Bicycle Safety Guide and Countermeasure Selection System
A raised crosswalk in Salt Lake City, Utah. http://www.pedbikeimages.org/ - Dan Burden
Speed humps are vertical traffic control measures. They are paved (usually asphalt) and approximately three to four inches high at their center, and extend the full width of the street with height tapering near the drain gutter to allow unimpeded bicycle travel. Speed humps should not be confused with the speed bump that is often found in mall parking lots. There are several designs for speed humps. A speed table is a term used to describe a very long and broad speed hump, or a flat-topped speed hump, where sometimes a pedestrian crossing is provided in the flat portion of the speed table.
The traditional 12 ft hump has a design speed of 15 to 20 mi/h, 14 ft hump a few miles per hour higher, and a 22 ft table has a design speed of 25 to 30 mi/h. The longer humps are much gentler for larger vehicles. The speed table can either be parabolic, making it more like a speed hump, or trapezoidal. Speed tables can be used in combination with curb extensions where parking exists. Speed humps can also be designed with two, one-foot slots to allow for vehicles with wide wheelbases such as buses and emergency vehicles to pass through them without having to go over the measure. These are typically called speed cushions.
In general, speed humps are a device of last resort. Other traffic calming solutions should be considered first. However, they may be the best solution in some situations, especially on long, straight residential streets where there are few intersections and no other visual cues to slow motorists.
Raised intersections, essentially a speed table for an entire intersection, may improve intersection safety by forcing vehicles approaching the intersection to slow down. This measure could be part of a street-wide traffic calming effort. Construction involves providing ramps on each vehicle approach, which elevates the entire intersection to the level of the sidewalk. Gradual approaches should reduce the impact on bicyclists.
Vertical measures tend to have the most predictable speed reduction impacts and are best used on local streets. Speed tables can also enhance the pedestrian environment at pedestrian crossings.
Costs can vary depending on the drainage needs of each site, the width of the road, and the specific design used.
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.
Authors and Acknowledgements