Abstract: In the event that a taxicab driver is threatened with robbery, he depresses a silent floor switch in his taxicab which initiates two different alarm or alerting mechanisms, neither of which are visible to the robber who is normally sitting in the back seat of the taxicab. First, a special warning light appears on the roof of the cab; and second, a coded signal is sent back to the dispatcher's office to both identify the serial number of the cab in which the robbery is threatened, and also to provide a continuous coded signal which may be employed to direct police or private detectives to the location of the robbery.