Wet evening, 3 mm tyres, 1.5 seconds behind a lorry
You are driving a five-year-old estate car on a wet autumn evening on a two-lane road with an 80 km/h limit. The front tyres have 3.5 mm of tread, the rear tyres 3 mm. You are following a heavy lorry with a gap of about 1.5 seconds - roughly 33 metres at your speed. You have been behind it for 20 km on a road that has been climbing and descending, and it has begun to brake intermittently on the downhill sections. You passed a deer warning sign a kilometre ago and the verge on both sides is dense spruce plantation. You have been driving for four hours, and you would describe your own reaction time as «about a second, on a good day». There is light oncoming traffic, currently none within 200 metres.
1. Calculate a realistic stopping distance at 80 km/h on this surface and compare it with your gap. Show the arithmetic.
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Speed: 80 km/h ÷ 3.6 = 22.2 m/s.
Reaction distance: 22.2 m/s × 1 second = 22 m. That is the best case, on a good day, before you have touched the brake. After four hours of driving, a realistic figure is 1.5 seconds, or 33 m - the whole gap you currently have.
Braking distance: using d = v² / (2 × μ × g) with μ ≈ 0.4 for wet asphalt and these tyres: 22.2² / (2 × 0.4 × 9.81) = 493 / 7.85 ≈ 63 m. The familiar Norwegian rule of thumb, (80/10)² = 64 m, gives the same answer - this is the standard figure for a wet road with worn rubber.
Total: 22 + 63 = about 85 metres from seeing the hazard to standing still, and more if the lorry's own braking distance differs from yours.
The comparison is the answer: your gap is 33 m, your stopping distance is 85 m. You are 50 metres short. Even at a perfect 1-second reaction you would still be moving at roughly 50–60 km/h when you arrived where the lorry was. At 80 km/h you need about 4 seconds - 89 m - just to have your own stopping distance in front of you.
2. The lorry's brake lights come on hard and it decelerates sharply. The oncoming lane is clear for about 150 metres. Brake or swerve?
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Brake, in your own lane. The oncoming lane being «clear» for 150 m is not the same as it being safe: at 80 km/h you cover 150 m in under seven seconds, and a car, a deer or a cyclist can appear in that window from a side road or the trees. A head-on collision at combined speed is the most lethal outcome available to you, and it is the one you would be choosing.
What to do instead:
- Brake firmly and progressively - hard enough to use the ABS, not so hard that you lose steering. With ABS you can brake and steer at the same time, so keep the escape route open.
- Aim for the gap behind the lorry, not through it, and look for the hard shoulder or verge on your own side as the secondary option.
- Watch the lorry's wheels as well as its lights; wheels tell you the truth about deceleration about half a second earlier.
What not to do: do not swerve left to «get a look» - that is how a following distance problem becomes a head-on. And whatever you do, do not follow the lorry into the gap it has just vacated at speed; it braked because there is something ahead, and deer do not show up on a brake light.
3. What should the following gap have been, and what specific changes should you make for the rest of this trip?
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Gap: on a wet road with these tyres, 3 seconds (67 m) is the minimum and 4 seconds (89 m) is what your actual stopping distance justifies. Your 1.5 seconds was about half the dry-road minimum and less than half of what the surface demanded.
Changes for the rest of the trip:
- Open the gap to 4 seconds immediately - pick a mark the lorry passes and count to four.
- Drop your speed to 70 km/h. At 70, the braking distance falls to about 48 m and the total to around 68 m; a small reduction in speed buys a large reduction in energy.
- Take a break. Four hours at the wheel with a reaction time already degraded is a fatigue decision, not a skill decision. Twenty minutes at the next rest area is cheaper than any of the alternatives.
- Treat the deer warning as active. Scan the verge and the far tree line, look for eyeshine, and expect a second animal behind the first.
- Deal with the tyres. 3 mm on the rear axle is where aquaplaning starts and where a loss of grip gives you oversteer you cannot catch. Rear tyres should be at least as good as the fronts; plan to replace them before winter.
- Lights and glass: dipped beam, clean windscreen, and keep the inside of the glass clear - glare from oncoming traffic on a wet screen hides exactly the things you need to see.