The target would be the rear of the vehicle. I would try to point it at a flat surface perpendicular to the sensor.
The code I found goes low for 2 microseconds, high for 10 microseconds and then low again. After the pulseIn there is a delay of 100.
I just need to find a way to try and eliminate false alarms.
The vehicle will always be moving away from the sensor, so I can pretty much eliminate any reading close than my saved reading if that helps.