Traffic engineering is a branch of civil engineering that uses engineering techniques to achieve the safe and efficient movement of people and goods. It focuses mainly on research and construction of the immobile infrastructure necessary for this movement, such as roads, railway tracks, bridges, traffic signs and traffic lights. Increasingly however, instead of building additional infrastructure, dynamic elements are also introduced into road traffic management. These use sensors to measure traffic flows and automatic, interconnected guidance systems (for example traffic signs which open a lane in different directions depending on the time of day) to manage traffic especially in peak hours. The relationship between lane flow (Q) (vehicles per hour) maximum speed (V) (kilometers per hour) and density (K) (vehicles per kilometer) is Q = KV. Observation on limited access facilities suggests that up to a maximum flow, speed does not decline while density increases, but above a critical threshold, increased density reduces speed, and beyond a further threshold, increased density reduces flow as well.