Speed is the primary factor in most vehicle accidents: it increases both risk and severity. As drivers move faster, they have less time to respond to road conditions, and any resulting collision causes more damage.
Municipalities can choose from a number of traffic calming tools to encourage safe driving: speed limits, speed bumps, speed humps, roundabouts, and signs, among many others.
Speed zones encourage safe driving, but they can be difficult and expensive to enforce on a large scale. In areas where speeding carries heightened risk, physical features can be used to impose speed reduction. Speed humps and speed bumps are widely used because of their ease of installation and low cost.
Despite the near-identical names, speed humps and speed bumps aren’t the same thing. Both use a 2 to 4 inch rise to force drivers to reduce their speed, but speed bumps are the more abrupt of the two. They measure up to 6 feet long, forcing serious speed reduction. Vehicles’ front wheels pass over the bump entirely before the rear wheels pass over—causing the driver to effectively experience two bumps. This requires drivers to slow to a near-stop to pass over them safely and comfortably.
Speed humps, on the other hand, are modular and span the entire width of a road if desired, meaning that vehicles can pass over them with less of a jolt—drivers will only feel one slow rise and then lower. Vehicles can safely maintain speeds of 15 to 20 miles per hour while driving over them, so they are better suited to local roads and lanes where low speeds are preferred, but full stops are not necessary or convenient.
Speed humps have been proven to reduce speed and make neighborhoods safer for children, but they still aren’t universally welcomed. Critics claim that they can cause damage to vehicles, increase emergency response time, and increase traffic noise and vibration.
To mitigate these potential issues, several factors should be surveyed and considered before installation.
How effective are speed humps?
Speed humps are intended to reduce driver speeds to 10 to 15 miles per hour over the hump, and to 25 to 30 miles per hour between humps in a series. They should be arranged to avoid disruption of cycling lanes and on-street parking.
Speed humps do reduce speed: several studies have shown a 40 percent speed reduction for most vehicles.1 Excessive speeders are also deterred. These effects translate to fewer accidents—children are much less likely to be struck by cars in neighborhoods where speed humps are installed.2
Most importantly, the results don’t revert over time. Other traffic calming measures like “slow” signs lose efficacy with age; the reduction in speed and traffic volume caused by speed humps remain long after local drivers become accustomed to their presence.3
Can speed humps cause damage to vehicles?
There is potential for speed humps to damage the undercarriage of vehicles, particularly if the vehicles are lowered. There are several methods that urban planners can use to mitigate any potential damage:
A change in incline can make the hump functionally higher, so speed humps should be placed on level roads. Other considerations should include placement relative to intersections, driveways, manholes, streetlights, and curbs.
Rubber and plastic speed humps can cause less damage to vehicles; rubber will compress under impact and plastic will sustain damage before a car. Concrete and metal speed humps are more likely to cause damage because of their lack of flexibility.
Drivers who are unable to see or anticipate speed humps can be at risk if they speed over the humps. Signage, paint, and reflective strips can all be used to increase speed hump visibility.
Speed humps can be hidden under snow, surprising drivers and interrupting snow removal operations. Signage at the roadside of speed humps help decrease the inconveniences caused by snow.
Will speed humps delay emergency response?
In most cases, decreasing speed increases safety—but what about emergency vehicles? For ambulances, fire trucks, and police vehicles, speed is everything. They need to get where they are going as quickly as possible. In emergencies, poorly placed speed humps can actually make the neighborhood less safe.
Fortunately, there are some simple ways for speed humps to accommodate emergency vehicles.
Speed cushion configuration
Ambulances and fire trucks have wider axles than passenger vehicles. Speed humps can be installed in speed cushion configuration, with cut-outs at designated widths to let emergency vehicles pass through unimpeded.
Speed humps can be installed in a staggered formation between opposing lanes. During an emergency, response vehicles can maintain higher speeds by swerving into oncoming lanes to effectively slalom around each hump.
Proper speed hump configuration
To further decrease impediments to emergency and transit vehicles and general traffic mobility, speed humps should only be placed on local roads and lanes. They should avoid collector and arterial roads (especially transit, truck, and major emergency response routes), cul-de-sacs, and steep or sharply curved roads.
Distance between speed humps
Speed humps should be installed in succession to be most effective. For local roads, speed humps are usually installed 150 to 250 yards apart, or 65 to 100 yards apart in lanes. Different municipalities may have different guidelines for installation.
The California Subcommittee of the California Traffic Control Devices Committee developed the following equation to determine optimal spacing between speed humps:
Hs = 0.5[2(V85)(V85)-700]
Where Hs is optimal spacing between 3-inch high speed humps (in feet) and V85 is the desired 85th percentile speed (in miles per hour) between speed humps.
For example, if you wanted to reduce interim speed to 25 miles per hour, you would need to place the speed humps 275 feet (about 90 yards) apart.
Speed hump configuration
Speed humps can extend from curb to curb, or be staggered along different sides of the road.
They should not be placed too close to an intersection, drainage, driveways, or in curves. Install speed humps beneath street lighting for increased visibility.
What is the best material for speed humps?
Speed humps are most commonly made from asphalt, concrete, metal, rubber, or plastic.
Concrete, asphalt, and metal are the most rigid of the material options, and will have the most efficiency at slowing traffic. Concrete and asphalt can be difficult to form into precise shapes, however, and require supervision while drying to prevent graffiti. Weather conditions, frequent use, and age can also cause concrete and asphalt to crack or chip over time. All three can also cause damage to vehicles if not installed properly and in appropriate configurations.
Rubber and plastic are much lighter than concrete, asphalt, or metal options, and are cheaper to transport. Both are resistant to sun, moisture, and oil damage. They can both be easily removed and reinstalled as needed.
Rubber is more flexible than any of the other materials, which means it may be slightly less effective at slowing traffic, but will cause less damage to vehicles. Its flexibility means it can conform to natural surface contours in the road, and won’t warp, crack, or chip over time. The increased density means it will be long-lasting and low maintenance.
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Conclusion: Speed humps succeed at making streets safer
Speed humps are highly effective at reducing speed without interrupting traffic flow. They are most useful in neighborhoods near where children often walk and play, such as near schools, parks, and residences. Though they can damage vehicles that don’t slow down adequately, that downside is easily outweighed by the safety benefits.
Information provided is for informational purposes only. If planning a traffic management installation, consult a certified engineer and/or relevant local authorities for details and information specific to any intended installation location.
- Center for Transportation Research and Education, Iowa State University. “Temporary Speed Hump Impact Evaluation.”
- Tester, June; Rutherford, George; Wald, Zachary; and Rutherford, Mary. “A Matched Case-Control Study Evaluating the Effectiveness of Speed Humps in Reducing Child Pedestrian Injuries.” Am J Public Health.
- City of Lakewood, Colorado. “Pros and Cons of Speed Bumps.”
- Speed humps: rachaelvoorhees, CC BY 2.0, via Flickr
- Bus at speed cushion: drdul, CC BY 2.0, via Flickr