The traffic reports you hear on the radio are generally derived from video cameras, which send pictures to police traffic control rooms.
The blue 'cameras' you refer to don't record any images (so they're not strictly 'cameras'). They're actually radar devices, owned and operated by Trafficmaster Ltd. They send out a radar beam, which is reflected back by passing vehicles. This indicates the speed of the traffic flow. (It's basically a 'speed camera' without the actual camera). The information is used by 'direct to car' traffic information systems, such as those incorporated into some SatNavs.
Chris