Abstract: A vehicle alarm for preventing injury to persons near the vehicle. Persons under and around a vehicle such as a school bus are detected by establishing an energy field and monitoring for disturbances therein. At least one transmitter establishes the energy field in areas of high risk. One or more receivers detect a signal resulting from the energy field. These signals are monitored by a processor unit for amplitude variations therein. The processor unit preferably stores the amplitude of the received signal when no persons are present and establishes one or more thresholds preferably in accordance with the stored amplitude. The received signal is subsequently monitored for variations that result in a threshold crossing indicating the unwanted presence of a person in a danger zone. When a threshold crossing is detected an alarm is activated.