BBC News:

The Thames Valley and Hampshire forces are rolling out the technology to show when motorists are using their phones.

But…

A sign will flash at the driver telling them to stop using their mobile – but the detectors cannot tell if it is a driver or passenger using the phone.

I totally get the value in stopping behind the wheel phone use. But not being able to distinguish between a driver and a passenger using their phones will create a lot of false positives which will keep this solution from accomplishing its goal.