Traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, and increased vehicular queueing. When traffic demand is great enough that the interaction between vehicles slows the speed of the traffic stream, congestion is incurred. As demand approaches the capacity of a road (or of the intersections along the road), extreme traffic congestion sets in.

Traffic congestion is one of those things we put up with in our lives of quiet desperation. It doesn't seem to get proper recognition as a problem, except to scream for wider roads. As if that will solve traffic congestion. Traffic congestion occurs when increasing road use goes beyond the capacity of the road. A saturated road cannot carry more cars, and instead the cars get clogged up, creating traffic slowdowns or even traffic jams.

Traffic congestion occurs when traffic volume demands space greater than available road capacity. There are many 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.

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-use them, up to the point where traffic collapses into a jam, when demand becomes limited by opportunity cost.

Perhaps rush hour traffic congestion is inevitable because of the benefits of having a relatively standard work day. If "everybody" goes to work or returns home at the same time, they're all crushing the road at the same time. If instead work days were staggered, there wouldn't be peak demand times in the morning and afternoon.

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. Increased fuel use may also in theory cause a rise in fuel costs.
• 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.

References

http://en.wikipedia.org/wiki/Traffic_congestion