Aquaplaning behind a truck at night in heavy rain
21:20 on a freeway with a 120 km/h limit and two lanes in your direction. It is raining hard enough that your wipers are on their fastest setting. You are in the left lane at 112 km/h behind a 22-wheeler, headlights on dipped beam. Spray from the truck limits your view of the road beyond it to roughly 40 m, and there are no streetlights for the next 15 km. You are holding a two-second gap because that is what your driving instructor taught you. Without warning your lane crosses a shallow dip that has collected water. For about half a second the car feels light, the steering goes vague, and the engine note rises slightly without the speedometer climbing. When the tyres bite again, the truck's brake lights bloom red through the spray and it begins to slow.
1. Why is a two-second gap the wrong tool here, and what gap and speed would you actually choose? Use numbers.
Read the worked answer
Two seconds is a dry-road, good-visibility minimum. Here the binding constraint is not the gap alone, it is sight distance: you can see 40 m.
At 112 km/h you travel about 31 m every second, so two seconds is roughly 62 m - already more than you can see. Worse, stopping distance from 112 km/h on wet tar (friction around 0.4) is about 120 m of braking, plus 25–30 m of reaction, so roughly 150 m in total. The gap is irrelevant when the stopping distance is four times the visible road.
The governing rule is that you must be able to stop within the distance you can see to be clear. To stop inside 40 m on wet tar you need to be down at about 50 km/h. A realistic compromise in moving freeway traffic: drop to 80–90 km/h, open the gap to at least four seconds (double the dry figure) and in spray, ideally five or six, because the truck can stop faster than you can see its brake lights.
Aquaplaning risk also rises with the square of speed - slowing 20 km/h cuts it substantially.
2. Explain the half-second of 'float' - what is happening at the tyres, and what do your hands and feet do?
Read the worked answer
A wedge of water has lifted the tread off the road so the tyres are riding on a film, not on tar. With no contact there is no grip, so steering has no effect and the engine note rises because the driven wheels are spinning slightly against water resistance. It feels light and vague because it is - you are a passenger for that half second.
Hands: hold the wheel straight and steady. Do not saw at it to 'find' the road; the correction is what spins the car the instant grip returns.
Feet: ease off the accelerator smoothly and progressively. Do not brake and do not lift abruptly - sudden load transfer to the front tyres on a water film is how a straight-line slip becomes a spin.
Eyes: look far ahead, not down at the bonnet.
Once the tyres bite again, reduce speed deliberately and then move out of the wheel tracks of the vehicles ahead, which is where the standing water collects. Then reassess whether to continue at all.
3. The truck brakes. Give your sequence of actions, including what you would not do.
Read the worked answer
- Mirror and over-shoulder check first, so you know what is behind and beside you before you change anything.
- Ease off the accelerator - that alone starts closing the problem before you touch the pedal.
- Brake progressively and earlier than feels necessary, adding gap rather than matching the truck's braking rate. Your wet stopping distance is far longer than his loaded rig's.
- Increase the gap to five or six seconds; if he slows hard, that buffer is your only crash-avoidance tool.
- Switch on your hazards briefly to warn the traffic behind, who also cannot see through your car.
What you would not do: pull out and overtake into 40 m of spray, where you would be blind and invisible; stop dead in a live lane behind the truck for a following vehicle to plough into; or stamp on the brakes on standing water, which invites a spin.
Best outcome: take the next off-ramp and wait out the storm. Continuing at 112 km/h in 40 m visibility is a decision you can only get wrong once.