Traffic congestion

Traffic jams are common in heavily populated areas.

Traffic congestion is a road condition characterized by slower speeds, longer trip times, and increased queueing. It occurs when roadway demand is greater than its capacity. A period of extreme traffic congestion is colloquially known as a traffic jam.

There are three main causes of traffic congestion:

High traffic volume
Construction
Accidents

Traffic congestion has several negative effects:

Sitting in traffic wastes the time of motorists and their passengers. Since drive time is a nonproductive activity for most, congestion reduces regional economic health, consequently reducing revenues.
Tardiness: traffic jams may result in late arrival at for employment, business meetings, education, or other settings for which prompt arrival is necessary, resulting in disciplinary action or lost business.
Lost fuel and pollution: congestion wastes fuel and increases air pollution due to increased idling, acceleration, and braking.
Wear-and-tear: idling in traffic, and frequent acceleration and braking for the prupose of traveling only short, creeping distances increases wear on vehicles, leading to more frequent repairs and replacements.
Stress: By increasing motorist stress and frustration, congestion can encourage road rage and cause reduced health of motorists.
Spillover effect: Motorists seeking alternatives to congested main arteries will use secondary roads and sometimes side streets in their place. This will lead to increased volumes of traffic in otherwise quiet residential communties, affecting neighborhood quality and real estate prices.

Traffic congestion in the United States

On Fridays in California, Interstate 5 is often congested as Los Angeles residents travel north for the weekend

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

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

Classification

Quantitative classification of traffic is done in the form of a six letter A-F level of service (LOS) scale defined in the Highway Capacity Manual and used by transportation engineers 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.^[1]

Mathematical theories of traffic congestion

Traffic engineers apply the rules of fluid dynamics to traffic flow, likening it to the flow of a fluid in a pipe. Traffic jam simulations have shown that in heavy but free flowing traffic, jams can arise spontaneously, triggered by minor events such as an abrupt steering maneuver by a single motorist. Such a situation is likened to the freezing of supercooled fluid by traffic scientists^[2].

In the three phase traffic theory of Boris Kerner, congestion is classified into two distinct phases: synchronised flow and wide moving jams. 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 of traffic congestion

Congested roads can be 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, where demand is limited by opportunity cost. Privatization of highways and road pricing are measures that can prevent recurring congestion through economic incentives. Congestion can also happen due to non-recurring highway incidents, such as a crash or construction, 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 market 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 traffic engineer's solution of making a "pipe" large enough to accommodate the total demand for peak-hour vehicle travel, either by widening roadways or increasing "flow pressure" via automated highway systems, Downs advocates greater use of road pricing to reduce congestion, in turn plowing the revenues generated therefrom into public transportation projects.

New evidence is emerging that building more freeways increases traffic congestion (see induced demand hypothesis). The logic being that with extra road capacity, more cars will use the road at any one time and as a result more traffic congestion will arise. It has also been likened to an obese person loosening their belt as a cure for obesity. Public Transport advocates argue that people still need to get from a to b and by adding more roads, it will inevitably create congestion because more people will use the roads. However, it should be noted that there is a finite number of cars, or people able to drive them, at any one time. In areas already with near sufficient road capacity, road building can alleviate congestion, as car use has peaked due to other factors.

Attempts to alleviate traffic congestion

Improvements of junctions, including grade separation and controlled access.
Broadcasting road conditions via traffic reports.
Building new roads and widening of existing ones (this practice is criticised by proponents of the induced demand hypothesis)
Reducing road capacity. This is a counterintuitive and controversial measure based on the reduced demand hypothesis. The argument is that by narrowing a road in corridor, the overall amount of traffic will decrease as drivers switch to alternative modes. Thus the other bottlenecks on the same corridor will also see less traffic and suffer from less congestion. Pedestrianisation of historic city centers, such as in Florence, Italy can alleviate congestion in crowded cities.
Road pricing, or tolls, such as the London congestion charge, a fee levied on vehicle drivers entering the centre of the city.
Restriction of on-road parking spaces. Traffic bottlenecks often occur where land is at a premium, so limited parking space is normally correlated with limited road capacity, for example in city centers. By curbing demand to parking spaces, either through a price mechanism or time limits, the amount of traffic destined to the area can be reduced. In Vienna, for example, public parking is limited to 90 min in the entire core of the city, where the historically narrow streets have limited capacity. This method has the additional benefit of reducing traffic of circling cars looking for a free parking space, which can make up a significant proportion of traffic in city centers.
Cheaper and better public transport offered by the local or provincial government, see Public Transport Funding
Setting of school opening times to avoid problems associated with the school run
High-occupancy vehicle lanes or "carpool lanes"
Quotas on the number of vehicles on the road. There are a number of different strategies:
- The "Cap and trade" method used in Singapore. ^[3] 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 on this densely populated island does not exceed road capacity while avoiding the negative effects of shortages normally associated with quotas. Since demand for cars tends to be inelastic, it results in exorbitant purchase prices.
- Number plate restrictions on alternating 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 drivers will simply purchase a second or third car to circumvent the ban.
Traffic management and prevention of accidents.
Doing away with turns across oncoming traffic at crossroads with traffic lights, which reduces the time needed to go through such crossroads.
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, traffic jams are often caused by curious drivers slowing down to gawk at a traffic 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.
Counterflow. Certain sections of highway operate in the opposite direction on different times of the day/ days of the week, to match asymmetric demand.
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.
ITS Use of Intelligent Transportation Systems, including VMS, CCTV, Traffic Operation Centers, and Remote Traffic Signal Control.

Notes and references

^ Traffic Engineering, Third Edition. Roger P. Roess, Elana S. Prassas, and William R. McShane. ISBN 0-13-142471-8
^ Critical Mass, Philip Ball, ISBN 0-09-945786-5
^ The high cost of motoring in Singapore, Rex S. Toh, Business Horizons, March-April, 1994

External links

[1] Traffic Engineering, Third Edition. Roger P. Roess, Elana S. Prassas, and William R. McShane. ISBN 0-13-142471-8

[ball-2] Critical Mass, Philip Ball, ISBN 0-09-945786-5

[toh-3] The high cost of motoring in Singapore, Rex S. Toh, Business Horizons, March-April, 1994

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