Traffic Calming
Roadway Design to Reduce Traffic Speeds and Volumes
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TDM
Encyclopedia
Victoria Transport Policy Institute
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Updated
August 28, 2007
This chapter describes the
concept of Traffic Calming, which refers to various roadway design features
intended to reduce traffic speeds and volumes.
Description
Traffic Calming (also called Traffic Management) refers to various design features and strategies intended to reduce vehicle traffic speeds and volumes on a particular roadway. Table 1 describes some of these strategies. Traffic Calming projects can range from minor modifications of an individual street to comprehensive redesign of a road network. Home Zones refers to an area with extensive Traffic Calming. Traffic Calming is becoming increasingly accepted by transportation professionals and urban planners.
Table 1 Traffic Calming Strategies and Devices
|
Type |
Description |
|
Curb extensions “pinch points” |
Curb extensions, planters, or centerline traffic islands that narrow traffic lanes to control traffic and reduce pedestrian crossing distances. Also called “chokers.” |
|
Speed tables, raised crosswalks |
Ramped surface above roadway, 7-10 cm high, 3-6 m long. |
|
Mini-circles |
Small traffic circles at intersections. |
|
Median island |
Raised island in the road center (median) narrows lanes and provides pedestrian with a safe place to stop. |
|
Channelization islands |
A raised island that forces traffic in a particular direction, such as right-turn-only. |
|
Tighter corner radii |
The radius of street corners affects traffic turning speeds. A tighter radius forces drivers to reduce speed. It is particularly helpful for intersections with numerous pedestrians. |
|
Speed humps |
Curved 7-10 cm high, 3-4 m long hump. |
|
Rumble Strips |
Low bumps across road make noise when driven over. |
|
Chicanes |
Curb bulges or planters (usually 3) on alternating sides, forcing motorists to slow down. |
|
Roundabouts |
Medium to large circles at intersections (Kittelson, 2000). |
|
Pavement treatments |
Special pavement textures (cobbles, bricks, etc.) and markings to designate special areas. |
|
Bike lanes |
Marking bikelanes narrows traffic lanes. |
|
“Road diets” |
Reducing the number and width of traffic lanes, particularly on arterials. |
|
Horizontal shifts |
Lane centerline that curves or shifts. |
|
2-lanes narrow to 1-lane |
Curb bulge or center island narrows 2-lane road down to 1-lane, forcing traffic for each direction to take turns. |
|
Semi-diverters, partial closures |
Restrict entry/exit to/from neighborhood. Limit traffic flow at intersections. |
|
Street closures |
Closing off streets to through vehicle traffic at intersections or midblock |
|
“Neotraditional” street design |
Streets with narrower lanes, shorter blocks, T-intersections, and other design features to control traffic speed and volumes. |
|
Perceptual Design Features |
Patterns painted into road surfaces and other perceptual design features that encourage drivers to reduce their speeds. |
|
Street Trees |
Planting trees along a street to create a sense of enclosure and improve the pedestrian environment. |
|
Woonerf |
Streets with mixed vehicle and pedestrian traffic, where motorists are required to drive at very low speeds. |
|
Traffic speed reduction programs. Increased enforcement of speeding violations. |
This table summarizes various Traffic Calming devices and strategies. For illustrations see www.pedbikeimages.org and DKS Associates, 2002.
Traffic Calming involves Context Sensitive Design practices, which means that roadway planners and engineers have flexible standards that can accommodate community values and balanced objectives. New Urbanism incorporates Traffic Calming features into the design of new developments and urban redevelopment. It can make urban streets safer and quieter. It can increase residential property values and local economic activity.
Figure 1 Speed Table
This illustrates a speed table used to limit traffic speeds on a residential street. (Photo curtsey of Urban Engineers)
Traffic Calming is one component of Area Traffic Management, which includes various strategies to control traffic volumes, control traffic speeds, manage transportation demand, educate and enforce traffic and pedestrian facility rules, improve the Streetscape design, and improve street environments (City of Ottawa, 2004).
Traffic Calming changes Streetscape design to give greater emphasis to pedestrians, cyclists and residents. It often involves Reallocating Road Space to increase the portion of right-of-way devoted to bicycle lanes, sidewalks and greenspace. Some features, such as wider sidewalks and improved crosswalks, support Universal Design objectives (making transportation systems accommodate people with disabilities and other special needs). Street Reclaiming emphasizes action by neighborhood residents to change the way their streets are perceived and used to better accommodate nonmotorized activities.
Some research indicates that improved roadway landscaping and tree planting encourages walking and reduces accident rates (Naderi, 2002). Trees can be particularly beneficial in hot areas where they provide shade.
Most Traffic Calming projects are implemented on urban streets with low to moderate traffic volumes, but some strategies can reduce traffic speeds and improve pedestrian conditions on suburban streets, arterials and highways. Ponnaluri and Groce (2005) describe how speed humps are successfully implemented on moderate volume suburban roads, significantly reducing traffic speeds. Highway traffic speed control strategies can include visual messages (Fildes, et al., 1999; Meyer, 2001), gateways and roundabouts (Hass-Klau, et al, 1992; Kittelson, 2000), and special design treatments for highways that bisect towns (DEA & Associates, 1999).
Road
Diets and Environmentally Adopted Through Roads refers to Traffic
Calming applied to higher-volume arterials (Burden and Lagerway, 1999; CORDIS,
1999; CTRE, 2006). Road diets typically involve converting four traffic lanes
to three traffic lanes, with a center turn lane and bicycle lanes, and various
pedestrian and aesthetic improvements. This is suitable for roads with up to
20,000 average motor vehicles per day. Stout, et al (2006) found that
conversion of four-lane undivided roadways to three-lane cross-sections in
typical
Table 2 Road Diet Crash Reduction
Impacts (
|
Roadway Location |
Date Change |
ATD Before |
ADT After |
Collision Reduction |
|
|
April 1995 |
11,872 |
12,427 |
24 to 10 (58%) |
|
|
December 1972 |
19,421 |
20,274 |
45 to 23 (49%) |
|
Ballard Area |
January 1994 |
10,549 |
11,858 |
18 to 7 (61%) |
|
|
January 1994 |
12,336 |
13,161 |
15 to 6 (60%) |
|
Queen Ann Area |
June 1991 |
13,606 |
14,949 |
19 to 16 (59%) |
|
NW 85th to NW 65th |
October 1995 |
9,727 |
9,754 |
14 to 10 (28%) |
This table summaries the crash reduction effects of road diets on
major arterials in
In previous decades many urban arterials were converted to
one-way traffic to maximize traffic speeds and volumes. Some of these are now
being converted back to two-way traffic in order to reduce traffic speeds and
create more pedestrian-friendly streets. One study of such conversions in 22
Modern RoundaboutsA roundabout is an intersection built with a circular island around which traffic rotates in one direction. Many older roundabouts (which were also called traffic circles or rotaries) were built primarily as a location for a fountain or statue, with little regard to traffic principles. As a result, there has been considerable variation in design features and traffic regulation, causing confusion and accidents. For many years roundabouts were unpopular with the public and traffic professionals. During
the late Twentieth Century, traffic engineering organizations developed
roundabout design standards and management practices to maximize traffic
efficiency and safety. These are called “Modern Roundabouts.” They have the
following features. ·
Yield at Entry. Traffic entering the roundabout yields the right-of-way to the
circulating traffic. This prevents traffic from locking-up and allows free
flow movement. ·
Deflection. The entry lane is designed with a small deflector island to
reinforce the yielding process and slow traffic. ·
Limited size. Modern roundabouts usually have just one, and never more than two,
rotating lanes. In
addition, there are mini-roundabouts, which are small traffic circles located
within local intersections. They still require yield-at-entry but do not have
a deflector island. Research
has shown that roundabouts can improve reduce vehicle stops and delays,
reduce traffic speeds, and increase safety compared with other intersection
designs. They are also used to provide a gateway or aesthetic feature. As a
result, roundabouts are once again being promoted by traffic engineers and
planners, and are an important Traffic Calming tool. They are increasingly
common throughout the world. To maximize safety and establish consistency it
is very important that all roundabouts be designed (and existing ones
redesigned) to reflect Modern Roundabout principles. ResourcesAlex Ariniello, Are Roundabouts Good for Business?, TRB (http://pubsindex.trb.org/document/view/default.asp?lbid=775405), Case study FHWA, Roundabout Safety Comes to America, Federal Highway Administration (www.tfhrc.gov/pubrds/fall95/p95a41.htm), 1995. Article about the history of the modern roundabout, its characteristics and safety improvement record. FHWA, Roundabouts: An Informational Guide, US Department of Transportation Federal Highway Administration (www.tfhrc.gov/safety/00068.htm) George Jacquemart, Modern Roundabout Practice in the United States, NCHRP Synthesis 264, Transportation Research Board (www.trb.org), 1998. Kittelson and Associates, Roundabouts: An Informational
Guide, NYDOT, Modern Roundabouts: Guidance for Design Engineers and Users, New York State Department of Transportation (www.dot.state.ny.us/roundabouts/howto.html). Detailed instruction for vehicle, pedestrian, and bicycle roundabout users with animated graphics. Roundabouts and Traffic Circles, ABNA Engineering (www.abnaengineering.com/abna), 2000. Information on roundabouts by leading designers, Michael Wallwork and Richard M. Barnett Jr. Roundabouts, an Informational Guide, Roundabout SIDRA Design (www.akcelik.com.au/SIDRA/roundabouts.htm), provides roundabout planning and design tools. WSDOT, Roundabout Information and Benefits (www.wsdot.wa.gov/Projects/SR539/I5_Access/Tenmile_Border/Roundabouts.htm#13) WSDOT, What is a Roundabout? Washington State Department of Transportation (www.wsdot.wa.gov/Projects/roundabouts), |
Traffic Calming measures must be carefully designed and managed to avoid degrading travel conditions for cyclists and visually impaired pedestrians. Unnecessary stop signs are a hindrance to cycling. On arterials, curb extensions and chicanes should not intrude into bicycle travel lanes (regardless of whether they are officially designated as bike lanes) and force cyclists to compete for road space with higher speed traffic. Street closures should allow access to nonmotorized modes. While small, slow speed, single lane traffic circles are easily negotiated by cyclists and people with visual disabilities, larger double-lane roundabouts with 20 km/h or higher traffic speeds can be difficult to negotiate.
How It Is Implemented
Traffic Calming programs are usually implemented by local engineering departments. These programs involve educating planners and traffic engineers about Traffic Calming strategies, establishing policies and guidelines for implementing Traffic Calming projects, and developing funding sources. Specific Traffic Calming projects may be initiated by neighborhood requests, traffic safety programs, or as part of community redevelopment. Street Reclaiming is initiated and organized by neighborhood residents.
Travel Impacts
Traffic Calming reduces vehicle traffic speeds and sometimes volumes. The table below summarizes the traffic speed impacts of various Traffic Calming devices. Even where speed reductions are small, Traffic Calming tends to reduce the highest traffic speeds (i.e., the fastest 5-15% of vehicles), which provides greater safety and noise reduction benefits than indicated by average reductions. Traffic studies find that for every 1 meter increase in street width the 85th percentile vehicle traffic speed increases 1.6 kph, and the number of vehicles traveling 8 to 16 kph [5 or 10 mph] or more above the speed limit increases geometrically (“Appendix,” DKS Associates, 2002). That study also found that as residential street traffic speeds increase, neighborhood livability ratings decline.
Table 3 Speed Impacts of Traffic Calming
Measures (
|
|
Sample Size |
Avg. Speed Afterward (mph) |
Avg. Speed Change |
Avg. % Change |
|
12' Humps |
179 |
27.4 |
-7.6 |
-22 |
|
14' Humps |
15 |
25.6 |
-7.7 |
-23 |
|
22' Tables |
58 |
30.1 |
-6.6 |
-18 |
|
Longer Tables |
10 |
31.6 |
-3.2 |
-9 |
|
Raised Intersections |
3 |
34.3 |
-0.3 |
-1 |
|
Circles |
45 |
30.2 |
-3.9 |
-11 |
|
Narrowings |
7 |
32.3 |
-2.6 |
-4 |
|
One-Lane Slow Points |
5 |
28.6 |
-4.8 |
-14 |
|
Half Closures |
16 |
26.3 |
-6.0 |
-19 |
|
Diagonal Diverters |
7 |
27.9 |
-1.4 |
-0.5 |
From www.trafficcalming.org.
Traffic Calming tends to reduce total vehicle mileage in an area by reducing travel speeds and improving conditions for walking, cycling and transit use (Crane, 1999; Morrison, Thomson and Petticrew, 2004). Residents in neighborhoods with suitable street environments tend to walk and bicycle more, ride transit more, and drive less than comparable households in other areas. One study found that residents in a pedestrian friendly community walked, bicycled, or rode transit for 49% of work trips and 15% of their non-work trips, 18- and 11-percentage points more than residents of a comparable automobile oriented community (Cervero and Radisch, 1995). Another study found that walking is three times more common in a community with pedestrian friendly streets than in otherwise comparable communities that are less conducive to foot travel (Moudon, et al, 1996). Where Road Diets include the addition of cycling lanes, bicycle travel typically increases 20-30%. For more information see Land Use Impacts on Transport and Evaluating Nonmotorized Transport.
Various studies indicate an elasticity of vehicle travel with respect to travel time of –0.5 in the short run and –1.0 over the long run, meaning that a 20% reduction in average traffic speeds will reduce total vehicle travel by 10% during the first few years, and up to 20% over a longer time period (for more information see Transport Elasticities). Of course, most Traffic Calming projects only affect a small portion of total vehicle travel, so their impact on total vehicle travel is small. However, a comprehensive Traffic Calming program combined with other TDM strategies may have a significant effect on total vehicle travel.
The following factors influence how much a Traffic Calming project affects travel:
· Magnitude of change. The more Traffic Calming reduces traffic speeds and
improves walking and cycling conditions, the more it will affect total travel.
Traffic Calming that significantly reduces a barrier to non-motorized travel
(for example, by making it easier to walk across an arterial from one major
activity center to another or creating a pleasant bicycle travel corridor where
none otherwise exists) may have significant travel impacts in an area.
· Walking and Cycling Demand. A Traffic Calming project will have the most travel
impacts if implemented near major pedestrian and cycling generators:
residential neighborhoods, commercial centers, schools, and recreation centers.
· Integration with other improvements. Traffic Calming complements other demand
management efforts. Traffic Calming can increase the effectiveness of Pedestrian and Cycling Improvements, Parking
Management, Transit Improvements, New Urbanism and many other TDM strategies.
· Land use effects. Traffic Calming supports Clustered, mixed-use,
infill, pedestrian-oriented land use development that further reduce automobile
use and automobile dependency over the long run.
Table 4 Travel Impact Summary
|
Objective |
Rating |