Heavy rain on the descent: spray, a tailgater and a side road
Heavy rain. You are descending one of the steep roads on the east side of the Rock at about 45 km/h - roughly 12.5 metres per second. Ahead of you a flatbed lorry carrying scaffolding is throwing up a curtain of spray; you can see its lights but not a great deal beyond them. Your wipers are on fast and there is standing water in the gutters. Your tyres have about 3 mm of tread. In dry conditions you would keep a two-second gap. A car behind you has been holding a gap of about 8 metres for the last half kilometre, and there is a side road 100 metres ahead on the left where a car is waiting to emerge. The lorry is braking gently for the bends.
1. What gap should you leave to the lorry, and how do you actually measure it in these conditions?
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Start from the dry two-second gap and double it: four seconds minimum, then add more for the spray, the downhill gradient and 3 mm of tread. Do the arithmetic - at 12.5 m/s a four-second gap is about 50 metres.
Measure it with a fixed point: when the lorry's rear passes a lamp post, a road sign or a marking, count "one thousand and one, one thousand and two..." until your front bumper reaches the same object. Counting out loud is more accurate than estimating metres.
Then apply the limit that overrides the arithmetic: you must be able to stop within the distance you can see to be clear. In a spray curtain behind a flatbed you may not be able to see 50 metres of clear road, and a downhill wet surface lengthens stopping distance further rather than keeping it constant. If you cannot both hold four seconds and see far enough to stop in, the gap is not the answer - reduce speed until the two match.
2. The car behind is sitting 8 metres off your bumper. What do you do about it?
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Do not brake to warn or punish him - that is precisely the event you are trying to avoid. His stopping distance is fixed by physics and by his own reaction time; your only lever is to remove the need for a sharp stop.
So increase your own gap to the lorry to five or six seconds. That gives you room to slow progressively for the bends using engine braking and the lightest possible brake pressure, which gives him several seconds of warning and a gentle rate of closure instead of a sudden one. Touch the brakes early and lightly rather than late and hard - the brake lights are his only information.
If the following distance does not improve and a safe place appears, pull over and let him go. A driver prepared to sit 8 metres back in standing water is a hazard you can remove from your day simply by not being in front of him.
3. Do you overtake the lorry on this descent? Give your reasoning.
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No - not here, not now. Overtaking is a contract with the road: you may only do it if you can see enough clear road to complete the move and be back on your own side before anything appears. The spray has already taken that away; you cannot see past the lorry, so you cannot know the road is clear.
Three further reasons stack on top of that:
- The gradient. Going downhill you will gain speed on the wrong side of the road, and the lorry is a long vehicle to pass.
- The side road 100 metres ahead. A car is already waiting there; if it pulls out while you are alongside the lorry, neither of you can see the other.
- The car behind at 8 metres. Anything you do, he will follow - probably immediately and probably without checking.
Wait until the road levels out, the rain eases and you can see a genuinely clear gap. A slow journey in this weather is the intended outcome, not a failure.