I find that the old "would you rather?" game helps liven up long motorway journeys. Here we go … "Would you rather that the car ahead of you was driven by Paul, who's just knocked back the hair of the dog at the motorway services pub before merging back into your lane from the slip road?" or, "would you rather be behind Rob, who is alert, has his eyes on the road, but shared a cannabis joint earlier in the day?"

The answer I'd expect is, "I'd rather drive behind Rob. He's probably as clear-headed as I am if a few hours have passed since the joint. Half-drunk Paul might be dangerous though. And what's a motorway services pub ? No one would allow that".

The government; however, sees things differently. It is considering removing rules preventing alcohol sales on highway agency sites, freeing up the development of motorway branches of Wetherspoon and other pub chains. If Paul can stay just south of our generous legal drink-driving limit, (not that he is likely to be breathalysed as he leaves the pub), his drug use is seen as an essential part of the economic recovery, although he is at least twice as likely to cause a crash as someone who hasn't been drinking.

What about Rob? Under government proposals, if a microgram of THC (from cannabis) can be found in a pint and a bit of his blood, (enough to be sure he hasn't simply walked too slowly past a cannabis smoker), then he is guilty of driving drugged. Whether Rob is illegally in possession of cannabis, actually intoxicated, or in any way a danger, will be considered immaterial. The government will ignore the recommendations of the expert panel it commissioned and take a "zero-tolerance approach" to drivers whose bodies contain definitive traces of any one of a blacklist of eight controlled drugs including cocaine, ecstasy and LSD. I have challenged government priorities before by referring to the risks that cats pose. While LSD has never been detected in a driver involved in a crash, unrestrained cats in a motor vehicle have distracted drivers and resulted in collisions. Yet it remains legal to drive with a free-roaming feline friend on board.

Rather than a consistent "risk-based" approach to drivers using drugs, the government would prefer to use the Road Traffic Act to get tough on people who test positive for an arbitrary selection of the wrong drugs, and in doing so send a clear message to everyone about the unacceptability of drug use. Well, some drug use. It thinks the "risk-based approach" – as recommended by its scientific advisers – is fine for alcohol, sleeping pills, morphine, just not for eight other controlled drugs. This is, they say, to avoid sending "mixed messages".

Sending messages isn't really what the Road Traffic Act is supposed to be for. Moreover, there is a plethora of evidence that tough drug laws have no clear effect on levels of drug use. Disproportionate toughness will not prevent car wrecks, but lives and careers will be wrecked through criminalisation. In US states where medical cannabis has been legalised, there are fewer deaths on the road.

The Independent Scientific Committee on Drugs has urged government to reconsider and accept the risk-based approach recommended by the expert panel led by academic Kim Wolff which advised the government. If you think your MP should support a rational and fair policy towards drug driving as proposed by the experts, rather than the zero tolerance, irrational approach being pushed by the government, it is not too late to let them know.

By being tough on cannabis-smoking Rob but soft on half-drunk Paul, the government's new laws could actually promote traffic accidents by encouraging Rob to swap a joint for a pint. Drivers with alcohol in their blood cause fatal crashes at more than 10 times the rate of those with cannabis in theirs.