Speed is the most significant factor that affects the risk and severity of vehicle accidents. Studies in the UK have identified that a 1-percent reduction in vehicle speed can reduce the risk of accidents by as much as 5 percent. Slower vehicles not only minimize the severity of impacts, they also increase reaction times for drivers to avoid collisions entirely.
Vertical deflection slows vehicles
Speed humps and bumps are some of the most common forms of traffic calming. These vertical obstacles are designed to slow drivers travelling at excessive speeds—by creating mild discomfort and increasing the risk of losing control. Speed humps are a milder form of speed bump with a longer traverse distance that lessens discomfort for drivers. Humps are more common in speed cushion installations.
Unfortunately, speed humps also slow emergency vehicles. When fire and ambulance services respond to an emergency, seconds count. Studies show that for every speed hump installation, emergency vehicles can be delayed up to 10 seconds. Heavy ladder trucks and medical vehicles carrying patients are especially susceptible to delays. When you consider how rapid a fire can spread, or the risk of brain damage for someone who isn't breathing, it's crucial to weigh the impacts of any traffic management strategy.
Speed cushion solution
Standard speed humps are especially difficult for long-axle vehicles to traverse. They have a greater effect on overall speed, as larger vehicles typically have slower acceleration rates. Larger emergency vehicles and buses are also more susceptible to damage from crossing speed humps. To avoid these issues, speed cushions are often promoted as an effective solution.
Speed cushions are essentially speed-hump installations with wheel cut-outs installed at designated widths. Emergency and other long-axle vehicles have a wider track width (measured from the center of each tire on the same axle) than standard personal motor vehicles, which gives them the ability to straddle wider obstacles. Speed cushions are designed to obstruct and slow ordinary passenger cars and trucks, while allowing free passage for wider emergency vehicles.
Speed cushions in Europe vs America
While speed cushions seem like an ideal solution, much of the literature on their effectiveness originates in Europe, where vehicle sizes differ. European vehicles are designed for lower occupancy and narrower streets, meaning they have a narrower track width. Vehicles in North America, however, have wider axles, and most fire trucks and ambulances have dual tires on either side of each axle, which diminishes their ability to straddle wider obstacles. In fact, most emergency vehicles have a narrower distance between their innermost tires. A typical passenger car or truck can straddle a width of about 50 to 59 inches without obstruction; whereas, an emergency vehicle can only clear about 48 inches between its innermost tires.
Speed cushions have other benefits—such as allowing unobstructed passage for bicycles and promoting drainage. However, they are limited in their ability to slow motorcycles, which will pass through wheel slots with minimal risk of discomfort.
Many North American researchers and authorities still approve the use of speed cushions to mitigate negative impacts on emergency vehicle response times, but planners should take all potential issues into account and consult their local emergency response providers for information most applicable to their situation.
In the meantime, there are other variations of traffic humps and speed cushions to consider.
Speed slots are a variation of speed cushions. In a speed slot installation, an especially narrow cushion is set over the centerline between opposing lanes. Emergency vehicles may drive partially in oncoming lanes during an emergency to straddle the center cushion. Speed slot installations risk unauthorized use by regular vehicles, but oncoming traffic will deter most drivers. Another disadvantage for emergency vehicles is that driving into oncoming traffic can increase risk of collisions when on route to an emergency, and there is no guarantee that oncoming vehicles will pull aside.
Offset speed humps
Offset speed humps are 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. Like speed slots, however, driving into oncoming lanes increases the risk of accident and there is no guarantee that oncoming vehicles will pull far enough to the side. Offset speed humps also invite unauthorized use by regular vehicles in low-traffic areas.
Planning vertical deflection installations
Vertical deflections are ideal forms of traffic control in many environments:
- Urban areas with closed drainage systems. Closed drainage systems feature designated street and curb areas that prevent vehicle circumvention around speed humps.
- Residential areas, schools, playgrounds, hospitals, care homes and anywhere else with pedestrians that may be more vulnerable to vehicle accidents.
- Areas where slower speeds (less than 20 mph) are desired.
Avoid using vertical deflections in the following environments:
- Arterial roads and streets designed for higher speeds and greater traffic volume—especially if they are used regularly by emergency vehicles.
- Areas with restricted vision, such roads with steep slopes and curves—which limit visibility and reduce stopping time.
Additionally, when installing vertical deflections, consider the following:
- Address the surrounding environment to ensure vehicles cannot circumvent speed humps and that changes in road patterns will not result in traffic spill into other areas.
- Depending on the average target speed, place speed humps in succession, spacing them close enough to prevent vehicles from reaching higher speeds, but far enough to avoid inconveniencing drivers.
- Ensure proper lighting and visibility. Many municipalities require signage for public roads. Signage ensures visibility in situations where humps may be covered by snow or other debris.
- Consult local residents, commercial businesses and emergency response authorities to ensure traffic calming installations won't disrupt local neighborhoods.
- Consider bolt-down installations, which can be removed during winter months to avoid obstructing snow plows or be reconfigured if needed.
Versatile bolt-down installation
Pre-manufactured speed humps ensure that succession installations will maintain the same shape and height to ensure consistent and predictable driver experiences. Bolt-down components also allow installations to be removed or reconfigured easily for weather considerations or to accommodate changing traffic patterns.
Reliance Foundry's rubber speed humps feature simple, bolt-down installation and can be configured to achieve a range of traffic control assemblies. Rubber materials are extremely durable and won't chip or crack like concrete or asphalt. Installations are much lighter and can be installed with minimal labor and resources—making them highly cost-efficient. All Reliance Foundry traffic products come manufactured with reflective markings for high visibility. Speed humps are manufactured entirely from recycled tires. For more information on Reliance Foundry's traffic management products, contact our Sales Department.
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.
Special thanks to the following companies and fire services that provided information on emergency vehicle specifications:
- 4 Guys Fire Trucks
- FWD Seagrave
- Hackney Emergency Vehicles
- Surrey Fire Service
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- Bunte, Leslie W., Jr. "Traffic Calming Programs & Emergency Response: A Competition of Two Public Goods." Graduate report presented to the Faculty of the Graduate School of the University of Texas. 2000.
- Chang, Kevin and Matthew Nolan. "An Evaluation of Speed Cushions on Neighborhood Streets: Balancing Emergency Vehicle Mobility with Traffic Calming Needs." 2006 ITE Annual Meeting and Exhibit. Milwaukee, Wisconsin. 2006.
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