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Choosing an ideal traffic calming strategy
Wherever people and cars are in close proximity, traffic safety is of heightened concern. Near playgrounds and schools, in parking lots and multiuse areas, aggressive driving is a safety risk for pedestrians. Speed reduction is one way to protect people in these areas. Slower traffic speeds reduce both the likelihood of an accident and the chance of a fatality should one occur.
Both speed bumps and humps are useful to encourage safe driving in pedestrian zones.
Though speed zones are often set up to encourage safe driving, posted limits can be missed or ignored, so traffic calming in these areas often incorporates physical design of the roadway or environment.
Speed bumps and speed humps are vertical obstacles used in traffic management—literal bumps in the road that jolt the occupants of a vehicle moving too quickly over them. They are the most commonly used structural traffic calming elements. They can be made from asphalt, concrete, plastic, rubber, or metal. Although these measures are related, and have many of the same benefits, they are not interchangeable solutions: they’re appropriate in different applications.
Speed humps, sometimes called road humps or undulations, are used for 10–15 mph speed zones. They’re often seen on local streets or connector roads where traffic needs to flow smoothly but excessive speed will endanger pedestrians. Playground and school zones often use these in traffic management.
A speed hump creates a gentle rocking sensation in a car passing over it at the posted speed limit. If a car is driving at unsafe speed, the hump will jar the vehicle and its contents, causing discomfort to the occupants and disruption to cargo. These obstacles usually span the lane they are placed in. This way, vehicles are encouraged to pass over them with both wheels, reducing the likelihood of bottoming out.
Speed humps come in a variety of profiles and travel lengths. These factors influence the experience of discomfort created in the speeding vehicle. Travel length varies between 3–20 feet. Any travel length longer than the vehicle creates only one up-and-down motion, whereas a travel length shorter than the vehicle creates two rocking movements as each set of wheels passes over. Speed humps are most often placed in a series, maintaining speed reduction through a long corridor.
These vertical deflections are not a good choice for arterial roads, emergency routes, or on any street where it is easy for a car to evade the hump by driving on a shoulder. Because of this, they’re usually installed in one or two lane local urban settings where there are curbs and closed sewers. If speed humps are installed in areas where there is a shoulder, they are often twinned with bollards or other obstructive measures to prevent cars from leaving the roadway.
Speed bumps are more aggressive traffic calming options than speed humps, and so are useful in places where pedestrians and cars share space closely, like parking lots and driveways. A speed bump generally slows traffic to 2–10 mph, giving both people and cars time to react safely to one another. Speed bumps are rarely used on public roads because they require vehicles to come to a near stop to pass over them, and can do damage to cars moving at regular speeds.
Speed bumps can be two to four inches high, but they have a much shorter travel distance than speed humps. These obstacles are under a vehicle’s tire for less than half of a full wheel rotation, with standard widths are between six inches and two feet. The height to travel-distance ratio creates an abrupt bounce in a vehicle, which can shake both occupants and cargo. Since a speed bump is always much smaller than vehicles passing over it, each axle will cross separately, meaning a car moving at excessive speed will receive two substantial jolts.
Speed bumps, like their more sedate siblings, can be placed at intervals to maintain speed reduction. They are often spaced judiciously as they are more uncomfortable to go over at any speed and are used in smaller geographical areas.
Speed bumps can deliver a shock, which is perhaps why they have developed colorful names around the world. They’re known as speed breakers (India), judder bars (NZ), sleeping policemen (UK), and road turtles (Southern US). In French, they’re known as “dos-d’âne,” or literally, “humpback.”
Benefits and cautions
One of the great strengths of speed bumps and humps is that they remain a deterrent no matter how accustomed drivers become to their presence. Flashing signs or intermittent traffic enforcement change behaviors for a while, but drivers often revert to previous behavior. One can disregard a sign, but not the road beneath. A speed hump or a speed bump creates the same discomfort for a driver going too quickly whether it is their first or their hundredth time over it.
A bump in the road is not perfect for every traffic calming scenario. Obstacles work best when the driver knows they’re coming and starts slowing down before arriving at their edge. Speed bumps and humps are usually well marked with contrasting colors or reflective surfaces to ensure visibility, but they also need to be installed in places where a driver can see them coming. They are not effective when used in the middle of a long curve, or on streets where the grade is greater than 8%. A steep grade can also change the effective height of a speed hump or bump, and create more of an impact than is intended. Visibility can also be an issue in some winter climates where snowfall is deep and local streets go unplowed.
Speed humps and bumps are also not recommended for arterial or emergency roads, where they can be a hazard. If unexpected or taken at too high a speed, these obstacles can cause a driver to lose control or damage a vehicle.
However, when used in the proper setting, these are the most adaptable tools in the traffic management toolbox.
Materials and installation
Speed bumps and speed humps are usually made from rubber or asphalt. Plastic, metal, and concrete options are available as well.
When made from rubber or plastic, these obstacles are easy to install on pre-existing surfaces, allowing for a rapid and effective set up. Both options are lightweight and can be installed temporarily. Very restrictive installations, with many placed close together in series, can create a challenge to snow removal in winter climates: both rubber and plastic speed bumps can be uninstalled to facilitate these operations.
Rubber has greater compression than asphalt or concrete, which can be a benefit to a vehicle that hits it too quickly, at an angle, or is low to the ground, because it is less likely to do the vehicle damage. This greater compression means that rubber obstacles of the same height are slightly less effective at slowing traffic than similar asphalt installations, and rubber or plastic are not ideal for roads on which there is constant passage of heavy industrial traffic. However, in normal conditions, rubber is tough and long lasting, and is inexpensive to buy and install.
Asphalt or concrete speed humps are often installed by municipal road crews or private paving companies. They must have precision and expertise in the area, or the installation can go poorly. With a sufficiently expert crew, asphalt can allow custom construction of obstacle shape and travel length. Both materials have less compression so are slightly more effective than rubber or plastic at speed reduction. Over time, and in variable weather, however, asphalt and concrete will tend to wear and crack.
Simple and effective traffic management
Both speed bumps and humps are useful to encourage safe driving in pedestrian zones. Speed humps are useful in places where traffic needs to flow, and speed bumps in small areas where pedestrians and traffic share space more equally. Both are less planning intensive compared to other physical traffic calming methods like roundabouts or one-way streets, and modular, prebuilt versions can be installed simply and inexpensively on existing surfaces. Larger speed humps and their small but forceful cousins, speed bumps, are traffic safety superstars, when put on the right stage.