Traffic congestion

By Matt Burdett, 11 May 2020

In this article, we look at traffic congestion patterns, trends and impacts.

  • Traffic congestion in Dhaka, Bangladesh. Traffic regularly snarls up into several hours worth of delays. Source: By the author.

Defining traffic congestion

While there is no internationally agreed definition of traffic congestion, most sources agree that there are at least three elements, which refer to the physical effects noticed when there is a lot of traffic (PWC, n.d.):

  • A breakdown in traffic flow
  • The reduction in vehicle speed
  • The increase in vehicles in the road space (‘crowding’)

The UK government’s Department for Transport (2012) recognises that, as well as these physical effects of congestion, there is also a relative judgement of what makes a ‘congested’ road. This relative judgement varies from person to person. A person from a small town in El Salvador might consider a delay of five minutes to be evidence of congestion, while a person from the megacity of Dhaka in Bangladesh will be used to two hour journeys across short distances. This means “congestion can therefore also be defined in terms of the difference between users’ expectations of the road network and how it actually performs” (Department for Transport [UK], 2012).

A further way to measure congestion is to consider ‘journey time reliability’. This is a comparison between the time it usually takes to complete a journey, compared to the variation of that journey time over a period of weeks or months. This helps determine not only if congestion is taking place, but the severity of the congestion.

Causes

PWC (n.d.), the international accountancy firm, identify six contributors to urban traffic congestion:

  • Economic expansion
  • Demographic change, and urbanization
  • Transport disruption
  • E-commerce and the growth of delivery vehicles
  • Lack of good infrastructure
  • Inconsistency in policies and programmes to solve the problem

The causes of congestion rarely happen in isolation. For example, economic expansion doesn’t necessarily mean an increase in congestion if the development of infrastructure is planned to meet demand, and inconsistent policies are likely to fail to account for demographic change.

Patterns of traffic congestion in urban areas

Patterns of traffic congestion can be seen spatially or temporally. Spatial patterns relate to the distribution of congestion over a physical space – in this case, where they are in the city. Temporal patterns are repeated over time. Temporal patterns are slightly different to trends, which show an overall increase or decrease over time. A temporal pattern may show a repeated event, such as a morning rush hour, but it doesn’t necessarily grow or shrink over a longer period of time.

Spatial patterns

Spatial patterns of congestion are commonly seen using tools such as Google Maps or Waze (with traffic congestion settings turned on). However, these show the ‘live’ pattern. Geographers are generally interested in the repeated patterns, which tend to show that there are locations of congestion where it is repeated.

These focus points are often junctions between major roads commonly known as bottlenecks. In fact, they are nodal points. A node is a meeting point between two or more links on a network. When traffic meets, it is forced to slow down as the incoming traffic joins existing traffic. This may mean traffic lights, roundabouts or regular junctions, or very large infrastructure such as the on/off-ramps from highways. These are generally designed to slow traffic down, which can help to reduce congestion by improving the flow of traffic through the nodal point.

Temporal patterns

Temporal patterns generally show how traffic congestion increases at certain times of day or the week. A daily pattern is known as a ‘diurnal’ pattern.

In almost every city there are two rush hours, as people travel to work in the morning and back to their home in the afternoon. The morning rush hour is usually denser, as most people begin work at around the same time between 8am and 10am.

The afternoon rush hour is often longer, and there are more vehicles on the road. This is for several reasons. Firstly, there are more people on the road doing non-rush hour travel, such as shopping. Secondly, there are more deliveries due to businesses sending the products they have worked on before the end of the day. Thirdly, the economy is transitioning between the day-time economy (shops, offices, factories, education and so on) into the night-time economy (cinemas, restaurants, bars, theatres) which means more people are moving between more points in the network.

A further consideration is the type of person on the road at each point in the day. During the morning rush-hour, most people are habitual travellers meaning they have a defined route to work. They find the most efficient route and it rarely varies. In the afternoon rush hour, there are more people travelling who don’t do the journey very often so they may use inefficient routes through the urban area.

Trends of traffic congestion in urban areas

Aggregate data is difficult to find, but cities around the world report that traffic congestion is going up.

This may be because of several things:

  • Increased urban population
  • Growing wealth encourages people to buy cars and move away from more efficient public transportation
  • Increasing formal sector employment requires travel to a place of work, rather than working from home
  • The size of urban areas is expanding, requiring more transport across the city to get from one place to another

Impacts of traffic congestion

Traffic congestion causes several problems for the urban area. The Department for Transport [UK] (2012) identified the following major impacts:

  • Slower speeds
  • Longer journey times
  • More queues at bottlenecks
  • Less predictable journey times

This all contributes to environmental problems such as air pollution (see the full case study of air pollution in New York City elsewhere on this site), and economic problems caused by the extra costs involved. Urban congestion cost 6.9 billion hours and made urban commuters use an extra 3.1 billion gallons of fuel in the United States alone in 2014 (Bopp et al., 2018).

Solutions to traffic congestion

Cities can do several things to combat traffic congestion. These can be split into ‘supply side’ or ‘demand side’ management. ‘Supply side’ management aims to reduce congestion by increasing the amount of road space for vehicles to use. ‘Demand side’ management aims to reduce congestion by decreasing the amount of traffic at any one time. Some examples of strategies are listed in the table below.

Supply side

Demand side

Improve roads.

Build new roads.

Create a ‘Green Zone’ in which polluting vehicles are not allowed to enter city centres, such as the one in London.

Limits on cars being allowed to drive. These include permission based on the number or letters of the registration plate on the car.

No car days.

Develop public transport.

Build better infrastructure

Traffic congestion is a major problem around the world. The worst traffic congestion is in cities which don’t have strong infrastructure (see graph below). This seems to suggest that the solution is to build better infrastructure. This is an example of ‘supply side’ management, i.e. reducing congestion by increasing the supply of roads for traffic to use, and is known as Transportation System Management (TSM).

However, this is a false economy: studies such as the UK’s ‘The Impact of Road Projects in England’ show that when cities focus on building better infrastructure, the amount of traffic simply grows to fill the available space (Sloman, Hopkinson and Taylor, 2017) because the shorter travel time is an incentive for more people to do more journeys. This is known as ‘induced demand’ (Campaign for Better Transport, n.d.). Eventually the new, larger road becomes clogged up with traffic too.

Active Traffic and Demand Management

An alternative solution being practiced in many cities is Active Traffic and Demand Management (ATDM). ATDM is an example of ‘demand side’ management: encourage fewer people to use the roads at the same time.

There are very strong links to the development of so-called Smart Cities. Strategies include:

  • Real-Time Traveler Information Systems like Waze and Google Maps. Individual travellers can see if the traffic is bad and made a decision not to add to it, or the app itself will direct users away from the congestion.
  • Ride-sharing apps to encourage individuals to share vehicles if they’re going in the same direction.
  • Road pricing, whereby sensors in the car are triggered and add a charge for the use of roads that are congested.

Sources

Bopp et al., 2018. Benefits and Risks of Bicycling. In Bicycling for Transportation: An Evidence-Base for Communities, Pages 21-44. https://www.sciencedirect.com/science/article/pii/B9780128126424000027 Accessed 8 May 2020.

Campaign for Better Transport, n.d. New roads create new traffic. https://bettertransport.org.uk/roads-nowhere/induced-traffic Accessed 8 May 2020.

Department for Transport [United Kingdom], 2012. An introduction to the Department for Transport’s congestion statistics. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/51125/An_introduction_into_the_Department_for_Transport_s_congestion_statistics.pdf Accessed 22 February 2020.

McCarthy, 2019. The World’s Worst Cities For Traffic Congestion [Infographic]. https://www.forbes.com/sites/niallmccarthy/2019/06/05/the-worlds-worst-cities-for-traffic-congestion-infographic/#f3279c312bcf Accessed 8 May 2020.

PWC, n.d. Mobility insights: tackling congestion. https://www.pwc.com/us/en/industries/industrial-products/library/mobility-insights-tackling-congestion.html Accessed 22 February 2020.

Sloman, Hopkinson and Taylor, 2017. The Impact of Road Projects in England. Available at https://www.cpre.org.uk/resources/the-impact-of-road-projects-in-england/ Accessed 8 May 2020.


Traffic congestion: Learning activities

Questions

  1. Briefly define ‘traffic congestion’. [1]
  2. Identify three metrics that could be used to measure traffic congestion. [2]
  3. Outline the typical spatial pattern of congestion in an urban area. [2]
  4. Explain the pattern you described in (3). [2]
  5. Describe the diurnal variation in traffic congestion in a typical city, starting from 12 midnight and continuing to the following 12 midnight. [3]
  6. Explain the diurnal variation you described in (5). [2+2]
  7. Describe and explain the impacts of traffic congestion. [2+2]
  8. For your nearest city, identify, describe and justify what the most effective traffic congestion management plan would include: supply or demand side management? [10]

Other tasks

Look at Google Maps, Waze or a similar mapping app and turn on the traffic heat map. Briefly look for the points of congestion in your nearest city and identify:

  • The general distribution (e.g. central / outskirts)
  • Whether congestion is focused on nodal points in a network
  • Whether this congestion is linked to the development of bigger roads e.g. highways
  • Suggest reasons for the distribution of current traffic congestion
  • Suggest whether this distribution is likely to be the same in six, twelve and eighteen hours from now.

Going further

This free-to-access article provides a wealth of information about the links between congestion and transport networks. https://www.sciencedirect.com/science/article/pii/S2352146516305750


© Matthew Burdett, 2020. All rights reserved.

All secondary material on this site is clearly referenced and may be subject to copyright restrictions by the original authors. All original material on this page is subject to copyright.