A curb extension is a traffic calming

Traffic calming

Traffic calming is intended to slow or reduce motor-vehicle traffic in order to improve the living conditions for residents as well as to improve safety for pedestrians and cyclists. Urban planners and traffic engineers have many strategies for traffic calming...

 measure, primarily used to extend the sidewalk, reducing the crossing distance and allowing pedestrians about to cross and approaching vehicle drivers to see each other when vehicles parked in a parking lane would otherwise block visibility.

A curb

Curb (road)

A curb, or kerb , is the edge where a raised pavement/sidewalk/footpath, road median, or road shoulder meets an unraised street or other roadway.-Function:...

 extension is an angled narrowing of the roadway and a widening of the sidewalk

Sidewalk

A sidewalk, or pavement, footpath, footway, and sometimes platform, is a path along the side of a road. A sidewalk may accommodate moderate changes in grade and is normally separated from the vehicular section by a curb...

 (pavement or footway in UK

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 usage). This is often accompanied by an area of enhanced restrictions (such as a "no stopping" or "no parking

Parking

Parking is the act of stopping a vehicle and leaving it unoccupied for more than a brief time. Parking on one or both sides of a road is commonly permitted, though often with restrictions...

" zone) and the appropriate visual reinforcement. This is achieved using paint

Paint

Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...

ed road markings (e.g. lines, coloured areas, or chevrons), barriers, bollard

Bollard

A bollard is a short vertical post. Originally it meant a post used on a ship or a quay, principally for mooring. The word now also describes a variety of structures to control or direct road traffic, such as posts arranged in a line to obstruct the passage of motor vehicles...

s, or the addition of pavement or street furniture

Street furniture

Street furniture is a collective term for objects and pieces of equipment installed on streets and roads for various purposes, including traffic barriers,...

 (e.g. planters, lamp standards

Street light

A street light, lamppost, street lamp, light standard, or lamp standard is a raised source of light on the edge of a road or walkway, which is turned on or lit at a certain time every night. Modern lamps may also have light-sensitive photocells to turn them on at dusk, off at dawn, or activate...

, or benches

Bench (furniture)

A bench is a piece of furniture, on which several people may sit at the same time. Benches are typically made of wood, but may also be made of metal, stone, or synthetic materials. Many benches have arm and back rests; some have no back rest and can be sat on from either side. In public areas,...

).

Curb extensions are often used in combination with other traffic calming measures such as chicane

Chicane

A chicane is an artificial feature creating extra turns in a road, used in motor racing and on city streets to slow cars to lower speeds.- Motor Racing :...

s, speed bump

Speed bump

A speed bump is a speed-reducing feature of road design to slow traffic or reduce through traffic, via...

s, or rumble strip

Rumble strip

Rumble strips, also known as sleeper lines, are a road safety feature that alert inattentive drivers to potential danger by causing a tactile vibration and audible rumbling, transmitted through the wheels into the car body...

s, and are frequently sited to "guard" pedestrian crossing

Pedestrian crossing

A pedestrian crossing or crosswalk is a designated point on a road at which some means are employed to assist pedestrians wishing to cross. They are designed to keep pedestrians together where they can be seen by motorists, and where they can cross most safely across the flow of vehicular traffic...

s. In these cases the "squeeze" effect of the narrowed roadway shortens the exposed distance pedestrian

Pedestrian

A pedestrian is a person traveling on foot, whether walking or running. In some communities, those traveling using roller skates or skateboards are also considered to be pedestrians. In modern times, the term mostly refers to someone walking on a road or footpath, but this was not the case...

s must walk.

Uses

The primary use of curb extensions are to improve visibility of pedestrians and reduce their exposure to motor vehicles.

Curb extensions are also in a number of special circumstances:

• To provide additional horizontal space to allow retrofitting of existing sidewalks with ramps, where the sidewalk would otherwise be too narrow.
• To provide additional visibility and protection for pedestrians (particularly child
  Child
  Biologically, a child is generally a human between the stages of birth and puberty. Some vernacular definitions of a child include the fetus, as being an unborn child. The legal definition of "child" generally refers to a minor, otherwise known as a person younger than the age of majority...
  
  ren) when leaving premises
  Premises
  Premises are land and buildings together considered as a property. This usage arose from property owners finding the word in their title deeds, where it originally correctly meant "the aforementioned; what this document is about", from Latin prae-missus = "placed before".In this sense, the word is...
  
  . The curb extension may contain a pedestrian barrier, preventing pedestrians from running
  Running
  Running is a means of terrestrial locomotion allowing humans and other animals to move rapidly on foot. It is simply defined in athletics terms as a gait in which at regular points during the running cycle both feet are off the ground...
  
   straight from the premises over the road.
• In combination with a controlled urban parking scheme, where parking space
  Parking space
  A parking space is a location that is designated for parking, either paved or unpaved.Parking spaces can be in a parking garage, in a parking lot or on a city street. It is usually designated by a white-paint-on-tar rectangle indicated by three lines at the top, left and right of the designated area...
  
  s are shielded from oncoming traffic by the extended sidewalk element.
• To slow and calm traffic, particularly fast traffic turning from a major to a minor road.
• To protect passengers embarking and particularly disembarking from tram
  Tram
  A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...
  
  s, bus
  Bus
  A bus is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers. The most common type of bus is the single-decker bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are...
  
  es, and level-grade urban light rail
  Light rail
  Light rail or light rail transit is a form of urban rail public transportation that generally has a lower capacity and lower speed than heavy rail and metro systems, but higher capacity and higher speed than traditional street-running tram systems...
  
   systems, particularly when retrofitting existing street
  Street
  A street is a paved public thoroughfare in a built environment. It is a public parcel of land adjoining buildings in an urban context, on which people may freely assemble, interact, and move about. A street can be as simple as a level patch of dirt, but is more often paved with a hard, durable...
  
  s.

Curb extensions are also used when retrofit

Retrofit

Retrofitting refers to the addition of new technology or features to older systems.* power plant retrofit, improving power plant efficiency / increasing output / reducing emissions...

ting existing streets to accommodate congestion charging schemes - hitherto wide (sometimes multi-lane

Lane

A lane is a part of the roadway within a road marked out for use by a single line of vehicles in such a way as to control and guide drivers for the purpose of reducing traffic conflicts. Most public roads have at least two lanes, one for traffic in each direction, separated by Lane markings...

) roads are deliberately narrowed to ensure that the charging equipment can see passing vehicles, and the charging equipment (and often bollards or other barrier devices) are placed in the expanded sidewalk area (to prevent drivers from circumventing the charging system's camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

s and detectors).

Design, advantages and disadvantages

The Pedestrian and Bicycle Information Center states that curb extensions must not encroach on travel lanes, bicycle lanes, or shoulders, and should not extend more than 1.8 m (6 ft) from the curb. Some curb extensions are built with the bike lane passing through (making the extension an island, separated from the main sidewalk by a narrow bike lane).

One study has found that pedestrians crossing at curb extensions have to wait for fewer vehicles to pass before a motorist yields to allow them to cross. The study was too small to determine whether other measures of effectiveness, such as percent of pedestrians crossing after a motorist yields, were significant.

Poorly designed curb extensions can pose a hazard to cyclists, as they force cyclists from their position at the road side (or in a roadside bike lane

Segregated cycle facilities

Segregated cycle facilities are marked lanes, tracks, shoulders and paths designated for use by cyclists from which motorised traffic is generally excluded...

) into the narrowed gap. They can also damage vehicles if the curbs extend too close to traffic lanes.

Curb extensions have not been found to affect the speed of vehicle traffic.

Author Randal O'Toole

Randal O'Toole

Randal O'Toole is an American public policy analyst. Although O'Toole studied economics at the University of Oregon, he did not receive a degree in economics...

 argues that curb extensions are used to increase traffic congestion to discourage the use of automobiles by reducing the number of lanes available for automobiles. In particular, curb extensions can be used to block turn lanes, forcing turning vehicles to remain with the forward moving traffic. Curb extensions do prevent drivers from using parking lanes or shoulders as right turn lanes.

However, reducing the crossing distance also reduces the time needed to cross the street. This allows a reduction in the length of the pedestrian phase at signalized intersection, and reduces the time needed to yield to pedestrians at stop sign controlled intersections. This at least partially compensated for any loss in vehicle capacity.

Curb extensions complicate drainage. They obstruct the gutter, so a catch basin is needed at the uphill end to keep a puddle from forming. An alternate solution is placing a gap in the curb, allowing the stormwater in the gutter to irrigate a rain garden

Rain garden

A rain garden is a planted depression that allows rainwater runoff from impervious urban areas like roofs, driveways, walkways, parking lots, and compacted lawn areas the opportunity to be absorbed...

 in the curb extension.

To facilitate street sweeping

Street sweeper

A street sweeper or street cleaner can refer to a person's occupation or a machine that cleans streets, usually in an urban area.-History of street sweeping in the United States:...

, the internal and external curvatures of the extended curb section are moderately graduated.

Retroactively adding curb extensions to older, established communities has been controversial in some instances, due to the installation of the extensions reducing the availability of on-street parking for some properties, particularly in cases where the curb extension has been installed on its own and not in conjunction with a crosswalk.

See also

• Bus bulb / boarder
  Bus bulb
  A bus bulb, also called a bus boarder or bus border, is an arrangement by which a sidewalk is extended outwards for a bus stop; typically the bus bulb replaces roadway that would otherwise be part of a parking lane...
  
  , a specific type of curb extension that is used as a bus stop

External links

• Transport SA (South Australia) explanation of curb extensions
• Portland Dept. of Transportation - document showing curb extensions and the circumstances of their use.
• WalkingInfo.org's detailed discussion of curb extensions
• Information and policy from Kingston, Ontario