Heavy rain on the expressway: gap, spray and aquaplaning
It is 8 pm and you are driving a five-year-old hatchback on a three-lane expressway in heavy rain. Your wipers are on maximum and you can see clearly for only about 60 m. You are in the centre lane at 90 km/h. A lorry 25 m ahead in the same lane is throwing up a wall of spray. In the right-hand lane, a car is closing on you from behind and is about 10 m off your bumper. In the left-hand lane, a slow van is about 40 m ahead of you. Your tyres are worn to about 2 mm above the tread-wear indicator.
1. Using the two-second rule, what gap should you keep to the lorry at 90 km/h, and what should it be in this rain? What does that imply about your speed, given the visibility?
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The arithmetic: 90 km/h is 25 m per second, so a two-second gap is 50 m. In rain the gap should be at least doubled, to four seconds - 100 m. You are at 25 m, which is roughly one second: a quarter of what the conditions demand.
The visibility check: you cannot see beyond 60 m, so a 100 m gap is physically impossible - you would be following something you cannot see. To keep a four-second gap inside 60 m of visible road, your speed must satisfy 60 ÷ 4 = 15 m per second, i.e. about 54 km/h.
Conclusion: the visibility, not the posted limit, sets your speed here. Ease off progressively rather than braking hard, because the car behind is only 10 m back and a sharp stop will be converted into a rear-end collision.
2. What are the first signs that you are aquaplaning, and what should you do?
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Warning signs:
- The steering suddenly feels light and vague, as though it is disconnected.
- The engine note rises as the driven wheels spin up on the water film.
- The rear steps out slightly, or the car wanders without input.
- The spray noise changes pitch as the tyres stop displacing water.
What to do: ease off the accelerator, hold the wheel straight, and do not brake hard or steer sharply - either will transfer weight and can spin the car. Let the speed bleed away until the tyres regain contact, then resume gentle control.
Why your tyres matter here: at 2 mm of tread the grooves cannot clear water fast enough, so aquaplaning occurs at a much lower speed than on new tyres. This is a maintenance issue, not a driving-technique issue - the tyres need replacing.
3. You want to move to the left-hand lane to get away from the car behind. What do you check, and is it a good idea here?
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Checks: mirrors, a blind-spot glance over the shoulder, and a signal applied early enough for the car behind to see it through the spray.
But the move is the wrong call right now:
- The van in the left lane is 40 m ahead and you are closing on it, so the move buys you a slow vehicle directly in front and a likely need to brake - just as the car behind is closing.
- In heavy spray you cannot read the van's brake lights clearly until you are close.
- Moving left does not remove the car behind; it simply follows you across.
Better plan: stay in lane, slow gently to around 55 km/h to open a genuine gap to the lorry, and let the car behind go past. Change lanes only when the left lane is clear well beyond the van, and signal well before you move.