Maybe there are other ways to detect the cars too besides what Joseph mentioned.

1) Put separate short loop antennas across each lane and then put the RF emitter on each go-cart under the vehicle and situate its antenna so that it is only 200 to 300 mm away. Low power can be used so that it does not couple until the transmitter is nearly over the detector loop.

2) Equip the lanes with the rubber hoses that used to be common at petro stations to announce the arrival of a car at the pump. A low pressure version of flexible hose should be able to detect a go-cart with a simple pressure switch.

3) Put a reflector on the side of each go-cart and use a reflective light beam approach to detect each car as it goes by.

4) Put RFID tags in several places in each go-cart and equip each lane with an adaptation of a standard RFID scanner. It can be made to work since the go-carts are not huge.

Michael Karas