Four vodkas and I'm a lethal weapon

After my second double vodka and orange in 15 minutes, I'm beginning to feel it. For some reason my tongue doesn't want to do quite what I ask it to: and I have to concentrate hard to make my speech come out clearly.

Still, I think, I'm sure no-one has noticed - and I bet I'm nowhere near the limit. No problem.

I stand up, a little bit more carefully than usual, and walk extra-steadily out to the car. For some reason, as I belt myself in, everything seems a little bit larger than usual - and when I turn my head suddenly it takes a split second for my vision to catch up. But I feel a surge of optimism. My head's perfectly clear - and anyway, it's not far. I'll be fine.

In no time, I'm heading serenely down the motorway at a steady 70mph. I'm feeling pretty pleased with myself and proud of the control I have over the car. OK, so the gears were a bit jerky at first - but I'm driving carefully, and I'm making sure to check the traffic all around. I look in the rear view mirror deliberately, just to prove it.

By the time I focus on the road ahead again, the lorry in front of me is slowing down, its brake lights flashing. It's suddenly very close. I freeze for a split second, then stamp belatedly on the brakes. The car slows and veers: but I manage to keep control and the lorry pulls away again.

Phew! I think, drawing a deep breath. Better to pull out into the fast lane perhaps and make sure it doesn't happen again.

I begin to pull across, for some reason not bothering to indicate - and a car flashes past on the outside.

My God! Where did that come from? I veer into the middle lane again, car swerving wildly, and draw a second deep breath. OK, just calm down, I think: drive sensibly. I frown and hunch over the wheel, peering forward, concentrating furiously and determined not to make another mistake.

"At the next junction, please prepare to leave the motorway," says a voice out of nowhere: and I coast across to the inside lane and then, following further instructions, bring the car to a smooth stop on the hard shoulder just short of the exit lane.

When I get out, traffic constable Martin Hemenway of North Yorkshire Police is waiting for me: and he's not impressed with my driving.

Neither is Samantha Jamson, a senior research fellow at Leeds University's Institute for Transport Studies, who is studying the effects of alcohol on driving.

"Once you increased your speed you really began wobbling around," she says, whipping out a breathalyser.

"And you had a real furrow in your forehead," adds Martin. "You were obviously trying to compensate for your body's inability to cope, but you weren't scanning the traffic around you the way you should have been.

"Your eyes became very tunnel visioned, which meant you would miss lots of things on the periphery of your vision."

Like the car that almost cut me up on the outside lane, I recall, sheepishly - or even the slow lorry I hadn't noticed until it was almost too late.

Luckily for me, the whole thing - apart from the two double vodkas (equivalent to two pints) which were all too real - is a simulation, designed to show how even a couple of drinks can seriously affect your driving.

The 'car' I've been driving is real enough in most respects - except it does not go anywhere. It is connected to a computer which monitors everything from steering to clutch, brake and accelerator control - and the 'motorway' I have been driving along is a giant moving image projected onto a huge screen in front of the car's bonnet. What you see through the rear view mirror is another projection on a screen behind.

What it reveals about my driving is frightening. Although I'm still nowhere near the limit - I register a blood/alcohol level of 0.05, where the legal limit is 0.08 - my driving after knocking back the vodkas is significantly worse than before.

Samantha says I have been showing many of the classic signs of the drink-driver who is struggling to drive normally: slow reaction times, tunnel vision and an over-confidence in my fitness to drive that can kill.

I'm actually quite shocked to find that legally I could have driven - in the UK at least, although not on the Continent - in the state I was in.

If I had pulled out into the path of that car in the outside lane for real, I might not be standing here now.

Samantha explains how alcohol affects you.

"It impairs your judgement in a number of ways, and that is even before you are over the limit," she says. "You overestimate your own skill and think you're a better driver than you are - and even think you're better than you think you are when sober.

"Your field of view shrinks, and you concentrate on the road just in front rather than paying attention to the periphery. That's because you are impaired by the alcohol, so you have to work harder to do the simple things and you have less attention to share around.

"And your basic perceptual process and reaction times are affected. You didn't notice the lorry ahead had put its brakes on. Not only are you braking later, the way you brake may not be the most effective. You slam on the brakes rather than braking gently - and that's a problem not only for you, but also for the people behind."

The most frightening thing, Martin Hemenway adds, is that when you have had a couple of drinks, you are the last person in the world to recognise that you are not in a fit state to drive.

I think of my foolish confidence of a few minutes before and have to agree.

The cost of all this, Martin says, is lives - and not necessarily just your own. When you are doing a steady 30mph, he says, you travel 44 feet in a second.

If it takes you a second longer to react because you've had a couple of drinks, it can be catastrophic.

"Instead of stopping this side of a zebra crossing, you could be stopping on the other side," he says.

"And there could be somebody on that crossing. We teach our children to use them, because they're safe."

Now that is a sobering thought.

It is the last time I have one for the road, that's for sure.