Traffic congestion is a condition on transport that is characterized by slower speeds, longer trip times, and increased vehicular queueing. Traffic congestion on urban road networks has become increasingly problematic in major cities across the globe. Traffic congestion occurs when a volume of traffic or modal split generates demand for space greater than the available street capacity. As demand approaches the capacity of a road (or of the intersections along the road), extreme traffic congestion sets in. When vehicles are fully stopped for periods of time it leads to a traffic jam.
Causes of traffic jams
Some of the causes of traffic congestion include:
- Obstacles in the road causing a blockage and merger. These can be any of the following:
- Double parking.
- Road work.
- Lane closure due to utility work.
- Road narrowing down.
- An accident.
- Road closure for political, sporting or other activities.
- Over-development in areas where the mass transit system is already overcrowded and the road system is inadequate.
- Too many cars for the roadway due to inadequate mass transit options or other reasons.
- Too many trucks on the road due to inadequate rail freight opportunities.
- Too many pedestrians crossing not permitting cars to turn.
- Traffic signals out of sync many times on purpose or occasionally when the computers are malfunctioning.
- Inadequate green time.
- Aggressive driving.
Positive impacts of traffic jams
Traffic congestion has a number of positive effects.
- Congestion has the benefit of encouraging motorists to retime their trips so that expensive road space is in full use for more hours per day.
- The standard response to congestion is to expand road space somehow, perhaps by widening an existing road or else by adding a new road, bridge or tunnel. However, that could well result in increased traffic flow, otherwise known as induced demand, causing congestion to appear somewhere else. Moreover, Braess’ paradox shows that adding road capacity might make congestion worse, even if demand does not increase.
- It has been argued that traffic congestion, by reducing road speeds in cities, could reduce the frequency and severity of road accidents.
Negative impacts of traffic jams
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, and education, 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 increasing air pollution and carbon dioxide emissions owing to increased idling, acceleration and braking.
- 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.
- 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.
- Higher chance of collisions due to tight spacing and constant stopping-and-going.
How to reduce traffic jams
Traffic congestion can be reduced by the following countermeasures.
a. Road infrastructure
- Junction improvements:
- Grade separation, using bridges (or, less often, tunnels) freeing movements from having to stop for other crossing movements.
- Ramp signaling, ‘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.
b. Urban planning and design
City planning and urban design practices can have a huge impact on levels of future traffic congestion, though they are of limited relevance for short-term change.
- Grid plans including fused grid road network geometry, rather than tree-like network topology which branches into cul-de-sacs (which reduce local traffic, but increase total distances driven and discourage walking by reducing connectivity). This avoids concentration of traffic on a small number of arterial roads and allows more trips to be made without a car.
- Zoning laws that encourage mixed-use development, which reduces distances between residential, commercial, retail, and recreational destinations and encourage cycling and walking. Cycling modal share is strongly associated with the availability of local cycling infrastructure.
- Car-free cities, car-light cities, and eco-cities designed to eliminate the need to travel by car for most inhabitants.
- Transit-oriented development are residential and commercial areas designed to maximize access to public transport by providing a transit station or stop (train station, metro station, tram stop, or bus stop).
c. Supply and demand
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.
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 is exorbitant purchase prices for the licenses, pricing out the lower levels of society.
- 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. 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. Some cities have imposed 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 which encourage greater usage of existing alternatives through promotion, subsidies or restrictions:
- Incentives to use public transport, increasing modal shares. This can be achieved through infrastructure investment, subsidies, transport integration, pricing strategies that decrease the marginal cost/fixed cost ratios, improved timetabling and greater priority for buses to reduce journey time e.g., bus lanes or bus rapid transit.
- Cycling promotion through legislation, cycle facilities, subsidies, and awareness campaigns. The Netherlands has been pursuing cycle friendly policies for decades, and around a quarter of their commuting is done by bicycle.
- Promotion of more flexible work place practices either through mode shift or peak spreading.
- Telecommuting encouraged through legislation and subsidies
- Online shopping promotion, potentially with automated delivery booths helping to solve the last mile problem and reduce shopping trips made by car.
d. Traffic management
Use of so-called intelligent transportation systems, which guide traffic:
- Traffic reporting, via radio, GPS and mobile apps, 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.
- Parking guidance and information systems providing dynamic advice to motorists about free parking.
- Active traffic management system to open hard shoulder as an extra traffic lane; it uses CCTV and VMS to control and monitor the traffic’s use of the extra lane.
e. Other countermeasures
- School opening times arranged to avoid rush hour traffic (in some countries, private car school pickup and drop-off traffic are substantial percentages of peak hour traffic).
- Considerate driving behaviour promotion and enforcement. 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.
- Visual barriers to prevent drivers from slowing down out of curiosity. This often includes accidents, with traffic slowing down even on roadsides physically separated from the crash location. This also tends to occur at construction sites, which is why some countries have introduced rules that motorway construction has to occur behind visual barrier.
- Speed limit reductions with lower speeds allowing cars to drive closer together, this 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.
- Lane splitting/filtering, in which some jurisdictions allow motorcycles, scooters and bicycles to travel in the space between cars, buses, and trucks.
- Reduction of road freight avoiding problems such as double parking with innovative solutions including cargo bicycles.