Traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, and increased vehicularqueueing. The most common example is the physical use of roads by vehicles. When traffic demand is great enough that the interaction between vehicles slows the speed of the traffic stream, this results in some congestion. As demand approaches the capacity of a road (or of the intersections along the road), extreme traffic congestion sets in. When vehicles are fully stopped for periods of time, this is colloquially known as a traffic jam or traffic snarl-up. Traffic congestion occurs when a volume of traffic or modal split generates demand for space greater than the available road capacity; this point is commonly termedsaturation. There are a number of specific circumstances which cause or aggravate congestion; most of them reduce the capacity of a road at a given point or over a certain length, or increase the number of vehicles required for a given volume of people or goods. About half of U.S. traffic congestion is recurring, and is attributed to sheer weight of traffic; most of the rest is attributed to traffic incidents, road work and weather events. 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 (a cascading failure) which then spread out and create a sustained traffic jam when, otherwise, normal flow might have continued for some time longer.[3 Traffic congestion has a number of negative effects:
* Wasting time of motorists and passengers ("opportunity cost"). As a non-productive activity for most people, congestion reduces regional economic health. * Delays, which may result in late arrival for employment, meetings, and education, resulting in lost business, disciplinary action or other personal losses. * Inability to forecast travel time accurately, leading to drivers allocating more time to travel "just in case", and less time on productive activities. * Wasted fuel increasing air pollution and carbon dioxide emissions owing to increased idling, acceleration and braking. * Wear and tear on vehicles as a result of idling in traffic and frequent acceleration and braking, leading to more frequent repairs and replacements. * Stressed and frustrated motorists, encouraging road rage and reduced health of motorists * Emergencies: blocked traffic may interfere with the passage of emergency vehicles traveling to their destinations where they are urgently needed. * Spillover effect from congested main arteries to secondary roads and side streets as alternative routes are attempted ('rat running'), which may affect neighborhood amenity and real estate prices.
Wild deer and coyotes cause traffic snarls, and domestic animals get into the act as well. July 5 tends to have an unusual number of animal-related traffic problems, as pets, spooked by the fireworks on the previous day, have a greater propensity to wander onto freeways. Sadly, animal-related traffic jams are not all accidental. Sometimes litters of unwanted puppies are deliberately left on freeways to be run over. The California Department of Transportation often dispatches work crews to go out and collect trash bags of dead roosters the day after cockfights. Even worse than animals dying on the roads are people. Freeways are a popular place to end it. People jump off bridges and overpasses, or deliberately drive into other cars or off the road; the frequency of highway suicides rises around Valentine’s Day. There are also higher rates of highway-related suicide in the days following some other well-publicized suicide. All of this can halt traffic. And then there are the homicides. Deranged individuals have been known to...
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