A proximity detection system (1) for detecting the proximity of a first mobile unit (2a) to a second mobile unit (2b). A transceiver (16) is provided to transmit a first signal from the second mobile unit (2b). The first signal is received by a transponder (12) on a first mobile unit (2a). After a selected time delay, the transponder (12) transmits a second signal in response to the first signal. The second signal is received by the transceiver (16). The time elapsed between transmission of the first signal and receipt of the second signal is computed. The computation is compared with a value, stored in a memory (22), by a computation unit (20). The value stored in the memory (22) represents a predefined distance that forms the boundary of an alarm condition zone (30a, 30b) around the second mobile unit (2b). A comparator (24) compares the computed time elapsed with the value stored in the memory (22) to determine whether the proximity of the first mobile unit (2a) to the second mobile unit (2b) is such that the first mobile unit (2a) is in an alarm condition zone (30a, 30b). If the first mobile unit (2a) is within an alarm condition zone (30a, 30b), a control unit (10) activates a device on the second mobile unit (2b), such as a warning light or siren or the brakes of the second mobile unit (2b) are activated.
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