MANY years ago a friend of mine momentarily fell asleep while driving down the M4, while I sat gob-smacked in the passenger seat.

We were returning from Wales, exhausted after two days of canoeing and two disturbed nights on a noisy campsite. And even though we had the sense to take regular coffee stops, the monotony of the motorway (and my conversation, probably!) eventually caused my pal to nod off. We had drifted into the hard shoulder before I managed to lift my chin from the floor to shout: "WAKE UP!"

Although my friend only lost consciousness for a few seconds, it is terrifying to think what could have happened if he had drifted the other way, or if he had been alone.

Government research shows that driver fatigue causes up to 20 per cent of accidents on motorways and other boring roads. If you feel tired at the wheel you should open your window, stop and have a nap of at least 20 minutes, then have a high-energy drink or strong coffee. If you are a passenger of a tired driver, keep them talking until they can stop for a rest. Don't fall asleep yourself or you might end up dead!

That incident was enough to persuade me that it's always a good idea to stay alert when you are a passenger. You never know when you might be able to point out a danger. However, you should not assist to the extent that you become an obtrusive, back seat driver.

I once drove to London with a melodramatic, elderly neighbour whose behaviour I found very distracting. She was a nervous passenger who didn't get out much but wanted to visit a particular Russian Orthodox church "before I die, darling".

Her eyes were darting in every direction as we made our way through the traffic and every so often she would flinch, grip the door handle and spin her head around, although I was driving perfectly safely the whole time. By the end of the day, her jumpiness was starting to grate on me, as was her insistence on pointing out every hazard.

"Red light, darling watch the busooo, there's a cyclist..."

I would have been glad if my passenger had fallen asleep I was close to stopping off at a chemist to buy some sleeping pills! It is ironic that passengers are often guilty of distracting drivers. Don't irritate, argue with, panic or distract them.Be supportive, calm and discreet. It is in your interests to pay attention too, just in case a situation arises when something you say really could prove helpful, such as "WAKE UP!"

Audrey Wixon is director of Wycombe Driving School www.wycombedrivingschool.co.uk