8% descent in freezing fog with a loaded C1 lorry
You drive a loaded 7.5 t C1 lorry down a mountain road with a 3 km descent at an average 8% gradient. The air temperature is -3 °C, freezing fog limits visibility to about 60 m, and the road is wet with ice patches in shaded sections. You are carrying 4.0 t. The lorry has a working engine brake, an exhaust brake, ABS and winter tyres with 5 mm tread. At the top, a sign requires snow chains; you carry chains but have not fitted them, and there is a lay-by 100 m before the sign. The service brakes are in good condition. You must decide whether to continue, what gear and speed to use, and how to brake on the descent without overheating the brakes.
1. Should you continue without fitting the chains?
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No. The sign requires snow chains, so the legal and safe option is to fit them in the lay-by before the sign. Five millimetre winter tyres are useful but do not satisfy a chain requirement and will not guarantee grip on ice under a loaded lorry. If you cannot fit the chains safely, or they do not fit the vehicle, do not proceed; turn back or choose another route. Continuing without chains would breach the sign and, more importantly, risk losing control on an 8% descent where stopping distances are already greatly increased.
2. What gear and braking technique should you use on the descent?
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Select a low gear before the descent begins, not after the lorry has already gained speed. Use the engine brake and exhaust brake as the primary retardation and keep the speed low, perhaps 20–30 km/h given the fog and ice. Apply the service brake intermittently and firmly enough to slow the lorry, then release it, rather than riding the pedal continuously. Never coast in neutral and do not use cruise control. If you smell hot brakes or the pedal becomes long, stop in a safe place and let the brakes cool; do not continue with fading brakes.
3. How should the fog and ice affect your speed and following distance?
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You must be able to stop within the distance you can see, which is only about 60 m. On ice that may mean crawling speed, well below the posted limit. Use low beam or fog lights as appropriate, keep a much larger gap than normal, and avoid sudden steering or throttle inputs. Do not use hazard warning lights as a substitute for appropriate speed while moving. If the fog thickens so that you cannot see a safe stopping distance, stop in a safe lay-by rather than pressing on.