Section 6.5.1 of the Right-of-Way Improvements Manual provides the following overview of traffic calming:
Traffic calming is a way to design streets to improve safety, reduce the amount of cut-through traffic traveling on residential streets, and generally encourage people to drive more slowly. Along with education and enforcement, traffic calming has been used in many Seattle neighborhoods to slow speeds on residential streets and improve neighborhood livability by reducing cut-through traffic and improving the environment for pedestrians.
Traffic calming relies on physical and visual cues in, and adjacent to, the roadway to induce drivers to travel at slower speeds. Traffic calming is self-enforcing. The design of the roadway results in the desired effect, without relying on compliance with traffic control devices such as signals, signs, and without enforcement. Street trees and lighting complement traffic calming devices and are often used to provide the visual cues that encourage people to drive more slowly.
Traffic calming is such a powerful tool because it is effective. Some of the effects of traffic calming, such as fewer and less severe crashes, are clearly measurable. Others, such as supporting community livability, are less tangible, but equally important. Experience throughout Europe, Australia, and North America has shown that traffic calming, if done correctly, reduces traffic speeds, the number and severity of crashes, and noise level. Research on traffic-calming projects in the United States supports their effectiveness at decreasing automobile speeds, reducing the numbers of crashes, and reducing noise levels in certain locations.
Typical traffic calming devices allowed in Seattle include: curb bulbs, on-street parking, streetscape improvements, signs, medians, crossing islands or pedestrian refuges, “road diets” (reducing the number of traffic lanes), speed cushions, gateway treatments, neighborhood speed watch program, vegetation, limited access roadways, all-way stop, raised crosswalks, raised intersections, speed limit reduction, chicanes, chokers, diverters, partial street closure, pedestrian districts (woonerfs), speed humps, and traffic circles. For more detail about traffic calming in Seattle, please see Chapter 6.5 in the Right-of-Way Improvements Manual.
Implementing traffic calming is essential toward achieving the goals of the SPMP. By slowing vehicular traffic, pedestrians feel more welcome into the public right of way which encourages walking, creates more vibrant business districts and promotes a more equitable public realm.