Techniques for displaying messages on top of a vehicle are disclosed. Although useful in many other areas, the techniques are particularly suitable for taxicab. According to one aspect of these techniques, a display unit is mounted on top of a vehicle. The display unit includes at least two display panels facing outward respectively. The display unit is controlled by a computing device to display various messages. In one embodiment, the vehicle is provided a Global Positioning System (GPS) that determines a location of the vehicle. Coupled to the GPS, the computing device sends the location information of the vehicle to a base station and receives returned information from the base station in accordance with the location information. The returned information, at least some of which are location based, is locally processed for suitable display on the display panels.