Dense fog is the one condition that can make a road you know by heart feel unfamiliar. Streetlights smudge into halos, hedges disappear, and the car ahead becomes a pair of soft red glows that seem to arrive later each time you look. Most of what goes wrong in fog isn't caused by the weather itself. It's caused by a handful of habits drivers carry into it — habits that are harmless in clear air and quietly dangerous in a grey one. Here's what to fix, and how.

Using main beam to see further

The instinct to flood the road with light is understandable, and completely counterproductive. Main beam throws light into the water droplets hanging in the air, and that light bounces straight back at you. The result is a bright wall a few metres past the bonnet, with the actual road hidden behind it. Dipped headlights aim down and to the left, keeping the beam under the worst of the fog so it lights the tarmac instead of the air.

Dirt makes this worse. A grimy headlight scatters light in every direction, including back towards your eyes, so give the lenses a wipe before a foggy drive. Keep the beam dipped, add front fog lights only when visibility genuinely demands it, and set your speed by what you can see rather than what you remember the road doing.

Tailgating the car in front

It happens for a simple reason: in thick fog, the car ahead is the only thing with any definition, so it becomes a target to follow. Call it the tail-light trap. You're no longer driving to the road — you're driving to someone else's judgement, and if that driver brakes hard, changes lane late or stops for something you can't see yet, you've left yourself nowhere to go.

Leave a much bigger gap than you would on a dry, clear day. Measure it in seconds rather than metres, and in fog make it several times the two seconds you'd normally aim for. Brake early and gently so the driver behind has time to react, rather than leaving it until the last moment.

At junctions, drop the window a little and listen. Sound carries further than sight in fog, and it's often the cyclist or pedestrian you never see that you hear first.

Starting the journey with a misted windscreen

Fog outside plus fog inside is a recipe for driving by guesswork. Damp coats, wet shoes and warm breath on cold glass will mist a windscreen within a minute or two, and squinting through a small clear patch isn't driving — it's hoping. You need a full view of the road, and that means sorting the glass before you pull away.

• Switch the demister on before you move off. Full heat clears the screen fastest; turn it down once it's clear.
• Run the air conditioning, even in winter. It dries the air and keeps the screen clear far longer than heat alone.
• Turn recirculation off. Left on, it recycles your own breath around the cabin.
• Clean the inside of the glass properly. A greasy film you'd never notice in daylight becomes a glowing white haze in fog.
• Clear the side windows and mirrors as well as the windscreen. Junctions are where you'll need them.
• Don't wipe the glass with your hand or a sleeve. It smears, and smeared glass mists up faster.

If it starts to mist again mid-journey, pull over somewhere safe before you start fiddling with the controls.

Getting fog lights wrong in both directions

Some drivers never switch them on, which makes them hard to pick out in a grey murk. Others switch them on and forget about them for a week, which is worse: a rear fog light is bright enough to dazzle, and it can be mistaken for a brake light, so the driver behind misreads what you're doing.

The general guidance is straightforward. Use front and rear fog lights when visibility is seriously reduced — broadly, when you can't see more than about 100 metres — and switch them off as soon as it improves. Check the bulbs now and then, too. A fog light that doesn't work gives you nothing at all.

Treating fog as though it were even

Fog comes in patches. You get a clear stretch, then a wall, then a stretch again. Drivers tend to carry the speed that felt comfortable in the clear bit straight into the wall, arriving far faster than they can stop.

Set your speed by your current visibility, not by what the road could handle in the dry. If you can see fifty metres, you need to be able to stop within fifty metres, and that's slower than most people guess. Use the left-hand edge of the road and the markings as your guide rather than the tail lights ahead.

Where you stop matters too. If the fog becomes too thick to be safe, leave the road at the next lay-by, service area or car park — don't pull onto the verge, and don't stop on the carriageway with your lights off. If you break down on a motorway, put your hazard lights on, get everyone out on the left-hand side and wait behind the barrier, away from the traffic.

Before you set off on a foggy day

1. Check

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