Traffic congestion is a condition in transport that is characterized by slower speeds, longer trip times, and increased vehicular queuing. Traffic congestion on urban road networks has increased substantially since the 1950s, resulting in many of the roads becoming obsolete. When traffic demand is great enough that the interaction between vehicles slows the traffic stream, this results in congestion. While congestion is a possibility for any mode of transportation, this article will focus on automobile congestion on public roads. Mathematically, traffic is modeled as a flow through a fixed point on the route, analogously to fluid dynamics.
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 known as a traffic jam, a traffic snarl-up (informally) or a tailback. Drivers can become frustrated and engage in road rage. Drivers and driver-focused road planning departments commonly propose to alleviate congestion by adding another lane to the road; however, this is ineffective as increasing road capacity induces more demand for driving.
Causes
Traffic congestion occurs when a volume of traffic generates demand for space greater than the available street capacity; this point is commonly termed saturation. Several specific circumstances can 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 volume of traffic; most of the rest is attributed to traffic incidents, road work and weather events. In terms of traffic operation, rainfall reduces traffic capacity and operating speeds, thereby resulting in greater congestion and road network productivity loss.
Individual incidents such as crashes or even a single car braking heavily in a previously smooth flow may cause ripple effects, a cascading failure also known as traffic waves, which then spread out and create a sustained traffic jam when, otherwise, the normal flow might have continued for some time longer.
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-use 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 argues that 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-served); 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 putting the revenues generated therefrom into public transportation projects.
A 2011 study in The American Economic Review indicates that there may be a "fundamental law of road congestion." The researchers, from the University of Toronto and the London School of Economics, analyzed data from the U.S. Highway Performance and Monitoring System for 1983, 1993 and 2003, as well as information on population, employment, geography, transit, and political factors. They determined that the number of vehicle-kilometers traveled (VKT) increases in direct proportion to the available lane-kilometers of roadways. The implication is that building new roads and widening existing ones only results in additional traffic that continues to rise until peak congestion returns to the previous level.
Classification and modeling
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. 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.
Another classification schema of traffic congestion is associated with some common spatiotemporal features of traffic congestion found in measured traffic data. Common spatiotemporal empirical features of traffic congestion are those features, which are qualitatively the same for different highways in different countries measured during years of traffic observations. Common features of traffic congestion are independent on weather, road conditions and road infrastructure, vehicular technology, driver characteristics, day time, etc. Examples of common features of traffic congestion are the features [J] and [S] for, respectively, the wide moving jam and synchronized flow traffic phases found in Boris Kerner's three-phase traffic theory. The common features of traffic congestion can be reconstructed in space and time with the use of the ASDA and FOTO models.
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.Critical Mass – Ball, Philip,
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 randomizing 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.
A team of MIT mathematicians has developed a model that describes the formation of "phantom jams", in which small disturbances (a driver hitting the brake too hard, or getting too close to another car) in heavy traffic can become amplified into a full-blown, self-sustaining traffic jam. Key to the study is the realization that the mathematics of such jams, which the researchers call "jamitons", are strikingly similar to the equations that describe detonation waves produced by explosions, says Aslan Kasimov, lecturer in MIT's Department of Mathematics. That discovery enabled the team to solve traffic-jam equations that were first theorized in the 1950s.
Road rage Road rage is aggressive or angry behavior by a driver of an automobile or other motor vehicle. Such behavior might include rude gestures, verbal insults, deliberately driving in an unsafe or threatening manner, or making threats. Road rage can lead to altercations, assaults, and collisions which result in injuries and even deaths. It can be thought of as an extreme case of aggressive driving. The term originated in the United States in 1987–1988 (specifically, from Newscasters at KTLA, a local television station), when a rash of freeway shootings occurred on the 405, 110 and 10 freeways in Los Angeles, California. These shooting sprees even spawned a response from the AAA Motor Club to its members on how to respond to drivers with road rage or aggressive maneuvers and gestures.
Economic loss
+AreaLoss in billionsNoteUS$305 UK$52.01 NYC$33.7LA$19.2Manila$18.615 Bangladesh$11.4SF$10.6Atlanta$7.1 Jakarta$5Dhaka$4.463GTHA$3.3
The US Urban Mobility Report estimated the annual cost of congestion at $1,480 per capita, however a Victoria Transport Policy Institute study found many studies overestimated the cost of congestion, and estimated a value of $131 per capita.
Countermeasures
Improving road infrastructure
Increasing road capacity is standard response to congestion, perhaps by widening an existing road or adding a new road, bridge or tunnel. However, this has been shown to result in attracting more traffic, otherwise known as induced demand. The result can be greater congestion on the expanded artery itself or on auxiliary roads. In a similar vein, Braess's paradox shows that adding road capacity might make congestion worse, even if demand does not increase. In his paper, "The Law of Peak Hour Express Way Congestion", published in 1962, Anthony Downs formulated this phenomenon as a "law": "on urban commuter expressways, peak-hour traffic congestion rises to meet maximum capacity." Junction improvements Grade separation, using bridges (or, less often, tunnels) freeing movements from having to stop for other crossing movements Ramp signaling, 'drip-feeding' merging traffic via traffic signals onto a congested motorway-type roadway Reducing junctions Local-express lanes, providing through lanes that bypass junction on-ramp and off-ramp zones Limited-access road, roads that limit the type and amounts of driveways along their lengths Reversible lanes, where certain sections of highway operate in the opposite direction on different times of the day(s) of the week, to match asymmetric demand. These pose a potential for collisions, if drivers do not notice the change in direction indicators. 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) Bus lanes as part of a busway system Express toll lanes HOV lanes, for vehicles with at least three (sometimes at least two) riders, intended to encourage carpooling Slugging, impromptu carpooling at HOV access points, on a hitchhiking or payment basis Market-based carpooling with pre-negotiated financial incentives for the driver
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. Cycling modal share is strongly associated with the availability of local cycling infrastructure. Carfree cities, car-light cities, and eco-cities designed to eliminate the need to travel by car for most inhabitants.