The figures just don't add up.
We know Britain has a serious drug habit, among the worst in Europe as a matter of fact. Yet our courts rarely deal with drug driving cases.
That's good news, surely? It means our drug users stay off the road, right? Er, no. Wrong.
The truth is we don't know how many drug users risk it on our roads. The police, the home office and the department for transport have been working on the issue for a decade, but their figures are largely based on guesswork.
We do know that in Germany, a country with a far smaller drug problem than ours, police prosecute more than thirty thousand drug drivers each year, while just two thousand or so pass through British courts.
We know that a recent survey revealed one in nine young British drivers have driven whilst on drugs in the past twelve months. And we know that at least one hundred and fifty people are killed by drugged drivers on the UK's roads every year. But the true figure could be far, far higher than that.
The issue is not being ignored. The authorities have been talking about it for more than a decade. But that's the problem: Ten years of talking has achieved comparatively little and all the while lives are being lost.
At present the police have a serious problem. Finding drugs in a driver's bloodstream is not enough for a prosecution, they have to prove beyond doubt that the drugs were affecting the way the car was driven.
If you fail a breath test after a couple of glasses of wine, you'll face a ban - and rightly so. But if you're stopped after snorting cocaine and smoking a cannabis joint there's not much the police can do. They're not obliged to test you, they're unlikely to have the right equipment and even if you do fail a test, it's up to them to prove you posed a danger to other drivers. Seems crazy, doesn't it?
We're told a new law will come into effect within a year which will bring drug driving into line with drink driving. In the coming weeks police forces in the UK will be given roadside testing kits to try out.
They're the biggest weapon the authorities have in the battle to keep drugs off our roads. They work on same the premise as breathalysers, testing saliva for traces of the most common illegal drugs.
We saw them at work on a filming trip to Hamburg. The police there routinely pull drivers over. Road users don't seem to mind, they accept that it's all in the greater good.
It seems crazy that we've taken so long to introduce such a simple system. A system that could potentially save dozens of lives every year.
I spent a heartbreaking afternoon with one bereaved mother.
Janice Ward's life fell apart three years ago. Her daughter Rachael was killed in a head on collision with a driver high on drugs. Rachael was her only child. In one awful moment Janice was denied everything she was looking forward to. She won't see her daughter grow up, she'll never have grandchildren. She told me:
I don't live any more, I just exist.
The new law can't come soon enough, but for Janice, and many others like her, it's already far too late.
Tonight: Drug driving is on ITV at 7.30pm
Useful links on drug driving: