Traffic Calming: Benefits, Limitations, and Design Guidelines

Mike Spack, PE, PTOE

Physical obstructions meant to slow down and possibly divert vehicles in residential areas are called traffic calming devices in our industry.
More generally, we refer to it as changing a road in an attempt to lower vehicle speeds, reduce traffic volumes, divert cut-through traffic, or some combination therein.

This guide provides background, limitations, and further resources for designing traffic calming installations.

TRAFFIC CALMING BACKGROUND

City engineering staff typically review the speeds and volumes on a local street after receiving a request from a concerned citizen (or from an elected official who fielded the citizen request). Traffic engineers then determine if traffic calming treatments are appropriate.

City engineering staff should also analyze traffic calming treatments as part of their street reconstruction projects. Traffic calming devices are often very cheap to build into a reconstruction project.

Not all traffic calming devices are equally effective. Speed humps placed every few hundred feet are better at slowing vehicles compared to road narrowing or raised intersections. Closing off a road so vehicles can’t “cut-through” a neighborhood can be very effective at reducing traffic on the road while a traffic circle may not change driver patterns.

Traffic calming treatments can be broken into the following four categories, each having their strengths and weaknesses.

Vertical impediments; designed to make fast driving uncomfortable. These treatments are the most effective for reducing vehicle speeds, but they also get the most complaints from residents who have to drive over them multiple times per day.

Horizontal impediments; designed to make a driver turn the wheel and reduce the sight lines of unending pavement, which usually results in slower speeds.

Road narrowing; via striping, parking, or curb to reduce the drive lane widths, which slightly lowers speeds. These treatments have the additional benefit of shortening pedestrian crossings, which lead to a safer multi-modal environment.

Closing the through road partially or fully to disrupt travel patterns. These treatments may not change vehicle speeds but are effective at lowering volumes along certain roads. The drawback is that the vehicles are then rerouted, sometimes onto other residential streets instead of out to an appropriate collector street.

A traffic engineer should analyze the road to determine if traffic calming is needed. From there, the traffic engineer should design a proposed plan with some of the above traffic calming measures that will correct the speed and volume problem.
Ideally, the process to install should include public input and meetings to discuss the proposed design as well as a temporary installation (using traffic cones, barrels, etc.) to test before permanent installation. The process of analysis, design, communication and temporary testing will ensure a successful traffic calming project.

TRAFFIC CALMING MEASURES

► Angled Slow Points
► Center Islands
► Chicanes
► Chokers
► Diverters/Diagonal Barriers
► Full Closures
► Half Closures
► Intersection Neckdowns
► Mid-Block Bulb-Outs
► Median Barriers
► Neckdowns
► Raised Crosswalks
► Raised Intersections
► Realigned Intersections
► Reduced Intersection Turning Radius
► Roundabouts
► Speed Humps
► Speed Tables
► Striped Bicycle Lanes to Narrow the Drive Lanes
► Textured Pavement
► Traffic Circles

LIMITATIONS OF TRAFFIC CALMING

► Can be costly to install, and remove if necessary
► Potential, local community opposition from other streets (where traffic may shift) or residents who don’t want the impacts on their travel route
► Reduced accessibility for residents/commuters
► Greater drive time/accessibility for semi-trucks and emergency vehicles (typically not appropriate in industrial areas or areas with high commercial access)
► Limited to local streets or minor collectors
► Not appropriate for busy roads (typically 4,000 vehicles per day is the cutoff)
► Posted speed limit should be 30 mph or less
► Typically for two-lane, two-way roadways

RESOURCES FOR TRAFFIC CALMING

Institute of Transportation Engineers Traffic Calming Measures
Seattle, Washington’s Traffic Calming Website
Lafayette, Lousiana’s Traffic Calming Website
Pennsylvania’s Traffic Calming Handbook

ABOUT THIS GUIDE

This Engineering Guide is part of a series created by the expert traffic engineers that write for Mike On Traffic and are part of the Spack Enterprise family of transportation companies. The Engineering Guides are part of Spack Enterprise’s vision to improve transportation globally. We believe in sharing our expertise, and providing practical content you can use in your professional life. This guide is provided under the Creative Commons Attribution License. Feel free to use, modify, and share this guide, but please give us credit in your document.
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  • Alex says:

    Here in Eugene, OR, there is traffic calming on some streets over 4k vpd and it works fine. Off hand, I can think of a street with an official count of 8500 vpd that the city put a raised intersection on last year, and another with 9100 vpd that chokers were installed on. (And the “official count” is an average of 2006-2013 so the current traffic level is certainly higher on both streets.) Both operate without problems and sees to have even better compliance in terms of yielding to crossing pedestrians than before. The 4k vpd cutoff seems arbitrary and prioritizing of traffic movement over safety.

  • David Lowe says:

    Dont know if you can help, my local council, thats Solihull MBC have brick blocked the centre of the main road going through my village, its been double white lines for years, now since an housing estate has been built off this main road the council have taken up the tarmac in the centre of the main road and replaced it with rough blocks running about half a mile, cannot see why. Regards D.G.lowe

  • Mike Spack says:

    Hi David, not sure we can help on this one as there as lots of unknowns (not to mention the distance and different driving habits). I’d encourage you to talk with the Council or local staff. They should be able to explain their reasoning and what benefits are achieved by this action.

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