Night rain behind a truck: 12 metres at 85 km/h on a road signed 80
At 21:10 on a wet winter night you are on a two-lane intercity road, one lane in each direction, signed at 80 km/h. Your speedometer reads 85. You are following a tractor-trailer at about 12 m. Rain is heavy; your wipers are on fast and the left blade smears across the middle of the windscreen. The truck's spray reduces your view to roughly the back of its trailer, and oncoming cars with bright headlights wash out what is left. There is no hard shoulder, only a narrow gravel verge, and the nearest lay-by is 2 km ahead. The truck's brake lights come on and it slows much harder than you expected. In the mirror there is a car at a normal distance behind you. You have been driving for 2 hours 40 minutes and your eyes feel gritty.
1. List the hazards in this situation and rank the two that most constrain your next move.
Read the worked answer
Hazards present:
- The truck braking hard for a reason you cannot see.
- Your sight distance: spray plus a smearing blade limits usable view to a few dozen metres.
- Oncoming headlights reflecting off a wet windscreen, producing glare and momentary blindness.
- The unseen reason for the truck's braking - a cyclist, a broken-down car, an animal, debris, standing water.
- Reduced grip: wet asphalt, polished wheel tracks, possible standing water.
- No hard shoulder: no escape space, and the verge is soft gravel.
- Your own state: 2 hours 40 minutes of night driving in rain, gritty eyes - reaction time already degraded.
- The car behind, which may be following too closely and will not expect a sudden stop.
Ranking. First: the truck's brake lights, because they define the time you have. Second: your own sight distance, because it means the truck is your only source of information about the road ahead - you are effectively driving blind behind someone else's eyes. The rain, the fatigue and the oncoming glare all matter, but they matter because they shrink the margin the first two already consumed.
2. Justify a sensible following distance at 85 km/h on this road, and compare it with the 12 m you are keeping.
Read the worked answer
At 85 km/h you travel about 23.6 m every second.
- Reaction distance: allow 1 second even when alert - about 24 m. At 2 hours 40 minutes into a wet night drive, 1.5 seconds is more honest: about 35 m.
- Braking distance: on wet asphalt assume a friction coefficient near 0.5 rather than the 0.8 of dry tarmac. Using v squared over 2 x mu x g: 23.6 squared is about 557; divided by roughly 9.8 gives about 57 m.
- Total: around 80 m with a 1-second reaction, closer to 90 m with a realistic one.
12 m is about half a second of travel. It is not a following distance; it is the gap at which the truck's rear is already inside your reaction time. The dry-weather 2-second rule gives 47 m here and is still not enough, because it ignores the wet-surface penalty and the fact that you cannot see past the trailer. On this road, in this weather, 4 seconds - roughly 95 m - is the minimum, and the practical answer is to drop to 70-75 km/h so that 4 seconds becomes a distance you can actually judge.
3. The truck brakes hard. Give your sequence of actions, and state what you must not do.
Read the worked answer
Do now:
- Brake immediately and firmly - do not wait to find out why the truck is slowing. Its brake lights are the only advance warning you will get.
- Glance in the mirror as you brake; if the car behind is close, modulate slightly to give it time, but never trade a rear-end impact for a head-on one.
- Reduce speed more than the truck does, so the gap grows rather than shrinks. If the truck stops, you want to stop behind it with room to steer out.
- Have an escape in mind: the gravel verge, straight ahead of your bonnet direction, chosen before you need it.
Must not do:
- Do not pull out into the oncoming lane to overtake. The truck is very likely braking because of something in that lane or across the road, and you would be overtaking into a blind hazard at 85 km/h in rain.
- Do not switch to high beam; on a wet windscreen it comes straight back at you.
- Do not aim for the trailer's rear as a 'soft' option - a rigid trailer does not deform.
Afterwards, the lesson is arithmetic: 10 km/h less would have bought you roughly 30 m of vision and cost about 90 seconds over 2 km.