Driving with a cold or flu could put motorists and other road users at risk, according to research.
A study by young driver champion Marmalade in association with Halfords estimated that a driver's ability dropped by more than 50%, equivalent to a motorist drinking four double whiskies*.
Reaction times dropped sharply and sudden braking became much more frequent as the motorist was less aware of surrounding traffic. Cornering also became erratic.
A dramatic increase in poor driving was found when victims of a cold were subjected to scientific tests. A participant who had an 'excellent' driving rating of 95% when healthy dropped to 60% when suffering from a cold.
The study provided a warning for motorists not to drive with heavy colds or flu. While there are no official figures for accidents caused by sneezing and cold and flu symptoms, the insurance industry suspects motorists are responsible for thousands of prangs when they are under the weather.
The findings back up work carried out by Cardiff University Common Cold Unit** which showed that those suffering from colds and flu suffered from poor reaction times and alertness, putting them at risk of being involved in an accident. They were a third more likely to hit the roadside kerb.
Halfords Winter Driving Expert Mark Dolphin said: "We want our customers to stay safe. You shouldn't drive if you are not feeling well. The best place to be when you have flu or a heavy cold is at home, but if you really must go out, get someone else to take you and avoid driving. Other drivers should be aware of those around them and if they see someone sneezing be prepared for the unexpected to happen and increase the distance between vehicles."
Crispin Moger, Managing Director of Marmalade said "Drivers need to be cautious if taking medication as the side affects can cause drowsiness, and some warn you not to drive as this can hamper your driving ability. Use the common sense approach, are you fit to drive? Stay safe at home, if it's vital you go out get someone to drive you."
The law states that the use of ANY drug by a driver that affects their ability to drive is an offence of driving a vehicle on a road whilst unfit through drugs contrary to section 4(1) of the Road Traffic Act 1988 and Schedule 2 to the Road Traffic Offenders Act 1988.
* Insurance studies say that drinking a double measure of spirits decreases reaction time by 11% and would add 1m (3.3ft) to stopping distance if travelling at 30mph (48km/h) - on top of a normal distance of 12m (40ft).
**Cardiff University " Effects of Common Cold on Simulated Driving" - Prof Alan Smith.