There are a lot of options, really.
You could use drive over sensors. There are several versions. Some lay on the ground and you drive over a tube which sets the trigger off. Others are like a heavy duty step pad. Still others mount in the road. You can search around for drive-thru sensors and find these fairly easily.
You could use optical beam break sensors that can tell when the hayride is passing because it breaks the line of sight between the emitter and receiver.
You could use some sort of Bluetooth, GPS, IR, etc. signal, but I think that's a bit overkill.
It's really just your preference and your budget...