Long-distance driving when the weather turns

Snow gets the attention and fog causes more disruption to a long journey. It forms in valleys and river plains, most reliably in the Thames Valley, the Vale of York and low ground in East Anglia, and it does so on clear still nights, which is to say on exactly the nights that look best in a forecast.

The practical effect is a journey that runs to time for two hours and then loses forty minutes in ten miles. There is no route around it and no way to make it up, so the plan for a foggy morning is simply an earlier departure. Airports are affected too, which is why the December diversion risk and the fog risk are the same risk.

• Fog forms on clear still nights, which look reassuring in a forecast
• Thames Valley, Vale of York and low-lying East Anglia are the usual places
• It costs forty minutes in ten miles and cannot be recovered
• The only defence is leaving earlier

More journeys are affected by heavy rain than by snow in an average winter, and the danger is not the rain but what collects in the nearside lane on older stretches of motorway. Aquaplaning at seventy is a genuine risk and the response is unglamorous: slower, further back, and out of the lane that holds water.

It also lengthens everything. Spray from lorries reduces visibility to the point where overtaking is unpleasant, and a wet M6 in the dark at five in the afternoon is a slower road than the same stretch in daylight. Add twenty per cent to a journey time in persistent heavy rain and the estimate will be about right.

Early, and from live information rather than a plan made the night before. On a bad morning the chauffeur checks the network before leaving base, not at the first junction, and the choice between the M6 and the A1 for a northern run is made at the start rather than at Birmingham.

The other decision is when to stop. On a day where conditions are deteriorating, breaking the journey at Carlisle and continuing in the morning is a decision that has to be made while options remain. Left too late it becomes a decision made in a queue on a closed motorway, which is not a decision at all.

When the destination is in hill country and there is a snow warning covering it. Getting there may be possible; getting out the following morning may not, and stranding a passenger is worse than moving a date. We would rather have that conversation two days out.

When a journey is discretionary and the forecast is bad across the whole window. A touring day in the Lakes in low cloud and driving rain has very little to show for eight hours, and a leisure journey is precisely the kind that can move.

And when the timing has no slack at all. A wedding at two with a four-hour drive in front of it, on a morning with a weather warning, is a plan with no margin in it. The right answer there is usually to travel the day before and stay locally, which costs a night and removes the entire risk.