| Traffic Engineering (transportation) |
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Increasingly however, instead of building additional infrastructure, dynamic elements are also introduced into road traffic management (they have long been used in rail transport). 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. Therefore, managing traffic density by limiting the rate that vehicles enter the highway during peak periods can keep both speeds and lane flows at bottlenecks high. Ramp Meter s, signals on entrance ramps that control the rate at which vehicles are allowed to enter the mainline facility, provide this function (at the expense of increased delay for those waiting at the ramps). Traffic engineering is closely associated with other disciplines:
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