I live near an intersection with 4-way stop. People are always running the stop sign, but my city doesn't make changes without there being data to back them up. I'd like to build a sensor that simply counts how many cars fail to slow to below, say, 5 MPH, at the stop line.
Due to design limitations, the project can't be done by measuring when two laser beams are cut. It has to be ultrasonic of some form (one sensor, installed on a lamp pole across the intersection). I understand the HC SR04 isn't a very accurate ultrasonic sensor. Is there a sensor someone can recommend that would get the job done reasonably accurately?
Parallax makes a small Doppler radar sensor which puts out 5v. square waves, at a higher rate when something is moving faster. The Parallax version is all nice and well-conditioned, but costs about $30 back when I bought one. But on EBay, you can get the raw radar sensor for about $6. If you're conversant with opamps, you can build the signal conditioning portion (amplifier/lowpass filter, comparator to put out square waves) for about $1 in parts. I am only using it for motion detection, and I find my homemade version of the backend gave me range out to about 20 ft.