Driving in fog: all you need to know

Driving in fog

Fog can cause drivers big problems. (Picture © IAM)

Being on the road in the winter can mean driving in fog which is responsible for some of the most treacherous conditions car owners face. Hardly surprising that it’s believed a large number of crashes every year are caused by poor visibility. In 2013, 60 drivers were injured (35 of them hospitalised) when 130 vehicles were involved in a series of accidents in heavy fog on the Sheppey Crossing in Kent. Here’s all you need to know about driving in fog and the steps you can take to avoid something similar happening to you. 

Driving in fog: Speed
Following crashes in fog, police frequently criticise drivers for going too quickly. The faster you’re travelling, the disproportionately longer it takes to slow down. According to the theory part of the driving test, it takes 53m to stop from 50mph but 96m from 70mph. In fog, with reduced visibility, you might not see stationary traffic in front of you until it’s too late to stop.

Driving in fog: Lights
At the very least, turn your headlamps on and ensure they’re dipped. Main beam will reflect off the fog and cause a ‘white wall’ effect, reducing your visibility rather than improving it. The majority of modern cars have fog lights fitted as standard. However, the switches can be in obscure places or require you to pull out the main light switch rather than twist it. Use the car’s handbook to familiarise yourself with the fog lights before you need them.

Driving in fog: When to use fog lights
The Highway Code states: “You must not use front or rear fog lights unless visibility is seriously reduced. You must switch them off when visibility improves to avoid dazzling other road users.” It is an offence to use fog lights unless visibility is seriously reduced. According to the police, this is defined as driving rain, snow or fog with visibility of less than 100m. Doing so can result in a £50 non-endorsable Fixed Penalty Notice.

Driving in fog: How much distance between you and the car in front
When visibility is reduced, follow the THREE-second rule. See when the car in front passes under a bridge or past a sign post then count ‘One hundred, two hundred, three hundred’. If you pass the same marker before you’ve finished counting, you should drop further back. It’s worth remembering that fog is moisture, so as well as hanging in the air, it will have coated the road surface in a wet layer which in turn will increase stopping distances further.

Driving in fog: Don’t get confused
The lack of external reference points can make it very easy to be fooled by fog into thinking you’re travelling slower than you actually are. Watch your speed and concentrate in front and behind. If someone is tailgating, let them pass. Equally, check that the car in front has its rear fog lights on. It can be easy to mistake fog lights for brake lights.

Driving in fog: Inside the car
Fog invariably means a high humidity level which can cause almost instant steaming up inside the car. Familiarise yourself with your car’s ventilation system so that you can react quickly. One way of rapidly demisting a car’s windows is to turn the air-conditioning on. This takes the moisture out of the air and clears fog from the inside of windows swiftly.

Driving in fog: What is fog?
Fog is made up of tiny liquid water droplets. According to the Met Office, the diameter of these varies between 0.01mm and 0.1mm and they cause light to be reflected off in different directions. This is why visibility can be so tricky in fog.

Driving in fog: What sort of fog is it?
There are three types of fog: aviation fog when visibility is limited beyond 1km; thick fog when you can’t see further than 200m; and dense fog when visibility is reduced to 50m.

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