Embodiments of the present invention disclose a mobile messaging device that is battery powered with no hard wire connection to the vehicle's electrical system. Signage is backlit for visibility in both low and high ambient lighting conditions. To conserve battery power, the signage is lit after braking (deceleration) of the vehicle for a brief period of time. To further conserve battery power, the signage is backlit at different intensity levels which depend on ambient lighting conditions. Deceleration sensing is provided by a sensor fully incorporated within the mobile messaging device, which requires no external wiring to the vehicle's electrical system. The deceleration sensor is user adjustable to minimize false triggering. Messaging can be provided by user replaceable transparent placards or by backlit, user programmable liquid crystal display panels. In one configuration, the mobile messaging device is incorporated into a vehicle license tag frame.