This work focuses on technologies for cyber physical systems (CPS) to mitigate traffic instabilities that adversely affect fuel consumption (e.g., stop-and-go waves) via precise velocity control of a small number of autonomous vehicles (AVs) on the highway. The main finding is that even a single autonomous vehicle may substantially reduce undesirable traffic waves in its vicinity when properly controlled. The general approach is to use AVs and their sensors to detect congestion events, and then close the loop by carefully following prescribed velocity controllers that are demonstrated to reduce the fuel consumption of the overall traffic flow.