Apparatus, methods, and computer programs for displaying information on mobile signs

A system of advertising information on one or more mobile vehicles as a function of the vehicle's location. The system comprises mobile units and a central system for controlling the displays on the mobile units. The mobile units include displays visible from the outside of the vehicle, a controller, and a wireless communication system for repeatedly transmitting a locator signal to and receiving display-selection messages from the central system. The central system includes a wireless communication system for receiving locator signals from the mobile units, a memory and a processor. The central system determines a zone in which the mobile unit is located, selects a display to be shown by the mobile unit based on the zone that the mobile unit is located, and transmits a display-selection message to the mobile unit identifying a selected display message to be shown on the mobile unit's display.