Late to the party, but this is something I've wanted to do for a long time. My background is in signal processing but the hardware has always been somewhat mysterious to me. Here is an interesting link from MIT:
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.