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 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, congestion is incurred. 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.
Traffic congestion occurs when a volume of traffic or modal split generates demand for space greater than the available road capacity. 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 throughput 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 works and weather events. Speed and flow can also affect network capacity though the relationship is complex.
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.
Some traffic engineers have attempted to 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
However, unlike a fluid, traffic flow is often affected by signals or other events at junctions that periodically affect the smooth flow of traffic. Alternative mathematical theories exist, such as Boris Kerner
's three phase traffic theory
Because of the poor correlation of theoretical models to actual observed traffic flows, transportation planners and highway engineers attempt to forecast traffic flow using empirical models. Their working traffic models typically use a combination of macro-, micro- and mesoscopic features, and may add matrix entropy effects, by "platooning" groups of vehicles and by randomising the flow patterns within individual segments of the network. These models are then typically calibrated by measuring actual traffic flows on the links in the network, and the baseline flows are adjusted accordingly.
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 Anthony Downs, in his books Stuck in Traffic (1992) and Still Stuck in Traffic (2004), offers a dissenting view: rush hour 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 systems, 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.
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 engineers
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 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 increases air pollution and carbon dioxide emissions (which may contribute to global warming) 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.
It has been suggested by some commentators that 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. Others link it largely to subjective lifestyle choices, differentiating between car-owning and car-free households.
- Junction improvements
- Grade separation, using bridges (or, less often, tunnels) freeing movements from having to stop for other crossing movements
- Ramp signalling, 'drip-feeding' merging traffic via traffic signals onto a congested motorway-type roadway
- Reducing junctions
- Reversible lanes, where certain sections of highway operate in the opposite direction on different times of the day/ days of the week, to match asymmetric demand. This may be controlled by Variable-message signs or by movable physical separation
- Separate lanes for specific user groups (usually with the goal of higher people throughput with fewer vehicles)
Urban planning and design
City planning and urban design practices can have a huge impact on levels of future traffic congestion, though they are of limited relevance for short-term change.
- Grid plans including Fused Grid road network geometry, rather than tree-like network topology which branches into cul-de-sacs (which reduce local traffic, but increase total distances driven and discourage walking by reducing connectivity). This avoids concentration of traffic on a small number of arterial roads and allows more trips to be made without a car.
- Zoning laws that encourage mixed-use development, which reduces distances between residential, commercial, retail, and recreational destinations (and encourage cycling and walking).
- Carfree cities, car-light cities, and eco-cities designed to eliminate the need to travel by car for most inhabitants.
- Transit-oriented development are residential and commercial areas designed to maximize access to public transport.
Supply and demand
Congestion can be reduced by either increasing road capacity (supply), or by reducing traffic (demand). Capacity can be increased in a number of ways, but needs to take account of latent demand otherwise it may be used more strongly than anticipated. Critics of the approach of adding capacity have compared it to "fighting obesity by letting out your belt" (inducing demand that did not exist before). Reducing road capacity has in turn been attacked as removing free choice as well as increasing travel costs and times.
Increased supply can include:
- Adding more capacity at bottlenecks (such as by adding more lanes at the expense of hard shoulders or safety zones, or by removing local obstacles like bridge supports and widening tunnels)
- Adding more capacity over the whole of a route (generally by adding more lanes)
- Creating new routes
- Traffic management improvements (see separate section below)
Reduction of demand can include:
- Parking restrictions, making motor vehicle use less attractive by increasing the monetary and non-monetary costs of parking, introducing greater competition for limited city or road space. Most transport planning experts agree that free parking distorts the market in favour of car travel, exacerbating congestion.
- Park and ride facilities allowing parking at a distance and allowing continuation by public transport or ride sharing. Park-and-ride car parks are commonly found at metro stations, freeway entrances in suburban areas, and at the edge of smaller cities.
- Reduction of road capacity to force traffic onto other travel modes. Methods include traffic calming and the shared space concept.
- Road pricing, charging money for access onto a road/specific area at certain times, congestion levels or for certain road users
- "Cap and trade", in which only licensed cars are allowed on the roads. A limited quota of car licences are issued each year and traded in a free market fashion. This guarantees that the number of cars does not exceed road capacity while avoiding the negative effects of shortages normally associated with quotas. However since demand for cars tends to be inelastic, the result are exorbitant purchase prices for the licenses, pricing out the lower levels of society, as seen Singapore's Certificate of Entitlement scheme.
- Congestion pricing, where a certain area, such as the inner part of a congested city, is surrounded with a cordon into which entry with a car requires payment. The cordon may be a physical boundary (i.e., surrounded by toll stations) or it may be virtual, with enforcement being via spot checks or cameras on the entry routes. Major examples are Singapore's electronic road pricing, the London congestion charge system, and the Stockholm congestion tax.
- Road space rationing, where regulatory restrictions prevent certain types of vehicles from driving under certain circumstances or in certain areas.
- Number plate restrictions based on days of the week, as practiced in several large cities in the world, such as Athens, Mexico City and São Paulo. In effect, such cities are banning a different part of the automobile fleet from roads each day of the week. Mainly introduced to combat smog, these measures also reduce congestion. A weakness of this method is that richer drivers can purchase a second or third car to circumvent the ban.
- Permits, where only certain types of vehicles (such as residents) are permitted to enter a certain area, and other types (such as through-traffic) are banned. For example, Bertrand Delanoe, the mayor of Paris, has proposed to impose a complete ban on motor vehicles in the city's inner districts, with exemptions only for residents, businesses, and the disabled.
- Policy approaches, which usually attempt to provide either strategic alternatives or which encourage greater usage of existing alternatives through promotion, subsidies or restrictions.
- Incentives to use public transport, increasing modal shares. This can be achieved through infrastructure investment, subsidies, transport integration, pricing strategies that decrease the marginal cost/fixed cost ratios, and improved timetabling.
- Cycling promotion through legislation, cycle facilities, subsidies, and awareness campaigns. The Netherlands has been pursuing cycle friendly policies for decades, and around a quarter of their commuting is done by bicycle.
- Telecommuting encouraged through legislation and subsidies.
- Online shopping promotion, potentially with automated delivery booths helping to solve the last mile problem and reduce shopping trips made by car.
Use of so-called Intelligent transportation system
, which guide traffic:
- School opening times arranged to avoid peak hour traffic (in some countries, private car school pickup and drop-off traffic are substantial percentages of peak hour traffic).
- Considerate driving behaviour promotion and enforcement. Driving practices such as tailgating and frequent lane changes can reduce a road's capacity and exacerbate jams. In some countries signs are placed on highways to raise awareness, while others have introduced legislation against inconsiderate driving.
- Visual barriers to prevent drivers from slowing down out of curiosity (often called "rubbernecking" in the United States). This often includes accidents, with traffic slowing down even on roadsides physically separated from the crash location. This also tends to occur at construction sites, which is why some countries have introduced rules that motorway construction has to occur behind visual barrier
- Speed limit reductions, as practiced on the M25 motorway in London. With lower speeds allowing cars to drive closer together, this increases the capacity of a road. Note that this measure is only effective if the interval between cars is reduced, not the distance itself. Low intervals are generally only safe at low speeds.
- Lane splitting/filtering, where space-efficient vehicles, usually motorcycles and scooters, ride or drive in the space between cars, buses, and trucks. This is however illegal in many countries, being perceived as a safety risk.
In Brazil the recent records of traffic jams over the major big cities are recognized by public authorities as one of the main challenges for São Paulo, Rio de Janeiro, Belo Horizonte, Brasilia, Curitiba and Porto Alegre, where due to the country's economic bonanza, the automobile fleets have almost doubled in several of these cities from 2000 to 2008.
According to Time Magazine, São Paulo has the world's worst traffic jams. In 2008, the accumulated tailbacks have reached in average more than 120 miles (190 km) during rush hours, and on May 9 2008, the historical record was set with 166 miles (266 km) of accumulated queues out of 522 mi (835 km) being monitored. Despite implementation since 1997 of road space rationing by the last digit of the plate number during rush hours every weekday, traffic in this 20 million city still experiences severe congestion. According to experts, this is due to the accelerated rate of motorization occurring since 2003, in São Paulo the fleet is growing at a rate of 7.5% per year, with almost 1,000 new cars bought in the city every day, and the limited capacity of public transport. The subway has only 38 miles of lines, though 22 further miles are under construction or planned by 2010. Every day, many citizens spend between three up to four hours behind the wheel. In order to mitigate the aggravating congestion problem, since June 30, 2008 the road space rationing program was expanded to include and restrict trucks and light commercial vehicles.
New Zealand has followed strongly car-oriented transport policies since after World War II (especially in the Auckland area, where about one-third of the county's population lives), and currently has one of the highest car-ownership rates per capita in the world, after the United States. Because of the negative results, congestion in the big centres is a major problem. Current measures include both the construction of new road infrastructure as well as increased investment in public transport, which had strongly declined in all cities of the country except Wellington.
In the United Kingdom
the inevitability of congestion in some urban road networks has been officially recognised since the Department for Transport
set down policies based on the report Traffic in Towns
Even when everything that it is possibly to do by way of building new roads and expanding public transport has been done, there would still be, in the absence of deliberate limitation, more cars trying to move into, or within our cities than could possibly be accommodated..
The Department for Transport sees growing congestion as one of the most serious transport problems facing the UK. On 1 December 2006, Rod Eddington published a UK government-sponsored report into the future of Britain's transport infrastructure. The Eddington Transport Study set out the case for action to improve road and rail networks, as a "crucial enabler of sustained productivity and competitiveness". Eddington has estimated that congestion may cost the economy of England £22 bn a year in lost time by 2025. He warned that roads were in serious danger of becoming so congested that the economy would suffer. At the launch of the report Eddington told journalists and transport industry representatives introducing road pricing to encourage drivers to drive less was an "economic no-brainer". There was, he said "no attractive alternative". It would allegedly cut congestion by half by 2025, and bring benefits to the British economy totalling £28 bn a year.
In the United States
, construction of new highway capacity has not kept pace with increases in population and car use and the resulting increase in demand for highway
travel. Between 1980 and 1999
, the total length of highways as measured by miles increased by only 1.5 percent, while the total number of miles of vehicle travel increased by 76 percent.
The Texas Transportation Institute estimates that in 2000 the 75 largest metropolitan areas experienced 3.6 billion vehicle-hours of delay, resulting in 5.7 billion US gallons (21.6 billion liters) in wasted fuel and $67.5 billion in lost productivity, or about 0.7% of the nation's GDP. It also estimates that the annual cost of congestion for each driver is approximately $1,000 in very large cities and $200 in small cities. Traffic congestion is increasing in major cities, and delays are becoming more frequent in smaller cities and rural areas.
In 2005, the three areas in the United States with the highest levels of traffic congestion were Los Angeles, New York and Chicago. The congestions cost for Los Angeles alone was estimated at US$ 9.325 billion.
- R. Wiedemann, Simulation des Straßenverkehrsflusses. Schriftenreihe des IfV, 8, 1974. Institut für Verkehrswesen. Universität Karlsruhe. (In German language).
- K. Nagel and M. Schreckenberg. A Cellular Automaton Model for Freeway Traffic. Journal de Physique I, 2:2221-2229, December 1992.
- K. Nagel. High-speed Microsimulations of Traffic Flow. PhD thesis, Universität zu Köln, 1994.
- D. Chowdhury, L. Santen, and A. Schadschneider. Statistical Physics of Vehicular Traffic and Some Related Systems. Phys. Rep., 329:199-329, 2000.
- D. Helbing. Traffic and related self-driven many-particle systems. Rev. Mod. Phys., 73(4):1067-1141, 2001.