Treadle

There is an inconspicuous treadle let into the floor and when you step on it an air-operated door-opening mechanism is automatically activated; [...].

As a car enters the parking area the front wheels pass over a treadle, causing a timber barrier to lift and allow the motorist to drive in and park. No money is required at this stage. As the car passes over a second treadle, just beyond the entrance, the barrier falls again.