page_title: Traffic congestion
section_title: Supply and demand
hierarchical_section_title: Traffic congestion / Countermeasures / Supply and demand
caption_attribution_description: English: Traffic jam &amp; an empty bike lane.
context_page_description: Traffic congestion is a condition on transport that is characterised by slower speeds, longer trip times, and increased vehicular queueing. Traffic congestion on urban road networks has increased substantially since the 1950s. When traffic demand is great enough that the interaction between vehicles slows the speed of the traffic stream, this results in some congestion. While congestion is a possibility for any mode of transportation, this article will focus on automobile congestion on public roads.
  As demand approaches the capacity of a road, extreme traffic congestion sets in. When vehicles are fully stopped for periods of time, this is known as a traffic jam or a traffic snarl-up. Traffic congestion can lead to drivers becoming frustrated and engaging in road rage.
  Mathematically, traffic is modeled as a flow through a fixed point on the route, analogously to fluid dynamics.
context_section_description: 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). For example, when new lanes are created, households with a second car that used to be parked most of the time may begin to use this second car for commuting. Reducing road capacity has in turn been attacked as removing free choice as well as increasing travel costs and times, placing an especially high burden on the low income residents who must commute to work.
  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, Stockholm congestion tax and the use of high-occupancy toll lanes, predominately in North America.
  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, Manila 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 Delanoë, 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 whic
caption: Bike lane constructed in areas of low space to encourage use of human-sized transportation
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