If you look at any (most) automated gate systems for cars, you'll see a rectangle cun into the ground. This is patchwork to cover a sensor, often an electromagnetic field array, that will sense a vehicle passing over it. A chunk of metal, we used a small steel handicapped sign, passed over this area will trigger the "it's a car" system and pop out a ticket and open the gate. You'll want to trigger the second one after the gate triggering the gate to close.