I have just read this interesting article which describes away to measure vehicle speeds using space-borne SAR sensors. The article explains very clearly how Doppler effect can be used, either with one single image, or with an interferometric pair, to estimate the speed of cars or ships. DLR's TerraSAR-X system has always impressed me. It has been providing very high quality images from its launch. For having used the images, I can say that their geometrical quality is very good. And as far as I know, this is the only operational system in flight which is able to perform one-pass interferometry. So far so good. However, the article forgets to mention that these satellites acquire only 2 images per day on a given point of the Earth's surface – typically in the morning and in the evening – so one can not expect to use this technology for real-time traffic monitoring. So you don't need to worry about getting a ticket for speed.