At the camp I worked at, the gates were magnetic. On one side you needed a code to get in, on the other the truck just had to get close and the gate would open. Whenever we walked past it with something big and metalic like a 2 wheeled cart it would open. Stop lights on the other hand are activated by weight.
1979 CJ5 project
"Life is an issue of mind over matter. Ya see, I don't mind, and well...you don't matter."
Red Jeep Club Member #2