Forward-looking: The UK is testing a new roadside system that can identify when drivers are using their mobile phones and show them a warning.

The signs, which have initially been installed in the county of Norfolk, work by scanning for phone signals as vehicles pass. A directional antenna picks up a handset's radio waves and measures the signal strength and length of activation. If phone use is detected, a “no mobiles” image made of LEDs is lit up by the roadside.

The system can even tell if a call is being made using a hands-free kit or via Bluetooth—both legal in the UK—in which case the warning sign won’t activate.

The system doesn’t record any video or take any pictures. It’s hoped that the warnings will deter drivers from using phones—they could decide it’s not worth the risk of being caught. However, statistics from the signs will be shared with local law enforcement, and the system could record specific number plates as a “future development.”

Using a hand-held phone while driving in the UK can lead to 6 penalty points on a driver’s license and a £200 ($265) fine. If the case goes to court, an offender could be banned temporarily from driving and faces a maximum penalty of £1000 ($1326) or £2000 for drivers of buses and trucks.

There are some caveats to the system. It cannot tell the difference between a driver and a passenger using a phone. It’s also unable to detect if a driver is using online services while behind the wheel; a common practice, thanks to the popularity of WhatsApp, Facebook, Messenger, etc.

"Any scheme which prevents this kind of behavior is welcomed. Using a mobile phone at the wheel is one of the fatal four road offenses which can have devastating consequences if it causes a fatal or serious collision," Insp. Jonathan Chapman, of Norfolk Roads Policing unit, told the BBC.