Out for lunch the other day I overheard someone at another table saying: “I’ll be OK to drive after a couple of glasses of wine, won’t I?”

And I thought: “Who knows?”

That’s the thing with drinking and driving. We have this legal limit that everyone has their own views about. “I’ll be OK after a couple of pints,” “nothing more than half a glass of wine for me thanks,” “if I spread it out over a few hours I’ll be fine” and so on. But in reality, do people genuinely know what amount of alcohol would impair their performance behind the wheel of a car?

Six years ago, car manufacturer Ford put this question to the test. Rather than using a simulator, Ford actually took a real driver, a real car and some real alcohol. To be safe, their experiment took place under strict test conditions at its European proving ground at Lommel, Belgium.

The car carried equipment measuring stopping distance and steering precision using GPS (Global Positioning System) to record any deviation from the selected route.

Twelve units of alcohol – 6 medium glasses of wine – were consumed over six hours. By the end, the driver’s breathalyser reading had reached 50 microgrammes of alcohol per 100 millilitres of breath. Britain’s legal limit to drive is 35.

As you might expect, emergency stop reaction times went from 0.48 seconds to 0.68 seconds, meaning that stopping distance at 70mph worsened by as much as 50%. On a slalom course the distance strayed from the best route through the 10 cones doubled. On his first sober run, the driver didn’t hit any cones, but on his final run he knocked down two.

It was a similar story when attempting a reverse manoeuvre. Without a drink there were no problems, but later in the test the Ford driver struck a post, despite taking around a quarter of the time longer to complete the task.

But what was even more interesting was that posts and cones were also struck in a variety of other tests after only half of the alcohol was consumed – a point at which the driver may not have exceeded the legal drink drive limit.

Asked whether he would have judged himself to be “under the limit” during these first couple of hours, the driver said “yes.” In reality his reaction times were deteriorating fast.

So there you have it. You may think you’re under the legal limit, you may actually be under the legal limit, but your ability to drive is still affected by alcohol – which at the end of the day is what really matters.

My view? Don’t drink and drive, means drink nothing if you’re driving.

About Teresa Budworth