Those of you outside London probably won't know, but London operates two separate taxi services. The familiar Black Cabs are easily spotted and can be hailed in the street. However, due to their high prices, the fact that you can't really phone one (you can, but they never come) and that they're difficult to find late at night, a parallel system of "minicabs" has grown up. These are just ordinary cars and you negotiate the fare with the driver before the journey starts. Inititally they were completely unregulated, with attendant problems, but they're now regulated by Transport for London.
TFL are running a campaign claiming that unlicensed minicab drivers commit ten rapes a month. I'm not going to challenge this statistic, but if true I don't see why this problem can't be tackled by some a very easy strategy.
Illegal minicabs are easily found. Stand outside a pub or club late on a Friday or Saturday evening and you'll be approached by loads of unlicensed cab drivers. If the police were to do this and arrest the illegal drivers for being unlicensed, they can take a swab of their DNA (we won't go into the privacy implications of this, as it's already the way things work). The swabs can then be run against the police DNA database to see if there are any matches for any of these "ten rapes a month" TFL are claiming.
So why aren't they doing this?