A roadside system that detects if a mobile phone is in use in a passing vehicle is being piloted in the UK. It is hoped it will act as a deterrent to people making calls behind the wheel. The technology picks up the phone signal in the vehicle and activates a roadside warning sign of a mobile with a red line through it. Using a directional antenna, the detector picks up radio waves emitted from a phone handset, and measures the signal strength and length of activation. When a signal is detected of a duration and signal strength sufficient to activate the system, the detector triggers the sign.