Do you know what the legal limit is when it comes to drinking and driving?

A quick straw poll of your friends in the pub might come up with a variety of answers: two pints of beer for a man, perhaps two glasses of wine for a woman, maybe two shots of spirits for either sex.

The real answer, according to the experts, is that there is no hard and fast rule to decide when enough is enough. And road safety campaigners and alcohol agencies say there is only one safe level of alcohol for motorists - absolutely none at all.

Yesterday, proposals for a new lower drink-drive limit for young adults were ruled out by ministers.

The Government's official drug policy advisers, the Advisory Council on the Misuse of Drugs, said the legal limit for drivers under 25 should be slashed by more than a third, meaning that some drivers would be over the limit after drinking just one pint.

Transport minister Stephen Ladyman said: "The Government has no plans to change the drink-drive blood alcohol limit from 80mg for young drivers or anyone else.

"We will continue to work with the police and others to drive home the message that drinking and driving kills and continue to believe that education, robust enforcement and tough penalties are the most effective ways of persuading people not to drink and drive."

Perhaps it's the fact that the letter of the law is so scientific that confuses people. The actual law says that the police can require a breath test from anyone who is driving, attempting to drive, or in charge of a motor vehicle on the road or in a public place (for example a car park or garage forecourt).

The prescribed legal limit for alcohol is 35 micrograms of alcohol per 100 millilitres of breath, or 80 milligrams of alcohol per 100 millilitres of blood.

But as you never hear the one about the man who went into a bar and ordered 35 micrograms of lager, what is all this in real terms? A couple of pints? A glass of wine? Campaigners say it's dangerous to think in those terms.

The Campaign against Drinking and Driving, which is based in Brighouse, says: "The legal limit for driving is 80mg of alcohol per 100ml of blood. There is no fail-safe guide to how much you can drink to stay under this limit. The amount and type of alcholic drink, your height, weight and metabolism will all play their part.

"It's a myth that you can calculate the amount of alcohol you are drinking in this way. In fact, the alcohol content of beers, wines and spirits varies so greatly that to rely on guesswork like this is risking your licence and your life."

And Yvonne Oliver, chief executive and founder of Ripple Services in Bradford, which tackles drug and alcohol abuse, said: "I would welcome a reduction in the drink drive limit but I don't think people should drink at all if they are driving.

"It's too difficult to quantify; some people can drink three pints and drive safely and others can have just half a lager and not be fit to drive, especially if they have also taken something else such as cannabis.

"You can have a skinful the night before and go out the next day for one lunchtime drink which can push you over the limit again, yet because you've only had the one drink you might think you're okay to drive.

"There is so much debate about this, it would be easier to have a blanket ban on drinking if you're driving. I know that would upset those who feel they can have a drink and drive safely but you have to think of the protection of those at the other end of the scale.

"You need all your wits about you on the roads these days, there are more cars than ever before. When the current drink drive limits were first introduced there weren't so many cars and the roads weren't as dangerous. These days you have to be so observant and have as few distractions as possible."

Yvonne, who is a recovered alcoholic, said drink driving was a particular problem among younger drivers because they were more likely to binge drink.

"I hate to use the term but they're not seasoned drinkers and they may feel they're fit to drive when they're not," she said.

If the numbers game in determining legal limits of alcohol is confusing, perhaps these statistics from the Department for Transport are easier to understand: 3,000 people killed or seriously injured every year in the UK in drink-drive related collisions; more than half a million breath tests carried out by police every year, 100,000 of which are found to be positive; a maximum penalty of six months in prison, a £5,000 fine and a 12-month driving ban for drink-driving, while causing death by careless driving when under the influence of drink and drugs can result in a prison sentence of up to 14 years.

Every year the Government unveils a series of TV, printed media and poster campaigns to drive home the message that drinking and driving don't mix.

CADD, on their website, reduce the message to a simple three-icon formula: a picture of a pint of lager, plus a photo of a steering wheel, equals an ambulance.

The rules about drink-driving might seem vague and confusing but, as CADD points out with this simple equation, the end results are all too often the same. It's not, as they say, rocket science.

The CADD website is at www.cadd.org.uk, while the Ripple Service can be contacted on 01274 693900