How Seattle is Rethinking Street Typologies
Henry Miller, Streetsmart Intern
US city planners and engineers have been building roads based on a set list of typologies for more than a century in order to increase the road network’s level of service for cars.
Even if they can’t identify them by name, most Americans today are familiar with the difference between a local, collector, and arterial street- and they know which are more dangerous to walk along.
However, there are transportation planners that have begun to reject car Level of Service (LOS) for a more comprehensive approach that seeks to provide better safety, accessibility, mobility, and equity, while lowering vehicle miles traveled (VMT). In Seattle, a geographically constrained city that is also one of the fastest growing in the country, this new approach has been paired with a complete reboot of its street typologies.
Seattle began this process by first identifying six core values for its transportation system: equity, safety, mobility, sustainability, livability, and excellence. These concepts are key themes in the city’s comprehensive plan, along with ensuring goods movement and using the right of way for multiple purposes, but one of the biggest shifts in thinking was swapping measuring car LOS for measuring mode-share. By focusing on mode-share, specifically on where the city can make streets more comfortable for non-automobile modes, engineers and planners felt they could more easily maintain the core values.
To provide a visual explanation for why this decision makes sense for a geographically restrained city, Dongho Chang, a traffic engineer at the City of Seattle, recently presented a slide during a
Portland State seminar
that reveals the difference between the amount of space 200 people in cars take up compared to other modes. While 200 people are shown fitting in one light rail train or three buses in a single lane in the space of one city block, 200 people in 177 cars take up five lanes and stretch out into the horizon.
Once it was determined that cars should play less of a dominant role in Seattle’s road network, the city began to rethink the traditional road types. While the traditional road type categories (local, collector, arterials) remain, they have been updated to make room for other modes and uses, resulting in nine new road types on top of the original three.
The 12 road types exist on a graph where the Y-axis represents movement (measured by travel speeds) and the X-axis represents place. Streets that are least suitable for non-automobile use (e.g. commercial alleys) land closest to zero on the “place” axis, while pedestrian, bike, and transit-heavy streets rank higher. The streets that rank highest on the “place” axis are located in what Seattle’s comprehensive plan refers to as “
”, which are neighborhood hubs that concentrate amenities in a small area in order to reduce the community’s reliance on cars.
Many of these new road types make clear efforts to reduce the speed of auto traffic, which they accomplish by using a new methodology for controlling car speeds. Rather than posting a speed limit based on how fast a car could drive down a street, the city picks a target speed and uses road diets and speed signs to achieve that speed. One example of this process in action is a one-mile stretch of Rainier Avenue S., where the city reduced the wide avenue from four lanes to three and lowered the speed limit 30 to 25 miles per hour. As a result, collisions were reduced by 15% and travel times were
improved by one minute in the southbound direction during the PM peak travel hour.
All of Seattle’s streets have been
according to the new street typology system, and there are already several major streets (including 2nd Avenue, Westlake Avenue, and NE 75th Street) that have undergone some amount of change in order to serve the core values of the transportation system. While it took US transportation planning decades before realizing the pitfalls of designing every street around car LOS, at least one city is moving to correct the mistake.