Wet rural road, short following gap and a one-lane bridge
You are driving a small hatchback on a rural two-way road at 7:30 am in July. The speed limit is 100 km/h. The road is wet, with patches of grit and a narrow one-lane bridge about 80 metres ahead. You are following a ute and trailer at about 35 metres. The ute's trailer has a loose tarpaulin flapping. The road curves left immediately after the bridge, and the low sun is in your eyes. A logging truck is approaching from the opposite direction, about 150 metres away. You are due at work in 20 minutes and have not had breakfast.
1. What speed should you be travelling at now, and why is 100 km/h not appropriate even though it is the limit?
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The 100 km/h sign is a maximum for good conditions, not a target. Here the road is wet, there is grit, the sun is low, the bridge is narrow and a large truck is approaching. At 100 km/h you travel about 28 metres every second, and your wet-road stopping distance is much longer than on a dry road. You should reduce speed well below 100 km/h, perhaps to 60-70 km/h or less, so that you can stop within the distance you can see to be clear. Slow further before the bridge and before the left-hand curve, because the curve hides oncoming traffic and the bridge narrows your escape space. The flapping tarpaulin is also a warning that the trailer may shed material or the driver may brake suddenly. Being late for work is not a reason to accept a higher crash risk.
2. Is 35 metres an adequate following distance at this speed? Show the reasoning.
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No. The two-second rule is the minimum in good, dry conditions. At 100 km/h, two seconds is about 56 metres. In the wet, you need roughly four seconds, which is about 111 metres. Even at 50 km/h, two seconds is about 28 metres, and wet conditions would push the safe gap well beyond that. At 35 metres behind a ute and trailer at 100 km/h, you have only about 1.3 seconds of gap. That is too close even on a dry road, and it leaves no margin for the trailer's loose tarpaulin, a sudden brake, or reduced grip from grit. The correct response is to slow down and drop back to a gap that gives you time to react and stop, not to stay at the limit and hope the ute driver is predictable.
3. The logging truck reaches the one-lane bridge at about the same time as you. What should you do?
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Approach the bridge slowly and check the priority signs before you enter. If your side has a give-way sign, stop and let the truck cross. If you have the blue priority sign, you may proceed, but you should still be ready to stop because the truck is large, heavy and may already be committed to the bridge. If there are no signs, approach with caution and give way to traffic already on the bridge. Never enter the bridge just because you have priority if doing so would force the truck to stop on or before the bridge. The safest judgement is to hold back, let the truck clear, and then cross at a speed that lets you deal with the wet surface and the left-hand curve immediately after the bridge.