Why fog is dangerous
Fog is made of tiny water droplets suspended in the air close to the ground. They scatter light, which shortens how far you can see and makes distance and speed harder to judge. The real danger is not the fog itself but how drivers behave in it: many go faster than their sight distance allows, follow too closely and drive in groups, which is how multi-vehicle collisions happen.
Fog can also change quickly. A road can be clear for a mile and then enter a dense patch in a valley, near water or at dawn. The same principles apply to heavy rain, smoke, dust and snow squalls, anything that cuts your visibility.
Slow down to match what you can see
The most important rule is to drive at a speed at which you can stop within the distance you can see. If your view is only a short distance ahead, then your speed has to be low enough to stop well within that space, even if a stationary object appears out of the fog.
Other points about speed:
- Ease off gradually when you see fog ahead rather than braking hard as you enter it.
- Do not rely on the speedometer alone. In fog, steady travel can feel slower than it is, so check your gauge now and then.
- Avoid sudden braking where possible, since drivers behind may not see you in time.
- Resist the urge to speed up to get through a patch quickly.
Use lights correctly
Lights help in two ways: they help you see, and they help others see you.
- Low beam headlights. Use these in fog, even during the day. High beams reflect off the water droplets and glare back at you, which reduces visibility rather than improving it.
- Fog lights. If your car has them, they are designed to light the road close to the vehicle with a wide, low beam. Rules on when they may be used differ by location, so check local laws. Switch them off when conditions improve, because they can dazzle others.
- Automatic lights. Some cars switch on only the daytime running lights, which may not turn on the rear lights. Make sure your tail lights are on, using the manual setting if needed.
- Hazard lights. Using them while moving is restricted or discouraged in many places. They can mask turn signals, so check local rules.
Many of the same habits apply to night driving, where headlight use and speed decisions are similar. Keep your lights clean and properly aimed, and make sure the windshield is clear inside and out.
Keep your distance
In fog, your following distance matters even more. A common guideline is a larger gap than usual, in time as well as space. Do not follow the tail lights of the vehicle ahead too closely just to have something to follow; you will have less time to react if it stops suddenly.
Equally, avoid stopping on the road if you can avoid it. A stopped vehicle in fog, even one with lights on, can be hard to see.
Practical techniques
- Open the window slightly, if safe, and listen. In fog, you may hear traffic before you see it, especially at junctions.
- Use road markings, such as the edge line, to keep your position, but do not stare at them so long that you drift.
- Be extra cautious at junctions, crossings and roundabouts. Creep forward carefully, since vehicles and pedestrians can appear suddenly.
- Do not overtake unless you are certain it is safe. In poor visibility this is rarely the case.
- Keep the defroster or air conditioning on to prevent the inside of the glass from misting. If this becomes a problem, defrosting windows is a topic of its own with steps that help.
- Wipers and washer fluid help with the film of moisture and road spray that builds up.
Stay calm and avoid distraction. Anything that takes your attention away from the road is a bigger risk when your margin for error is smaller, and distracted driving needs more attention than usual in these conditions.
If you have to stop
If the fog becomes so dense that you cannot continue safely, find a safe place to leave the road, such as a parking area or a side road. Do not stop on a traffic lane or the shoulder of a high-speed road unless there is no other option.
If you must stop on or near the road:
- Pull as far off the roadway as you safely can.
- Switch off the headlights and tail lights if you are well off the road, because other drivers may follow your lights and drive into you. Keep the parking brake on and your foot off the brake pedal so the brake lights are not lit.
- Turn on the hazard lights if local guidance recommends it for your situation.
- Get out and move away from the vehicle, to a safe spot away from traffic, if you can do so safely.
This is general advice and recommendations vary by place, so check the guidance from your local authority. If you break down, a highway breakdown in fog is especially risky, so call for help and stay clear of traffic.
Before you set off
Check weather and traffic reports if fog is forecast. If the trip is not essential, delaying it for an hour or two is often enough for the fog to lift. If you must go, allow extra time so you do not feel pressure to hurry, and make sure the phone is charged and the fuel tank is not low.