At the outset of any redesign or reconstruction, designers set forth key criteria that govern the ensuing design of the street. These parameters, referred to here as “design controls,” critically shape design decisions.
High-quality design for city streets and intersections relies on a keen understanding of the analytical processes and assumptions underlying those technical decisions that shape streets. Design controls, from peak-hour traffic demand to level of service, should always be driven by the intended outcome of a design and the specific set of issues a project strives to resolve.
Passive vs. Proactive Design
A passive design approach assumes, and strives to account for, the worst case scenario, both in terms of user behavior and traffic congestion. For many years, roadways have been designed with a “passive” approach, allowing drivers to travel unpredictably at high speeds. While a passive approach to system design is sound in parallel fields of engineering, such as stormwater management or seismic engineering for earthquake zones, its consequences for ordinary city streets have been disastrous. Overdesigned buffers, clear zones, and setbacks intended to account for fixed-object crashes have created streets that not only account for, but encourage, unsafe speeds.
Whereas storms and earthquakes are environmental factors whose impact can be tempered through design, human behavior, which governs traffic engineering, is fundamentally adaptable, not fixed. People adapt to their conditions. Changing streets change behavior, meaning that a street designed for the fastest and worst driver may very well create more drivers who feel comfortable at faster and more unsafe speeds. A proactive approach uses design to affect desired outcomes, guiding user behavior through physical and environmental cues.1
- Eric Dumbaugh and Wenhao Li, “Designing for the Safety of Pedestrians, Cyclists, and Motorists in Urban Environments,” Journal of the American Planning Association. 77:1 (2011): 69–88.
In 2010, 4,280 pedestrians were killed—an increase of 4% from 2009. Approximately 70,000 pedestrians were injured in 2010.
Traffic Safety Facts—2010 Data (Washington, D.C.: National Highway Traffic Safety Administration, 2012). ↩︎