A sonar detector system for exiting passengers on public conveyances such as buses and trolleys which detects the presence of passengers at unattended exits and opens the exit doors is disclosed. The system employs two sonar transducers/receivers at each door of the exit with narrow, electronically truncated detection zones. The sensors are positioned so as to prevent operation of the doors due to echoes from passengers on the aisle floor of the public conveyance or receipt of echoes from outside the conveyance when the doors are open. It also will not operate the doors when echoes are received from packages or debris in the stairwells of the exits. Special circuitry is used to prevent a sensor from detecting echoes from the transmission of the other sensors at the door or in the conveyance and the system is designed so that the presence of persons who are very close to the senor, i.e. within the dead space of the sonar sensor, can be detected using a single sensor. By automatically closing unattended doors, the system will prevent unauthorized entry of fare evaders through the exit. When used on subway trains, the system also prevents closing of doors on passengers entering or leaving the conveyance.