Wireless mobile traffic lights are often used to secure construction siteswhen roads are partially blocked. Some day, when a pair of them was placedclose to our home, I set off to explore how they are working. In this talk, Iwill describe how I used a cheap RTL-SDR together with GQRX, Inspectrum, andGNU Radio to reverse engineer the modulation and frame format of differenttypes of wireless traffic lights. With some patience, I could also make somesense out of the bits. In particular, I was able to extract the signal stateand display it in a web interface, mirroring the traffic light. A closer lookat the frame format and the apparent absence of any authentication might leaveone with a bit of a worrying impression regarding the security of thosesystems.