Traffic calming

Traffic calming consists of physical design and other measures, including narrowed roads and speed humps, put in place on roads for the intention of slowing down or reducing motor-vehicle traffic and thereby improving the living conditions for residents living along the road as well as to improve safety for pedestrians and cyclists. Urban planners and traffic engineers have many strategies for traffic calming. Such measures are common in Australia and Europe (especially Northern Europe), but less so in North America. Traffic calming is a literal translation of the German word Verkehrsberuhigung - the term's first published use in English was in 1985 by Carmen Hass-Klau.

In its early development in the UK in the 1930s, traffic calming was based on the idea of residential areas protected from through traffic. Subsequently, it was mainly justified on the grounds of pedestrian safety and reduction of the noise and local air pollution that traffic produces. However, car traffic severely impairs the social and recreational functions that streets are now recognized to have. The Livable Streets study by Donald Appleyard (1981) found that residents of streets with light traffic had, on average, three more friends and twice as many acquaintances as the people on streets with heavy traffic which were otherwise similar in dimensions, income, etc.

For much of the twentieth century, streets were designed by engineers who were charged only with ensuring smooth traffic flow and not with fostering the other functions of streets. The basis for traffic calming is broadening traffic engineering to include designing for these functions.

Measures
Traffic engineers refer to three "E's" when discussing traffic calming: engineering, (community) education, and (police) enforcement. Because neighborhood traffic management studies have shown that residents often contribute to the perceived speeding problem within their neighborhoods, instructions on traffic calming (for example in Hass-Klau et al, 1992 ) stress that the most effective traffic calming plans entail all three components--that engineering measures alone will not produce satisfactory results.

Engineering measures
Engineering measures involve physically altering the road layout or appearance to actively or passively retard traffic by increasing the cognitive load of driving. Measures include speed humps, chicanes, curb extensions, and living street and shared space type schemes. The town of Hilden in Germany has achieved a rate of 24% of trips being on two wheels, mainly via traffic calming and the use of 30 km/h or 20 mph zones. As of 1999, the Netherlands had over 6000 woonerven where cyclists and pedestrians have legal priority over cars and where a motorised speed limit of "walking speed" applies. However, some UK and Irish "traffic calming" schemes, particularly involving road narrowings, are viewed as extremely hostile and have been implicated directly in death and injury to cyclists and pedestrians.

A number of visual changes to roads are being made to many streets to cause more attentive driving, reduced speeds, reduced crashes, and a greater tendency to yield to pedestrians. Visual traffic calming includes lane narrowings (9-10'), road diets (reduction in lanes), use of trees next to streets, on-street parking, and buildings placed in urban fashion close to streets.

Physical devices include speed humps, speed cushions, and speed tables, sized for the desired speed. Such measures normally slow cars to between 10 and 25 mph. Most devices are made of asphalt or concrete but rubber traffic calming products are emerging as an effective alternative with several advantages.

Traffic calming can include the following engineering measures, grouped by similarity of method:
 * Narrowing: Narrowing traffic lanes differs from other road treatments by making slower speeds seem more natural to drivers and less of an artificial imposition as opposed to most other treatments, which physically force lower speeds or restrict route choice. Such means include:
 * Narrower traffic lanes — streets can be narrowed by extending the sidewalk, adding bollards or planters, or adding a bike lane or on-street parking.
 * Curb extensions (also called bulbouts) that narrow the width of the roadway at pedestrian crossings
 * Chokers, which are curb extensions that narrow the roadway to a single lane at points
 * Road diets: actively remove a lane from the street.
 * Allowing parking on one or both sides of a street to reduce the number of driving lanes.
 * Pedestrian refuges or small islands in the middle of the street.
 * Converting one-way streets into two-way streets.
 * Vertical deflection: These include:
 * Speed bumps, sometimes split or offset in the middle to help emergency vehicles reduce delay
 * Speed humps, parabolic devices that are less aggressive than speed bumps and used on residential streets.
 * Speed cushions, two or three small speed humps sitting in a line across the road that slow cars down but allows (wider) emergency vehicles to straddle them so as not to slow emergency response time.
 * Speed tables, long flat-topped speed humps that slow cars more gradually than humps
 * Raised pedestrian crossings, which act as speed tables, often situated at intersections.
 * Changing the surface material or texture (for example, the selective use of brick or cobblestone).
 * Horizontal deflection, i.e. make the vehicle swerve slightly. These include:
 * Chicanes, which create a horizontal deflection that causes vehicles to slow as they would for a curve.
 * Pedestrian refuges again can provide horizontal deflection, as can curb extensions and chokers.
 * Block or restrict access. Such traffic calming means include:
 * Median diverters to prevent left turns or through movements into a residential area.
 * Converting an intersection into a cul-de-sac or dead end.
 * Boom barrier, restricting through traffic to authorised vehicles only.
 * Closing of streets to create pedestrian zones.
 * Other means

Enforcement and education measures
Enforcement and education measures for traffic calming include:
 * Reducing speed limits near institutions such as schools and hospitals (see below)
 * Vehicle activated sign, signs which react with a message if they detect a vehicle exceeding a pre-determined speed.
 * Watchman, traffic calming system

Speed limits
Speed reduction has traditionally been attempted by the introduction of statutory speed limits. Traffic speeds of 30 km/h (20 mph) and lower are said to be more desirable on urban roads with mixed traffic. The Austrian city of Graz, which has achieved steady growth in cycling, has applied 30 km/h limits to 75% its streets since 1994. Zones where speeds are set at 30 km/h (or 20 mph) are gaining popularity as they are found to be effective at reducing crashes and increasing community cohesion. Speed limits which are set below the speed that most motorists perceive to be reasonable for the given road require additional measures to improve compliance. Attempts to improve speed limit observance are usually by either education, enforcement or road engineering. Education can mean publicity campaigns or targeted road user training.

Speed limit Enforcement techniques include: direct police action, automated systems such as speed cameras or vehicle activated signs or traffic lights triggered by traffic exceeding a preset speed threshold. One cycling expert argues for placing direct restrictions on motor-vehicle speed and acceleration performance. An EU report on promoting walking and cycling specifies as one of its top measures comprehensive camera-based speed control using mainly movable equipment at unexpected spots. The Netherlands has an estimated 1,500 speed/red-light camera installations and has set a target for 30 km/h limits on 70% of urban roads. The UK has more than 6,000 speed-cameras, which raised more than £100 million in fines in 2006/07.

Europe
Traffic calming has been successfully used for decades in cities across Europe. For example, a living street (sometimes known as Home zones or by the Dutch word woonerf, as the concept originated in the Netherlands) towards the end of the 1960s, initially in Delft, is a street in which the needs of car drivers are secondary to the needs of users of the street as a whole; traffic calming principles are integrated into their design. From the Netherlands, application spread rapidly to Germany, starting in North-Rhine Westphalia in 1976, becoming very widespread by the early 1980s.

North America
More recently, in response to growing numbers of traffic accidents and speeding problems, cities across North America have begun creating traffic calming programs to improve safety and liveability on residential streets. Many municipalities create asphalt or concrete measures, although preformed rubber products that are easier to install and consistently meet standardized requirements are becoming increasingly popular.

Studies
A Cochrane Review of studies found that there is evidence to demonstrate the efficacy of traffic calming measures in reducing traffic-related injuries and may even reduce deaths. However, the review found that more evidence is needed to demonstrate its efficacy in low income countries.

According to right-wing economic commentator and smart growth critic Randal O'Toole, the main goal of traffic calming is to increase congestion of motor traffic for the social engineering. He claims that some traffic calming measures such as reverting one way roads into two-way roads or creating "bump outs" have increased motor traffic congestion, resulted in more accidents and increased pedestrian fatalities. According to Florida urban planner Dom Nozzi "[c]ongestion is a powerful disincentive for sprawl; sprawl that steamrolls outlying ecosystems. With congestion, the sprawl market wither."