Q+A: Bumps, Humps, Cushions, Tables — What’s the Best Way to Slow Down Traffic?

The last three years have been three of Philadelphia’s worst for traffic fatalities, according to the city’s Vision Zero annual traffic safety report. Citywide efforts in partnership with the Pennsylvania Department of Transpiration are attempting to slow traffic flow to safer speeds – particularly in areas that are prone to traffic accidents. These strategies range from enforcement measures like signage, lights and cameras, to physical reminders, such as grooves and bumps.

While new speed control measures are rarely popular among drivers, understanding when, where and why they are being implemented can ease the friction of adapting to safer new traffic patterns, according to Zhiwei Chen, PhD, an assistant professor in Drexel University’s College of Engineering who studies transportation systems. Chen’s Connected & Automated Mobility Lab researches sustainable, equitable, efficient and resilient systems and was recently tapped to examine the transit effects of the I-95 collapse in Philadelphia.

Chen recently shared his insights on traffic calming strategies and Philadelphia’s efforts to improve safety by getting drivers to slow down.

What is the difference between a speed hump, a speed bump, a speed cushion and a speed table?

These are all traffic claiming devices designed to slow down vehicles.  They can be distinguished by their physical dimensions, particularly their height and length along the direction of travel, as well as their specific design intentions.

Speed bumps are typically the highest (up to 6 inches) with round tops, yet the shortest in length (1-2 feet) in the direction of travel. This combination makes speed bumps the most disruptive in speed reduction. Their aggressive design is well-suited for parking lots and commercial driveways. These are typically not used on public roadways.

Speed tables are the longest (22 feet) in the direction of travel and feature a flat, plateau-like top, which is why they have the least height (3-3.5 inches) among these features. Their design brings the least disruption to traffic, allowing vehicles to pass over at a more consistent, controlled speed (25-35 mph). Thus, they are ideal for public roads like pedestrian crossings or school zones where consistent slow traffic movement is necessary.

Speed humps stay in between speed bumps and speed cushions. They are not as aggressive as speed bumps nor as flat as speed tables. They typically extend to medium lengths (12-14 feet) and heights (3-4 inches), creating a moderate slowing effect (15-20 mph) without causing too much discomfort to drivers. They are common in residential areas, effectively reducing vehicle speeds while maintaining a relatively smooth traffic flow.

Speed cushions are probably the easiest to identify. Whenever you see two or more speed humps/tables put side by side across the width of a road, you have found a speed cushion! 

The gap between humps/tables is specifically designed to enable a vehicle (usually emergency vehicles and buses) to pass through without being much affected. Thus, they are mostly used in routes where there are frequent emergency vehicles and bus operations.

What are some other traffic-calming methods?

Other traffic calming options include speed limits; raised pedestrian crossings and intersections; “road diets,” such as road narrowing and reducing the number of lanes, restricted movement, such as one-way streets; roundabouts; and textured pavements — using materials like cobblestones or other pavement markings that create slight driving discomfort to alert drivers that they are in a special zone, often encouraging slower driving speeds.

What factors might be considered when engineers are deciding on the appropriate traffic-calming strategy?

There are numerous technical, financial and community considerations when selecting traffic calming measures, among others.

From a technical perspective, it is important to identify the type of road and the specific objectives you aim to achieve. The traffic volume, traffic composition, current and target speed limits and crash history are also important factors to consider. Different traffic-calming devices have different effects on speed reduction. For instance, if the goal is to decrease speeds to 25 mph in a sensitive area like a school zone, a speed table is good choice. On the other hand, if your road sees a substantial amount of transit traffic, a speed cushion could be more appropriate. The Traffic Calming ePrimer published by the Federal Highway Administration has a very good discussion on this aspect.

Cost is another critical factor. We cannot build anything without money. And in a city, there are usually many roads where traffic calming measures need to be implemented. Thus, it’s essential to find a balance between effectiveness and affordability.

Lastly, but equally important, is getting community support. It is usually a make-or-break aspect for traffic calming initiatives. Implementing traffic calming measures can be a significant change, and it’s vital that residents and businesses are on board. I have seen transportation projects get halted due to the lack of community support. When it comes to traffic calming, some community members may say “no,” because speed humps can make driving uncomfortable and can damage the underside of vehicles. This is actually where community opposition came from in most past efforts to promote measures such as speed humps.

Fortunately, the Streets Department in Philadelphia is imposing a bottom-up approach — where community members actively request traffic calming safety studies — ensures these interventions address genuine local needs. However, it is important to make sure all communities are aware of this opportunity.  

What kinds of roads or traffic situations are good candidates for this type of intervention?

I would say that any roads with a track record of high crash rates for pedestrians, cyclists, e-scooter users, among others should probably be considered for this type of intervention.

Here a practical challenge is how to define “high crash rates.” We obviously want to prevent all accidents, but there are always budget limitations. Thus, the real question that we should be asking is: how should transportation practitioners use the information at hand — crash history data, traffic data, cost information, community needs — to systematically evaluate different roads in a city and allocate the limited resources based on the evaluation outcomes?

In Philadelphia, the Streets Department has created a High Injury Network, which identifies corridors with highest rates of fatality and serious injuries. This should be a good starting point to identify local roads that need traffic calming interventions.

What strategies are used to safely acclimate drivers to the new infrastructure before, during and after installation?

I’m actually not aware of any systematic strategies that have been proposed or implemented to safely acclimate drivers to the new infrastructure before, during, and after installation. Usually, there’s an expectation that traffic calming measures, such as speed humps, are inherently noticeable — especially since they’re often painted a bright color — so that drivers can easily see them and react accordingly. However, reality proves otherwise at times, as there have been instances where drivers, myself included, have been caught off guard by these structures, leading to discomfort or even accidents.

There are, however, ad-hoc measures being taken. More cities are recognizing the need for some level of community engagement before implementing these traffic calming measures. They’re not only seeking input from residents and businesses, but also posting information on websites and social media to raise public awareness. This prepares people for the changes they’ll see on the roads and can help reduce any potential negative reactions or incidents.

After installation, authorities typically install signs to indicate the new changes to the driving landscape, particularly if there’s been the addition of bumps, humps, cushions, or tables. These signs serve as a warning for drivers to reduce their speed or pay more attention to the road.

These all could help, but I think more attention is probably needed to ensure that the information is properly communicated so that drivers are aware of the new infrastructure. One solution could be the integration of these infrastructure updates into mapping applications, such as Google Maps. This feature could provide real-time audio warnings for drivers, alerting them of upcoming speed humps, tables, or cushions, thereby enhancing road safety and awareness. Opposition to traffic-calming measures such as speed humps arises primarily due to three concerns: they interrupt the flow of traffic, necessitating even law-abiding drivers to reduce speed; they can cause considerable discomfort to drivers, particularly those who fail to notice them in advance; or they pose a risk of potential damage to vehicles. It would be interesting to explore how smart sensing and AI technologies can be leveraged to address these challenges and promote community buy-in.

How do engineers study the effectiveness of traffic-calming infrastructure?

There is a large body of literature on this topic. The most straightforward way is to conduct a “before-after study.” Engineers typically collect data to calculate performance metrics, such as travel time and accident rate, both prior to and following the installation of traffic-calming measures. A longitudinal comparison of the performance metrics would help us understand the effectiveness of the infrastructure.

However, there are instances where evaluation must be performed before an infrastructure is installed.  Here, engineers and researchers may leverage simulation technologies as surrogate environments for real-world driving conditions. Researchers will set up specific driving scenarios and ask participants to drive on the same road with and without the traffic-calming infrastructure. The same set of performance metrics can be measured to evaluate the effectiveness. Emerging technologies such as virtual reality, mixed reality, and metaverse could improve the effectiveness of this approach.

In situations where driving simulators or similar resources are not accessible, researchers might employ stated preference surveys. These surveys ask for information on participants’ hypothetical changes in their driving behavior in response to new traffic-calming measures. While this method provides predictive insights, studies in behavioral economics have indicated a discrepancy between individuals’ projected behaviors — called behavioral intention — in surveys and their actual behaviors under real-world circumstances. Despite this, stated preference surveys remain a viable, resource-efficient tool for anticipating the outcomes of traffic-calming initiatives.

Reporters who would like to speak with Chen should contact Britt Faulstick, bef29@drexel.edu or 215.895.2617.

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