Howdy MacM,
For vehicles or persons leaving the compound, the gates could be activated by a passive infra red (PIR) sensor. Though would recomend a dual technology type using microwaves as these are better for detecting vehicles in motion. A second device watching the immediate area of the gate would provide the safety feature you require.
Controlled entry into the site using personal transmitters is normallay cost prohibitive but modern culture offers a posibble loophole....
What's cell-phone reception like at the gates location?
No fancy electronics just enough to detect the "ringing" of an old cheap cell-phone (Pre paid tariff) wired into the gate motor controller circuit.
Call charges only begin when the recipient anwers but in this case one simply lets the phone ring a couple of times, the gates open up and the person wanting to get in, simply terminates the call.