A development is the use of the RDS system of car radios, whereby the vehicle can be fitted with a short range FM transmitter, set to RDS code 31, which interrupts the radio of all cars within range, in the manner of a traffic broadcast, but in such a way that the user of the receiving radio is unable to opt out of the message (as with traffic broadcasts). This feature is built into all RDS radios for use in national emergency broadcast systems, but short range units on emergency vehicles can prove an effective means of alerting traffic to their presence, although is not able to alert pedestrians and non-RDS radio users.