Overtaking a slow truck on a wet highway
A driver in a rigid truck limited to 100 km/h is travelling on a two-lane rural highway (one lane each direction) at 90 km/h in steady rain. Ahead is a semi-trailer doing 80 km/h. The road has a broken white centre line, a 3.5 m lane width, and a clear straight section of about 500 m before a gentle curve. The truck's tyres are in good condition. Visibility is reduced by spray, and the road surface is wet with some standing water near the edges. The driver wants to overtake before the curve. There is no oncoming traffic visible at the moment, but the curve limits how far ahead can be seen.
1. What factors should the driver weigh before deciding to overtake, and what is the safest decision?
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The driver should weigh:
- Available sight distance. About 500 m of straight is visible, but the curve cuts off the view beyond it. Overtaking must be completed with a clear margin before sight distance runs out. Rain and spray shorten how far the driver can actually see, so the usable distance is less than 500 m.
- Speed differential. Overtaking a vehicle doing 80 km/h while travelling at 90 km/h gives only a 10 km/h advantage. At that differential, passing takes a long time and a long distance - far more than a 10 km/h closing speed suggests at first glance.
- Road surface and weather. Wet road, standing water and reduced grip increase stopping distance and the risk of aquaplaning or loss of control during a lane change.
- Spray and visibility. Spray from the semi-trailer can hide oncoming traffic and obscure the driver's own view.
- Legal markings. A broken white line permits overtaking if it is safe, but the line does not override the safety judgement.
Safest decision: do not overtake here. With only a 10 km/h speed advantage, wet roads, spray and a curve limiting sight distance, the margin is too thin. The driver should hold back, increase following distance, and wait for a straight section with full clear visibility, no oncoming traffic and a dry or well-drained surface - or simply stay behind and accept the slower trip.
2. If the driver does decide to overtake, what minimum safe practices apply during the manoeuvre?
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If the driver proceeds, the manoeuvre must be done with these safeguards:
- Confirm it is permitted and safe: broken white line, no overtaking prohibition signs, and a clear view of oncoming traffic for the whole pass.
- Check mirrors and blind spots, and signal clearly before moving out.
- Build speed before pulling out so the pass is completed quickly, not by creeping past at 10 km/h.
- Leave a safe lateral gap - at least 1 m from the overtaken vehicle at low speed, more at highway speed.
- Do not return to the lane until the overtaken vehicle is visible in the interior mirror (or both mirrors), so there is safe clearance.
- Abort if conditions change: oncoming traffic appears, spray worsens, or the curve approaches before the pass is complete - slow and drop back rather than force the pass.
- Watch for the overtaken driver speeding up, which is common and can strand the overtaking vehicle in the oncoming lane.
In wet conditions the driver should also reduce speed generally and allow extra following distance, because stopping distances are longer.