Motorists who insist on driving when they are struggling with a heavy cold or flu could be causing thousands of accidents a year.
Safety experts have found a dramatic increase in poor driving when cold sufferers were subjected to scientific tests.
Reaction times dropped sharply, sudden braking became much more frequent and cornering became erratic as the motorist was less aware of surrounding traffic.
Experts put the drop in driving ability as being equivalent to downing over four double whiskies.
Insurance company Young Marmalade, which carried out the research jointly with Halfords, said a participant who had an “excellent” driving rating of 95 per cent when healthy dropped to 60 per cent when suffering from a cold.
At this level an insurance company would expect the victim to be involved in an accident.
The experiment was carried out using a black “telematic” box, which records drivers’ speed, braking and cornering.
Young Marmalade said the smallscale trial provided a warning for motorists not to drive with heavy colds or flu.
The findings back up work done in south Wales by Cardiff University Common Cold Unit.
Its research showed cold and flu sufferers had poor reaction times and alertness, putting them at risk of being involved in an accident.
They were a third more likely to hit the kerb.
“In the case of this one particular driver this was totally out of character and we arranged for someone to take him home,” said Nigel Lacy, of Young Marmalade, speaking of the small-scale trial.
“We would advise a commonsense approach. A heavy cold can impair a driver’s mood, concentration and judgment, if you don’t feel well don’t drive.”