I was impressed with how easy it is to hook up an HB100 doppler radar module (normally used for things like automatic door sensors) to an Arduino in order to measure speed of nearby moving targets. I decided to make a halloween costume replica of a radar speed sign, like the ones commonly find besides roads which attempt to guilt you into slowing down. It worked out pretty well!

Primary project writeup is over here: http://johnmcnelly.com/arduino-radar-speed-sign/