Wet rural road, 25 metres behind a braking car
At 17:40 on a rural road near Ringkøbing the speed limit is 80 km/h. Rain has been falling for an hour and the asphalt is wet, with standing water in the wheel tracks. A rider on an A2 motorcycle with ABS follows a car at about 25 m. The rider has a reaction time of 1.0 s and is tired after a nine-hour workday. Ahead, a tractor with a trailer begins to turn left into a field, and the car brakes hard. The rider's visor is partly fogged, and the front tyre is a touring model with 2.8 mm tread. The rider sees the brake lights late. The road is straight, there is no oncoming traffic, and there is a wet grass verge on the right. The rider's speed is 80 km/h.
1. Using the figures given, calculate the reaction distance and the wet braking distance at 80 km/h, and decide whether 25 m is an adequate gap.
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First convert speed: 80 km/h ÷ 3.6 = 22.2 m/s. Reaction distance at 1.0 s is 22.2 m. For wet asphalt, a realistic maximum deceleration for a motorcycle is about 0.5 g, or 4.9 m/s². Braking distance is v² / (2a): 22.2² / (2 × 4.9) = 493 / 9.8 ≈ 50 m. Total stopping distance is therefore about 72 m, before allowing for the rider's late brake-light detection and fogged visor. A 25 m gap is only about 1.1 seconds at 80 km/h. By the time the rider has reacted, the motorcycle has already covered 22 m, leaving only 3 m for braking. Even with perfect ABS operation, the impact speed would be close to the original speed. The conclusion is not that the rider should brake harder; it is that the gap was too small before the hazard appeared. ABS prevents wheel lock but does not shorten the physical stopping distance on a wet road.
2. What following distance should the rider use in these conditions, and how should it be maintained in practice?
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A minimum dry-road rule is two seconds, but in heavy rain and poor visibility the rider should use at least four seconds, which at 80 km/h is 4 × 22.2 = about 89 m. Three seconds, about 67 m, is an absolute minimum for a alert rider on a wet road, and it is still less than the calculated 72 m stopping distance. The practical method is to pick a fixed object beside the road, such as a sign or a tree, and count 'one thousand and one, one thousand and two' when the car passes it. If the rider reaches the object before the count ends, the gap is too short. The rider should also look through and past the car, not only at its rear lights, and should identify escape routes such as the verge before they are needed. In rain, the rider should reduce speed as well as increase the gap, because stopping distance rises with the square of speed. A four-second gap is not excessive; it is the space needed to see and react.
3. If the rider still has to avoid the car, what are the main risks of swerving onto the wet grass verge, and how should the decision be made?
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Swerving gives up most of the braking capability because the tyres must use grip for cornering as well as slowing. On wet grass the available grip drops sharply, and the front wheel can tuck or slide before the rider reaches the verge. The verge may hide a ditch, a culvert, a fence post or soft ground that stops the bike abruptly. ABS will not save a rider who asks the front tyre for more grip than the surface can provide. The decision should be made early: if the car is already braking hard and the gap is 25 m, the rider should brake progressively with both brakes, keep the bars straight, and use the verge only if there is a clear, smooth line and no better option. Looking at the car's rear bumper increases the risk of target fixation; the rider should look at the gap or the escape path. After stopping, the rider should move off the carriageway and switch on hazard lights before checking on the car.