The paper forms a continuation of earlier work which established the need and feasibility of introducing a dedicated traffic information service, using a single medium frequency in time division multiplex, to serve the United Kingdom. A study of the effectiveness of a muting system in the receiver relying entirely on signal strength for restricting the coverage of each transmitter to local areas, revealed that variations of signal with vehicle movement gave spasmodic mute operation. This deficiency would have been particularly prevalent in fringe areas, causing only parts of messages to be heard, leading to possible confusion and ambiguity when interpreting the information. A field trial was therefore set up to pursue the development of a system still based on the original concept, but which would overcome the problem of message fragmentation when motoring in an urban environment or leaving a service area. A description is given of the various listening modes offered by a prototype receiver, designed to respond to tone codes in transmission. An analysis of results, based on the probability of communicating with a moving vehicle, shows that the problem of message interruption due to local field-strength variations has been overcome.This work represents an improvement stage in the development of the system and, although alternative muting arrangements have been developed since the work described was completed, the use of ‘open’ and ‘close’ codes has been retained as a valuable part of the system