Traffic Calming

Definitions of traffic calming vary, but they all share the goal of reducing vehicle speeds, improving safety, and enhancing quality of life. Some include all three "Es," traffic education, enforcement, and engineering.  Most definitions focus on engineering measures to change driver behavior. Some focus on engineering measures that compel drivers to slow down, excluding those that use barriers to divert traffic. The following are some example definitions.

Traffic calming involves changes in street alignment, installation of barriers, and other physical measures to reduce traffic speeds and/or cut-through volumes, in the interest of street safety, livability, and other public purposes

Traffic calming involves altering of motorist behavior on a street or on a street network. It also includes traffic management, which involves changing traffic routes or flows within a neighborhood

Traffic calming consists of operational measures such as enhanced police enforcement, speed displays, and a community speed watch program, as well as such physical measures as edge lines, chokers, chicanes, traffic circles, and (for the past four years) speed humps and raised crosswalks.

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                      Types of Traffic Calming Measures

Traffic calming measures can be separated into two groups based on the main impact intended. Volume control measures are primarily used to address cut-through traffic problems by blocking certain movements, thereby diverting traffic to streets better able to handle it. Speed control measures are primarily used to address speeding problems by changing vertical alignment, changing horizontal alignment, or narrowing the roadway. The distinction between the two types of measures is not as clear as their names suggest, since speed control measures frequently divert traffic to alternate routes, and volume control measures usually slow traffic.

                             Speed Control Measures

Speed Humps: Speed humps are rounded raised areas placed across the roadway. They are generally 10 to 14 feet long (in the direction of travel), making them distinct from the shorter "speed bumps" found in many parking lots, and are 3 to 4 inches high.

Speed Tables: Speed tables are flat-topped speed humps often constructed with brick or other textured materials on the flat section. Speed tables are typically long enough for the entire wheelbase of a passenger car to rest on the flat section.

Raised Crosswalks: Raised crosswalks are Speed Tables outfitted with crosswalk markings and signage to channelize pedestrian crossings, providing pedestrians with a level street crossing.
Raised Intersections: Raised intersections are flat raised areas covering an entire intersection, with ramps on all approaches and often with brick or other textured materials on the flat section. They usually raise to the level of the sidewalk, or slightly below to provide a "lip" that is detectable by the visually imparied.

Textured Pavement: Textured and colored pavement includes the use of stamped pavement or alternate paving materials to create an uneven surface for vehicles to traverse. They me be used to emhasize either an entire intersection or a pedestrian crossing, and are sometimes used along entire street blocks.

Traffic Circles: Traffic circles are raised islands, placed in intersections, around which traffic circulates. They are good for calming intersections, especially within neighborhoods, where large vehicle traffic is not a major concern but speeds, volumes, and safety are problems.

Roundabouts: Roundabouts require traffic to circulate counterclockwise around a center island. Unlike Traffic Circles, roundabouts are used on higher volume streets to allocate right-of-way between competing movements.

Chicanes: Chicanes are curb extensions that alternate from one side of the street to the other, forming S-shaped curves. Chicanes can also be created by alternating on-street parking, either diagonal or parallel, between one side of the street and the other.

Realigned Intersections: Realigned intersections are changes in alignment that convert T-intersections with straight approaches into curving streets that meet at right-angles. A former "straight-through" movement along the top of the T becomes a turning movement.

Neckdowns: Neckdowns are curb extensions at intersections that reduce the roadway width from curb to curb. They "pedestrianize" intersections by shortening crossing distances for pedestrians and drawing attention to pedestrians via raised peninsulas.

Chokers: Chokers are curb extensions at midblock locations that narrow a street by widing the widewalk or planting strip. If marked as crosswalks, they are also known as safe crosses. Two-lane chokers leave the street cross section with two lanes that are narrower than the normal cross section. One-lane chokers narrow the width to allow travel in only one direction at a time, operating similarly to one-lane bridges.

Center Island Narrowings: A center island narrowing is a raised island located along the centerline of a street that narrow the travel lanes at that location. Center island narrowings are often landscaped to provide a visual amenity. Placed at the entrance to a neighborhood, and often combined with textured pavement, they are often called "gateway islands." Fitted with a gap to allow pedestrians to walks through at a crosswalk, they are often called "pedestrian refuges."

                          Volume Control Measures

Full Closures: Full street closures are barriers placed across a street to completed close the street to through-traffic, usually leaving only sidewalks open. They are good for locations with extreme traffic volume problems and several other measures have been unsuccessful.

Half Closures: Half closures are barriers that block travel in one direction for a short distance on otherwise two-way streets. They are good for locations with extreme traffic volume problems and non-restrictive measures have been unsuccessful.

Diagonal Diverters: Diagonal diverters are barriers placed diagonally across an intersection, blocking through movements and creating two separate, L-shaped streets. Like half closures, diagonal diverters are often staggered to create circuitous routes through the neighborhood as a whole, discouraging non-local traffic while maintaining access for local residents.

Median Barriers: Median barriers are islands located along the centerline of a street and continuing through an intersection so as to block through movement at a cross street.

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                        History of Traffic Calming

    European traffic calming began as a grassroots movement in the late 1960s. Angry residents of the Dutch City of Delft fought cut-through traffic by turning their streets into woonerven, or "living yards."  This was followed by the development of European slow streets (designed for 30 kph or 20 mph) in the late 1970s; the application of traffic calming principles to intercity highways through small Danish and German towns in the 1980s; and the treatment of urban arterials in area wide schemes, principally in Germany and France, also in the 1980s.

    In the U.S., a version of traffic calming was practiced as early as the late 1960s and early 1970s in such places as Berkeley, CA, Seattle, WA and Eugene, OR.  The first national study of traffic calming was completed circa 1980.  It explored residential preferences related to traffic, collected performance data on speed humps, and reviewed legal issues.

A fantastic site for all that is Traffic Calming is www.trafficcalming.org