road crosses a bridge over a river, then continues straight for 2 km. The next scheduled stop is 7 minutes away and you are already 4 minutes late. Two of the standing passengers are moving towards the front to talk to you.
1. What speed will you actually drive at, and how do you justify it with numbers?
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Start from the distance you can see - 50–60 m - and work backwards.
At the 70 km/h limit you travel 19.4 m/s. One second of reaction covers 19 m; braking on wet asphalt at about 0.35 g adds roughly 55 m. Total around 75 m, which is more than you can see. The 14 standing passengers make it worse: you cannot use full braking without injuring them, so your usable deceleration is lower and your stopping distance longer than the textbook figure.
Now work upwards. With a 1 s reaction and a gentle 3 m/s² (about 0.3 g, the sort of braking standing passengers can survive):
- 40 km/h → 11 m + 21 m ≈ 32 m
- 50 km/h → 14 m + 32 m ≈ 46 m
- 55 km/h → 15 m + 39 m ≈ 54 m
So roughly 40–45 km/h is the maximum that still leaves a margin inside the visible distance, and slower over the bridge and on the shaded approach. That also matches the lorry's 45 km/h, so you keep the gap instead of closing it.
Drive on the engine brake, no overtaking, dipped beam on. The legal limit is a ceiling, not a target; in fog the visible distance sets the speed.
2. Assess the 25 m gap at 55 km/h and say what gap you will hold, and why.
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25 m at 55 km/h (15.3 m/s) is about 1.6 seconds. That is below the 2-second minimum for good conditions, and it is far too short in fog.
The governing rule is: you must be able to stop within the distance you can see to be clear. In fog, double the normal time gap. At 45 km/h (12.5 m/s), a 2-second gap is 25 m, so 3–4 seconds is roughly 40–50 m - that is the gap to hold.
There is a second reason a short gap is worse than it looks: you are steering by the lorry's tail lights. If he brakes, you react to the lights, so your effective reaction starts late and the closing speed adds to your own stopping distance. He is also a loaded lorry with a trailer and can brake harder than you can with 14 people standing.
Practical handling: drop back until you can see his tail lights clearly and still have 40 m or more, and use his path only as a guide rather than following his exact line - the gap gives you room to steer as well as stop. If a car overtakes and fills the gap, drop back again rather than closing up. Overtaking is out of the question: you would be in the oncoming lane with no view at all.
3. Deal with the lighting, the standing passengers, the bridge and the time pressure.
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Lighting. Dipped beam is correct and compulsory outside a built-up area; main beam in fog just reflects back and blinds you. The rear fog lamp may be used only when visibility is seriously reduced - around 50 m or less. At 50–60 m you are on the borderline, so switch it on if visibility drops below 50 m, and switch it off again when a vehicle is behind you, because it masks your brake lights.
Standing passengers. 14 is legal (30 permitted), but the duty changes. At 40–45 km/h with a 40–50 m gap you have created room for gentle braking only. Warn the two passengers who are walking towards you that they must sit or hold a rail; they must not stand beside the driver's area, and you should not be answering questions while driving in fog.
Bridge. At −1 °C with a wet road, a bridge deck freezes before the road does because it loses heat from below. Expect ice in the first 50 m and on the shaded approach. Before it: speed down to around 30 km/h, hold the wheel straight, and do no braking or steering on the bridge. If the wheels go light, ease off the accelerator - do not brake.
Time pressure. Four minutes late is not a reason for anything. A schedule can be recovered; a bus in a ditch cannot. Tell the controller you are running late due to fog and let them deal with the stop.