Farm gate autocloser

The other day I was watching a farmer come out of a field. He got out of his vehicle, opened the gate, got back in his vehicle, drove through, got out of his vehicle, closed the gate, got back in his vehicle. This strikes me as very inefficient. An expensive solution would be a powered remote controlled gate (perhaps powered by solar power). But I'm sure it must be possible to have a sprung gate with some kind of delay, such that the driver has enough time to drive through before it closes. Or perhaps one where the spring kicks in once the car is past - a mechanism is triggered perhaps by the driver clipping a lever with his wing mirror (obviously only gently).