Very Long Range Beam Break Trigger Sensor
One idea: Use a CW microwave radar motion detector (=an intrusion alarm device). Place the sensor in front of a big enough parabolic reflector to get narrow enough transmitting beam and big enough echo signal catching area. You must find the needed reflector size and focal length by making experiments. 

There's a couple of traps: Vehicle speed can be out of the detection range (=too high doppler shift) and your local law can limit the allowed beam power densities. Detector's transmitter can be legal as is, but when all power is directed to a narrow beam, the beam can be illegally strong.