Electronic – How Do Vehicle Backup Sensors Work

"Vehicle detection loops, called inductive-loop traffic detectors" are one pretty common method. A groove is cut in the pavement, wire is layed in the groove, the groove is filled in with a sealant, and the wires go to a processor which makes the wire act like a metal detector - the vehicle interacts with the e-mag field, and the processor detects that.

Are you allowed to sawcut the area in question? If not, then a different method should be selected. Maybe just tubes across the road as one finds leading into or out of a town so the local municipality can count how many cars are using a road. I imagine they just detect change in air pressure as cars drive over the tubes.

How large is an obstruction? I think your options are probably either "build a RADAR set" (difficult, pricey) or "modify a laser rangefinder to do what you want". There exists an open source laser rangefinder base unit: http://www.lightware.co.za/shop/en/ - that $150 sounds expensive but realistically you're not likely to do it for less unless you have a really great supply of scavenged electronics and optics.