I think it'd be fun to plot speeds and courses, just for science, so to speak.
By "build" I mean make an antenna, signal generator, signal processor, and record echos to output range and velocity for fusing, association, and tracking, possibly using an MCU to record or broadcats tracks (position and speed over time).
I'm intimately familiar with tracking math and code (EKF, Batch Filters, old Bar Shalom methods, track association, etc).
I'm mildly familiar with MCU programming and PCB layout (have 3-ish decent PCB+MCU projects done in last 15 years).
I'm not familiar with FPGA, DSP, or complex signal processing.
I'm a Ham General and can manage electronics, oscilloscopes, etc.
Presumably the hardest part is timing and signal generation, and then signal processing.
Roadside ranges are < 100m, speeds < 20m/s.
Lakeside ranges are 100-200m, speeds as high as 20m/s.
Is this even legal? I live in the USA.
Not radar, but last year there was a thread from someone who uses a webcam to record trains, and measures speed just through their pixel changes and framerate: https://news.ycombinator.com/item?id=35738987
Might work well enough for fixed-route travel... railways especially, but maybe roads are close enough? Probably not as good if the vehicle has more directional freedom and can move diagonally or straight away from the camera. It's less likely to run into any radio licensing issues, though.
http://ocw.mit.edu/resources/res-ll-003-build-a-small-radar-...
Other notes: https://hackaday.com/2015/04/07/build-a-phased-array-radar-i...
https://makezine.com/article/craft/diy-phased-array-radar-fr...
I really like the idea of using wifi signals, such that you'd have to build a receiver only. I'm not sure what the range would be but it seems like getting traffic speeds in front of the house would be possible.
IANAL, not even remotely, but I have some amount of familiarity with this stuff. Don't take me at my word, though. Get expert advice instead.
FCC rules absolutely apply here, and the FCC has explicit guidance for radar transmitters (https://docs.fcc.gov/public/attachments/DA-96-2040A1.pdf). But which specific laws apply depends on a lot of things, such as what frequencies you're using.
The laws are a lot more lenient if you aren't selling the thing you make, but they still exist. In general, they boil down to "you can't cause interference with licensed transmitters" and "you can't use more than a weak signal without a license".
If I were doing a project like this, I'd probably look very hard at non-radio solutions, such as perhaps laser ranging, where there isn't much legal risk.
Is this even legal?
If it matters, ask your lawyer. If it doesn’t matter, it doesn’t matter. Good luck.