Handling a phone while driving is asking for trouble, but plenty of people still do it every day.

A recent poll revealed that 26 percent of drivers in the U.K. use their phone behind the wheel, whether to make calls, check social media, or send messages.

In a bid to combat drivers’ phone use in the car, the county of Norfolk, near London, is piloting a new roadside system that flashes a warning to a driver when it detects they’re making a call.

It works via a scanner capable of detecting radio signals emitted by a mobile phone when it’s on a call. When it does so, an electronic sign further along the highway flashes a warning to the driver, telling them to stop using their handset.

According to Westcotec, the company behind the system, the scanner is able to differentiate between mobile phone radio signals and Bluetooth signals. As Bluetooth-connected calls are legal in the U.K., the sign won’t show a warning in such cases.

But in its current form, the technology has a number of limitations. First, it can’t tell whether it’s the driver or a passenger who’s using the phone, so on some occasions the sign will be displaying the warning unnecessarily.

Second, it won’t work if a driver is using internet services on their handset, arguably a far more dangerous practice than making a call as such use causes drivers to direct their gaze away from the road.

But Westcotec’s Chris Spinks hopes the system will nevertheless serve as a useful reminder to drivers about their phone use, and help to gradually change behavior over time.

“So many people, by force of habit, can’t resist using their phone,” Spinks told the BBC. “The system … goes some way toward remotely warning drivers that they can be detected using their phone.”

There are no current plans to use the system to prosecute drivers. Rather, the idea is to simply issue a warning to remind them to focus on the road rather than on their handset. Four of the signs are currently operational in Norfolk, and there are plans to install more next month.

In the U.K., drivers caught using a handset behind the wheel can be hit with a fixed penalty of 200 British pounds ($265), an amount that was raised from 100 pounds last year. The case also has the potential to end up in court where the fine could reach up to 1,000 pounds, together with a temporary driving ban.

Drivers of buses or goods vehicles who are caught using their handset behind the wheel can be find as much as 2,500 pounds.

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