integrated results as a standard at 46 during the setup it is necessary to adjust the length of time following a pulse that echoes will be received this may be accomplished by measuring the distance from the transmitter receiver to the floor and dividing by the speed of sound as the speed of sound can vary slightly with ambient conditions the time period should be somewhat longer or shorter than calculated from the measurement this may be a first step 48 in the setup mode according to a preferred embodiment the setup is run for both the door closed condition and the door open condition enabling the determination of a passenger waiting to exit as well as whether a passenger has cleared the doorway referring to fig3 b the microprocessor is programmed to operate in the monitoring mode to emit a trigger pulse or a plurality of trigger pulses at 50 to receive echo signals caused from each pulse during a period of time insufficient to receive an echo from the floor at 52 to integrate the echo signals at 54 and compare the integrated results to a standard at 56 if the integrated result is sufficiently close to the stored standard control returns to emitting a new trigger pulse at 50 if the integrated result is not sufficiently close to.