page_title: Traffic congestion
section_title: Road infrastructure
hierarchical_section_title: Traffic congestion / Countermeasures / Road infrastructure
caption_attribution_description: A38(M) For Birmingham - Near The End, Birmingham Skyline
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: 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(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
caption: The A38M Aston Expressway in Aston, towards central Birmingham - the lanes are controlled via the overhead gantries, which reverse the flow of one lane (making 4 in one direction, 2 in the other and a central buffer lane) during peak times accordingly.
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