By Eric Sundquist
In the last decade a number of project development and design guides, such as ITE’s “Designing Walkable Urban Thoroughfares,” NACTO’s “Urban Street Design Guide,” and city design guide manuals, have emerged. A new article by Eric Dumbaugh of Florida Atlantic University and Michael King of BuroHappold Engineering, reviews these updated practices.
The article finds four general principles of livable streets engineering:
1. Examine networks and person capacity during the alternatives analysis process.
.…When establishing a project’s scope and evaluating project alternatives, planners should include not only the identified corridor but also the surrounding network in their analysis. Conventional planning practice strives to direct traffic onto streets designated as “arterials.” Yet in urban environments, a significant portion of traffic is likely directed not for distant destinations but toward local “microdestinations” in the surrounding area ( Jacobs 2004). Indeed, it is for this reason that destination accessibility (by automobile) has proven to be the design variable with the most profound effect on reducing VMT followed closely by street network connectivity (Ewing and Cervero 2010). A significant portion of the traffic delay along an urban corridor may be addressed by simply providing more direct means for allowing people to get where there are trying to go….
2. Establish target speeds between 20 and 35 mph.
Twenty mph is the maximum speed at which a pedestrian or cyclist can be struck by a vehicle and survive, leading many European countries, as well of the World Health Organization and the OECD, to recommend the adoption of 20 mph as the target speed for urban streets where pedestrians are likely to be present. The use of 20 mph target speeds has yet to be adopted as a model practice in the United States, with the design guidelines and practices reviewed in this article recommending target speeds of 25 to 35 mph. A review of the scholarly literature suggests that the upper limit for safe urban streets is 35 mph. On-street parking appears to reduce total and injurious crashes at speeds up to thirty-five mph, but exacerbates crash risk at higher speeds. Similarly, crosswalks tend to have detrimental effects on pedestrian safety at speeds of 40 mph or greater and no effect whatsoever on encouraging motorists to yield to pedestrians. As such, 35 mph is likely the upper bound for desirable target speeds on urban streets.
3. Develop consensus on design controls during the early phases of the project.
The design of a street project can change dramatically as the project vision enters the engineering phases of design. This can create conflicts between planners and engineers, as the initial project concept is modified based in the design controls applied by engineers. Many of these problems can be avoided by developing consensus on the project’s traffic volumes, design vehicle, and design speed during the visioning and alternatives analysis phases of the project and being attentive to available strategies for addressing the challenges associated with application of specific design controls….
4. Focus on intersections and safe crossings, rather than lane widths and curb radii.
[L]ane widths and curb radii, by themselves, are not a silver bullet for creating walkable streets. … The spacing and timing of controlled intersections, the use of roundabouts and traffic circles, bulb-outs, and the provision of protected pedestrian crossings are strategies that will likely be more effective at reducing vehicle operating speeds and addressing the broader design goal of ensuring that pedestrians can cross streets safely and comfortably….
Eric Sundquist is Director of SSTI.