Crosswind, a lorry and a swaying box trailer on the M1
You are driving a 1,520 kg estate car towing a 1,300 kg maximum-mass box trailer on the M1 towards Győr; the combination's permitted maximum mass is 3,400 kg and you hold a category BE licence. The trailer carries 250 kg of camping gear and its nose weight is at the permitted 75 kg. It is 14:20 on a dry but very windy Tuesday: gusts of 70 km/h are forecast from your left, and the next 5 km is flat, open carriageway with no windbreak. You are in the right-hand lane at 80 km/h, about 25 m behind a 40-tonne articulated lorry doing a steady 74 km/h. Its spray and the crosswind make the trailer feel light and vague through the steering. The left-hand lane behind you is clear for about 400 m, your exit is 3 km ahead, and you have driven this combination for two years.
1. What is the fastest you may legally drive this combination on this stretch, and which lane may you use? What would change if the section had three lanes in your direction?
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80 km/h, and normally the right-hand lane. The cap for a vehicle towing a trailer on a motorway is 80 km/h, and it comes from the category of the vehicle, not from a sign - a 130 km/h sign does not raise it, and a clear, dry road does not raise it either. The left-hand lane is an overtaking lane: you may enter it to pass, and you must return to the right when the manoeuvre is finished. On a section with three or more lanes in your direction, a vehicle towing a trailer may not use the leftmost lane at all, so a slow lorry would have to be passed using the middle lane, never the inner one.
At 80 km/h you cover about 22 m every second, so 25 m is barely one second of gap - far too little behind a 40-tonne lorry whose driver cannot see you, and far too little to react if it brakes for a gust or debris. The overtake is legal, but it is not advisable here: you would spend a long time alongside a lorry in a 70 km/h crosswind, at your maximum speed, with a light trailer.
2. As you draw level with the lorry's cab the trailer begins a rhythmic side-to-side sway. Describe your actions in order and explain why the instinctive reactions make it worse.
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Order of actions: hold the steering wheel firmly and keep it straight; ease off the accelerator smoothly and let the speed bleed away; do not brake sharply and do not steer against the swing; once the sway has died down, drop back behind the lorry, leave the motorway at the next opportunity and stop somewhere safe to check the load, the nose weight and the tyre pressures.
Why the instincts fail. Hard braking throws weight onto the front axle, unloads the trailer's wheels and can lock them, so the trailer's tail overtakes its nose - the classic jackknife. Accelerating simply puts more energy into the oscillation and pushes you past the speed at which the combination is stable. Counter-steering out of phase with the sway feeds the next swing and can build a resonance the trailer cannot damp.
Why it happened. A tall box trailer with a large flat side is a sail in a 70 km/h gust; the 250 kg load was probably not centred over the axle, which lightens the nose and makes the trailer unstable; and you were driving at the combination's absolute limit rather than a speed suited to the conditions. The legal maximum is not a target.
3. You abandon the overtake and stay behind the lorry. What following distance should you keep, and why is a trailer combination's safe gap longer than a solo car's?
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At least 40 m, and in this weather 70–90 m. The working rule on a motorway is half the speedometer reading in metres, so 80 km/h means 40 m as an absolute floor in good conditions. Add rain, spray and gusts and you should be thinking in seconds rather than metres: three to four seconds is 65–90 m at this speed.
Why the combination needs more room.
- The combination weighs 3.4 t and carries far more momentum than a solo car.
- The trailer's brakes are overrun type: they are applied by the car pushing back on the drawbar, so they bite a fraction of a second after the car has begun to slow. Add that delay to your reaction time.
- The lorry's spray destroys your view of the road surface and of anything lying on it, so you cannot see the hazard before you are in it.
- You need a long, clear view to plan a pass of a vehicle that is only 6 km/h slower than you, and while sitting close you are invisible in the lorry's mirrors.
Dropping back costs you nothing and gives you both a view and an escape route.