These are known as phantom traffic jams—stop-start waves that travel backwards through traffic even though every vehicle is moving forwards. One of the simplest relationships in traffic flow theory is q = ρv, where q is the traffic flow (the number of vehicles passing a point each hour), ρ is the traffic density (amount of cars on the road) and v is their average speed. Initially, adding more vehicles obviously increases the overall traffic flow, as more vehicles pass along the road. In traffic, the disturbances we seek to prevent are the stop-start waves that create phantom traffic jams. These ideas are central to the design of intelligent transport systems, where connected vehicles and infrastructure must cooperate to improve traffic flow.