A choke plate on a shaft is arranged at a discharge end of a snowplow blade and is rotationally driven, selectively to a closed position for blocking the discharge of snow from the plow blade, and an open position for allowing the discharge of snow. Switching between the positions preferably involves successive quarter turns of the shaft in a consistent rotation direction. A controller receives inputs from a manual control switch, a speed-dependent timer, and an external signal receiver, and accordingly controls the opening and closing of the choke plate. External wireless signals are provided by transmitters located before and after a driveway that joins into a road that is being plowed. The choke plate is automatically controlled to temporarily block the discharging of snow from the snowplow as the plow passes the driveway, to prevent the formation of a discharge snowbank across the mouth of the driveway.