+1 on
@GCoco and
@bigredfish 's suggestions.
Inductive loop detectors require a physically sound road bed (such as concrete, AC or asphalt), as once tuned and stable (no-call/no vehicle), any vibration or movement can cause it to lock up for a period of time. Dirt or gravel would NOT be suitable. They do make and sell PVC tubing of 6 ft. x 6 ft. squares and longer-sided rectangles prewired with 3 to 4 turns of wire for drop-in use to be paved over but they are expensive and would take LOTS of prep labor for your situation. The detector amplifier is not cheap either and generally requires a 24VDC power supply. Shielded twisted pair cable has to run from the loop to the amplifier if the amplifier is to be more than 75 feet from the loop.
Unless a quadra-pole loop (figure 8) is used, the circuit can miss high axle vehicles and vehicles with low iron/steel content (like motorcycles with alloy wheels). I don't advise it.