What is Traffic Congestion?

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 or modal split that generates demand for space greater than 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, or increase the number of vehicles required for a given throughput of people or goods. The former causes include on-road parking, construction works, accidents and emergencies or unsafe road conditions (due to weather or other factors); the latter causes include conditions where the mode share between high and low occupancy vehicles primarily consists of low occupancy vehicles or of types of vehicle that take up a large quantum of network space per person. 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.^[1]

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.^[2]

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 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.

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 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.^[3]

Negative impacts

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, education etc. - 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 owing to increased idling, acceleration and braking. Increased fuel use may also in theory cause an imperceptible 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.

Countermeasures

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.

Road infrastructure

• 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
• 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/ 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)
• Bus lanes as part of a busway system
• 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

Supply / Demand

• Increasing road capacity by adding more lanes or new routes - classical method of dealing with traffic congestion
• Reduction of road capacity - alternative to above, intended to force traffic onto other travel mode

Both these strategies are now widely disputed. Adding road capacity has been compared to "fighting obesity by letting out your belt" (causing demand that did not exist before),^[4][5]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.

• Parking restrictions, making motor vehicle use less attractive by intentionally not providing parking where it is wanted
• Public transport improvements, a 'pull'-method to change modal shares, often connected with 'push'-methods like discouraging road use
• Road pricing, charging money for access onto a road/specific area at certain times, congestion levels or for certain road users
• 'Cap and Trade', where 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 tends to be exorbitant purchase prices for the licenses, pricing out the lower levels of society.^[6]
• 'Cordon 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 tooll stations) or it may be virtual, with enforcement being via spot checks or cameras on the entry routes. A major example is the London congestion charge system.
• Rationing, where some types of vehicles are not allowed to drive under certain circumstances or in certain areas
• An example is number plate restrictions based on days of the week, as practiced in many large cities in the world, such as Athens, Mexico City and São Paulo. In effect, the cities are banning a different part of the automobile fleet from roads each day of the week. Mainly used to combat smog, it also reduces congestion. A weakness of this method is that many richer drivers will purchase a second or third car to circumvent the ban.

Traffic management

• Traffic reporting, via radio or possibly mobile phones, to advise road users
• Variable message signs installed along the roadway, to advise road users
• Navigation systems, possibly linked up to automatic traffic reporting
• Traffic counters permanently installed, to provide real-time traffic counts
• Automated highway systems, a future idea which could reduce the safe interval between cars (required for braking in emergencies) and increase highway capacity by as much as 100% while increasing travel speeds

Other associated

• School opening times arranged to avoid peak hour traffic (in some areas, school pickup and drop-off traffic are substantial percentages of traffic)
• Promotion of more considerate driving behaviour. 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.
• Promotion of utility cycling through legislation, cycle facilities, subsidies, and awareness campaigns. The Netherlands has been pursuing a cycle friendly policy for decades, and around 1/3 of commuting is done by bicycle in that country.
• On highways, congestion may be caused by curious drivers slowing down to observe a motor vehicle accident on the opposite lane (often called "rubbernecking" in the United States). To prevent this, visual barriers are often placed in the central section of the highway by the police when there is a major accident.
• Reduction of speed limits, as practiced on the M25 motorway in London. The argument is that a lower speed allows cars to drive closer together, which 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.
• Park and ride - encourages people to park their car outside the main town/city and transfer by public transport, shuttle bus, or carpool. Park-and-ride lots are common at freeway entrances in suburban areas to encourage ride sharing.
• City planning practices that avoid concentration of traffic on a small number of arterial roads and allow more trips to be made without a car. One measure is arranging streets in a fused grid geometry, rather than a tree-like network topology branching into cul-de-sacs.
• Lane splitting or filtering. Space-efficient vehicles, usually motorcycles and scooters, ride or drive in the space between cars, buses, and trucks.^[7]
• ITS Use of Intelligent Transportation Systems, including VMS, CCTV, Traffic Operation Centers, Remote Traffic Signal Control and Parking Guidance and Information systems.

By country

United States

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.^[8]

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 2003, the ten areas in the United States with the highest levels of traffic congestion in order are: [1]

• Los Angeles, California
• New York City, New York
• Boston, Massachusetts
• San Francisco, California
• Houston, Texas
• Washington, D.C.
• San Diego, California
• Miami, Florida
• Dallas/Fort Worth Metroplex
• Chicago, Illinois

Due to dramatic population increases, San Diego, Sacramento and Las Vegas have seen their congestion levels increase by more than 50 percent since 1982.

See also

• Carpooling
• Congestion, more general usage of the word congestion
• Gridlock
• Journey to work
• Queueing theory
• Roadway air dispersion modeling
• Rush hour
• Traffic break, the deliberate separation of traffic
• Traffic flow
• Traffic forecasting
• Traffic Message Channel, broadcasting of digitally encoded traffic info
• Vehicle infrastructure integration

References

External links

• Institute of Transportation Engineers
• The New Mobility Agenda
• Traffic Bulldog - Commuter Advocacy
• Anatomy of a traffic jam (entry explaining a type of traffic jam from a private blog)