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during peak hour traffic.]] Traffic congestion is a condition on any Network as use increases and is characterized by slower speeds, longer trip times, and increased Queueing . The most common example is for physical use of roads by vehicles. There it occurs when traffic demand is greater than the capacity of a road (or of the intersections along the road). Extreme traffic congestion, where vehicles are fully stopped for periods of time, is colloquially known as a '''traffic jam'''. DEFINITION Causes .]] There are several main causes of traffic congestion. The prime cause is simply a volume of traffic too high for the road capacity, but there are a number of specific circumstances which cause or aggravate congestion - most of which reduce the capacity of a road at a given point or over a certain length, while others increase the number of vehicles required for a given throughput of people or goods. The former causes include construction works, accidents and emergencies or unsafe road conditions (due to weather or other factors); the latter would be defined by an change in mode share between high and low occupancy vehicles to more low occupancy vehicles. Generally, reduced speeds also mean less road capacity (though this does not in reverse mean that higher speeds create higher capacity - in fact, with very high speeds the capacity falls again). Traffic research still cannot fully predict under which conditions a 'traffic jam' (as opposed to heavy, but smoothly flowing traffic) may suddenly occur. It has been found that individual incidents (such as accidents or even a single car braking heavily in a previously smooth flow) may cause ripple effects which then spread out and create a sustained traffic jam when otherwise, normal flow might have continued for some time longer. Science Hobbyist: Traffic Waves (from the 'Science Hobbyist' website) Mathematical theories Traffic engineers therefore apply the rules of Fluid Dynamics to traffic flow, likening it to the flow of a fluid in a pipe. Congestion simulations and real-time observations have shown that in heavy but free flowing traffic, jams can arise spontaneously, triggered by Minor Events ('butterfly Effects') , such as an abrupt steering maneuver by a single motorist. Traffic scientists liken such a situation to the sudden freezing of Supercooled Fluid .''Critical Mass'' - Ball, Philip , ISBN 0-09-945786-5 In the Three Phase Traffic Theory of Boris Kerner , congestion is classified into two distinct phases: ''synchronised flow'' and ''wide moving jams'' (in addition to the first phase, ''free flow''). In synchronised flow, the speeds of the vehicles are low and vary quite a lot between vehicles, but the traffic flow (expressed in vehicles per time unit) remains close to free flow. In wide moving jams, vehicle speeds are more equal and lower, and time delays can be quite large. Economic theories Congested roads can be seen as an example of the Tragedy Of The Commons . Because roads in most places are ''free at the point of usage'', there is little financial incentive for drivers not to over-utilize them, up to the point where traffic collapses into a jam, when demand becomes limited by Opportunity Cost . Privatization Of Highways and Road Pricing have both been proposed as measures that may reduce congestion through economic incentives and disincentives. Congestion can also happen due to non-recurring highway incidents, such as a Crash or Roadworks , which may reduce the road's capacity below normal levels. Economist traffic congestion is inevitable because of the benefits of having a relatively standard work day. In a Capitalist economy, goods can be allocated either by pricing (ability to pay) or by queueing (first-come first-serve); congestion is an example of the latter. Instead of the traditional solution of making the "pipe" large enough to accommodate the total demand for peak-hour vehicle travel (a supply-side solution), either by widening roadways or increasing "flow pressure" via Automated Highway System s, Downs advocates greater use of Road Pricing to reduce congestion (a demand-side solution, effectively rationing demand), in turn plowing the revenues generated therefrom into Public Transportation projects. Road Pricing itself is controversial, more information is available in the dedicated article. Classification Qualitative classification of traffic is often done in the form of a six letter A-F Level Of Service (LOS) scale defined in the Highway Capacity Manual , a US document used (or used as a basis for national guidelines) worldwide. These levels are used by Transportation Engineer s as a Shorthand and to describe traffic levels to the lay public. While this system generally uses delay as the basis for its measurements, the particular measurements and statistical methods vary depending on the facility being described. For instance, while the percent time spent following a slower-moving vehicle figures into the LOS for a rural two-lane road, the LOS at an urban intersection incorporates such measurements as the number of drivers forced to wait through more than one signal cycle.''Traffic Engineering'', Third Edition. Roger P. Roess, Elana S. Prassas, and William R. McShane. ISBN 0-13-142471-8 NEGATIVE IMPACTS Traffic congestion has a number of negative effects:
ATTEMPTS TO ALLEVIATE In many respects, the level of congestion that society tolerates is a rational (though not necessarily conscious) choice between the costs of improving the transportation system (in infrastructure or management) and the benefits of quicker travel. Methods currently used Road infrastructure
Supply / Demand
Both these strategies are now widely disputed. Adding road capacity has been compared to "fighting , United States)while reducing road capacity has been attacked as reducing free choice as well as increasing travel costs and times. Both strategies are linked to the Induced Demand hypothesis.
Traffic management
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