Raised traffic calming devices are typically used on local streets, primarily to reduce traffic speeds. Raised devices may provide the greatest impact of traffic calming devices on lowering speeds, but effectiveness is dependent on the geometrics of the devices and how widely spaced they are.2 Some traffic may also be diverted through the use of raised devices, depending on how much of current traffic is non-local, the availability of alternate routes, the extent of area-wide treatment, and the type of treatment implemented (that is, humps may divert more traffic than longer and greater tables). Designs should consider bicyclist needs. More gradual and/or longer humps are less uncomfortable for bicyclists as well as other vehicle drivers and passengers, but also tend to have somewhat less slowing effect. Bicyclists may pass between speed cushions, but this and the other devices should be clearly marked for visibility.
Speed humps are paved (usually asphalt), approximately 7.6 to 10.2 cm (3 to 4 in) high at their center, and usually extend the full width of the street with height tapering near the gutter for drainage. (ITE suggests an approximate 3.5 in maximum height due to the jarring that occurs at 4 in.1) Space near the curb may also be provided to allow unimpeded bicycle travel or for a bike lane (but motorists may be tempted to use the area). (Speed humps should not be confused with the narrow speed "bump" that is often found in mall parking lots.) There are several designs for speed humps. The traditional 3.7 m (12 ft) hump has a design speed of 24 to 32 km/h (15 to 20 mi/h), a 4.3 m (14 ft) hump a few miles per hour higher.
Speed table is a term used to describe a very long and broad, or flat-topped, speed hump. Sometimes a pedestrian crossing is provided in the highest or flat portion of the speed table. A speed table can either be parabolic, making it more like a speed hump, or trapezoidal, which is used more frequently in Europe. A 6.7 m (22 ft) table has a design speed of 40 to 48 km/h (25 to 30 mi/h). The longer humps/tables are much gentler for larger vehicles. Speed tables can also be used in combination with curb extensions, where parking exists, to create pedestrian crossings.
Speed cushions, resembling a cushion or pillow placed longitudinally in the
travel lane, are modified speed humps that do not span the entire roadway or
lane width. The intent is to slow most motor vehicles similarly to speed humps
and tables, but allow wide-axled vehicles such as buses and fire trucks to
span and pass over the traffic calming device. These devices have been used
to slow motor vehicles in Vancouver, WA, on a collector street used by emergency
response and transit (see case study #30). Bicyclists typically ride between
Speed humps and tables should probably be considered as "Plan B" on streets that are thoroughfares for bicyclists. Speed cushions may be somewhat more suitable for bicyclists. Use of other treatments such as mini circles, chicanes or chicane-like parking treatments, median islands, and curb radii reduction should also be examined. Bicyclists may, however, be more concerned with traffic speeds on local streets than with traversing raised devices, but should be included in traffic calming planning processes.
- Reduce vehicle speeds. Raised measures tend to have the most predictable speed reduction impacts.
- Enhance the pedestrian environment at crossings.
- May divert some (cut-through) traffic.
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- Raised treatments are not typically suitable for use on arterial streets.
- Do not use if on a sharp curve or if the street is on a steep grade.
- The effect on speed reduction is inversely related to the comfort of the device. Higher and shorter devices have the greatest slowing effect, but are the most uncomfortable to traverse.
- Markings and signs should promote nighttime visibility of raised devices for bicyclists and motorists.
- If the street is a bus route or primary emergency route, the design must be coordinated with operators. Speed cushions show promise here. Usually, some devices are acceptable if used prudently — one device may be appropriate and may serve the primary need (e.g., if there is a particular location along a street that is most in need of traffic slowing).
- The aesthetics of speed humps and speed tables can be improved through the use of color and special paving materials. Designs that complement neighborhood aesthetics will be more readily accepted by the public.
- Noise may increase, particularly if trucks use the route regularly, but some noise assessments have found little impact, and noise may be reduced overall because of cars traveling at lower speeds.
- Raised treatments such as speed tables may contribute to drainage problems on some streets.
- Speed humps, tables, and cushions should be properly designed and installed to reduce the chance of back problems or other physical discomfort experienced by vehicle occupants.
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The cost for each speed hump is approximately $1,500 including markings. Speed tables are $2,000 to $15,000, depending on drainage conditions and materials used. Speed cushions also cost approximately $2,000 each.
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