Wet motorway at night: gap, aquaplaning and the lorry that brakes
It is 22:30 on the A1 near Bern. The limit is 120 km/h, but rain has been falling for an hour and the road is soaked. Spray from the lorry-and-trailer ahead of you cuts visibility in your lane to roughly 60 m. You are in the right lane at 110 km/h, about 25 m behind the lorry. On your left a hatchback is overtaking at about 115 km/h and is currently beside your rear wheel. Your front tyres have 3.0 mm of tread, the rears 4.0 mm. Your reaction time is a normal one second. Suddenly the lorry's brake lights come on hard and its trailer begins to snake slightly.
1. At 110 km/h, what following distance do these conditions actually require? Estimate reaction distance, braking distance and total, and say why the two-second rule is not the answer here.
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Convert first. 110 km/h ÷ 3.6 ≈ 30.6 m travelled per second.
Two-second rule. 2 × 30.6 ≈ 61 m. That figure was derived for dry asphalt and mostly covers reaction plus a small margin; it is a minimum fair-weather bubble, not a wet-weather one.
Braking distance in rain. d = v² / (2μg). With μ ≈ 0.5 for a soaked road: 30.6² / (2 × 0.5 × 9.81) ≈ 936 / 9.81 ≈ 95 m. On dry asphalt (μ ≈ 0.8) the same sum gives about 60 m, which shows how much the rain costs you.
Total. Reaction 1 s = 31 m plus 95 m braking ≈ 126 m, i.e. about four seconds of travel. The 25 m you currently hold is roughly one fifth of that. The 3 mm front tread makes it worse: a worn tyre clears far less water, so the aquaplaning threshold arrives at a lower speed and the effective μ can drop below 0.5.
Practical rule. Three seconds (about 92 m) is the absolute floor in heavy rain at this speed; four seconds is realistic. If you cannot see 126 m ahead because of spray, you are driving too fast for the conditions regardless of the posted 120.
2. What happens if you hit a patch of standing water at 110 km/h, and what are the correct inputs?
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What happens. The tyre cannot displace the water fast enough, the wedge of water lifts the tread off the surface and you aquaplane: steering and braking go light or dead, the car follows the water film rather than your input. With 3.0 mm of tread this can occur at motorway speeds in relatively shallow water, especially in the ruts where lorries have polished the surface and water collects.
Correct inputs. Keep the wheel straight and hold it firmly with both hands; ease off the accelerator smoothly so engine braking and rolling resistance slow you; do not brake hard and do not steer sharply, because a tyre that regains grip mid-manoeuvre will convert that input into a sudden swerve or spin. Do not switch off the ignition or yank the handbrake. Wait for the front tyres to bite again - usually within a second or two - then reduce speed deliberately and, if the car has one, keep the stability programme on; it cannot create grip where there is none, but it helps at the moment grip returns.
Afterwards. Reduce speed to a level where you can see and stop within the visible distance, move out of the deepest ruts where the road allows, and treat every bridge, underpass and shaded stretch as a place where the water sits.
3. The lorry brakes hard, you cannot see past it, and the hatchback is level with your rear wheel. What do you actually do in the next few seconds?
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Priority order: slow, do not swerve. Brake firmly but progressively so the car stays stable and the driver behind reads your intention; switch on the hazard lights immediately, because decelerating from 110 km/h to a stop in a motorway lane is exactly the situation they exist for. The hazard lights also warn the traffic you can no longer see.
Why not the left lane. The hatchback is alongside your rear wheel. A sudden move left is a collision with a car you have already seen and possibly with one behind it in the spray. By the time he has passed, the gap may be gone; if it is not, indicate, check the mirror again, and change lane only with a clear margin - never as an evasive reflex.
What you must not do. Do not overtake the braking lorry into the unknown ahead of it: a lorry braking that hard usually has a reason - a queue, a lost load, an accident - and overtaking puts you into the hazard first at the highest speed.
If it becomes a stop. Come to rest as far right as possible, hazard lights on, and consider the emergency lane only as a last resort; if you stop there, high-visibility vest before opening the door, warning triangle well behind (roughly 100 m on a motorway), and stand behind the barrier, not between the car and the traffic.