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.
This section defines the Seattle Department of Transportation's (SDOT) traffic calming policy, including appropriate tools for use on residential and arterial streets. Also included is a description of the steps that community members must take to get SDOT to evaluate traffic calming requests and prioritize them for design and construction.
Policy Guidance for SDOT’s Neighborhood Traffic Control Program (NTCP)
Seattle’s Neighborhood Traffic Calming Program (NTCP) was established in 1978 as part of the City's annual Capital Improvement Program (CIP). Since then, Seattle’s residents, in partnership with the City, have been involved in the installation of over 800 traffic circles and other traffic calming devices on neighborhood streets. The purpose of the NTCP has been to reduce accidents and speeds on residential streets, thereby creating safer, more pleasant neighborhoods. Traffic circles are the most common tool used and can be seen in most residential neighborhoods throughout the City. SDOT’s NTCP is guided by specific goals and policies in the City of Seattle Comprehensive Plan (2008) and the Transportation Strategic Plan (2005) as follows:
Manage the street system safely and efficiently for all modes and users and seek to balance limited street capacity among competing uses.
Protect neighborhood streets from through traffic.
Use neighborhood traffic control devices and strategies to protect local streets from through traffic, high volumes, high speeds, and pedestrian/vehicle conflicts. Use these devices and strategies on collector arterials where they are compatible with the basic function of collector arterials.
Continue Seattle's Neighborhood Traffic Control Program.
Consider requests from neighborhood organizations and citizens and consequently design and implement traffic circles and other neighborhood traffic control devices. These devices can be very effective to slow speeds and reduce collisions on neighborhood streets. In fact, to date, Seattle's traffic circles have resulted in a substantial reduction in accidents and speeds in neighborhoods. They can also encourage through traffic to stay on arterial streets, reducing the impact of cut through traffic on neighborhoods.
Traffic calming projects shall improve neighborhood livability in balance with transportation efficiency and the safety needs of the communities.
The NTCP shall takes a holistic approach to traffic management, resulting in streets that provide access to neighborhood destinations for all modes, including walking, bicycling, transit and automobiles.
Traffic calming devices shall complement the overall transportation network and not result in shifting the problem to an adjacent street.
Traffic calming is not designed to address dangerous intersections, mitigate traffic noise, redesign the overall transportation and street classification system or effect a modal shift.
Considerations for Traffic Calming on All Streets
Although traffic calming is typically used on residential streets, there are certain tools that are appropriate for use on some arterial roadways. When a traffic calming approach is considered for any street, SDOT applies the following guidance:
Vehicle speed is more critical than volume in terms of safety and should be addressed first where there are constraints.
Neighborhood involvement is important to successful implementation. Rationale for traffic-calming and management measures should be explained clearly to community residents and installation of these treatments should incorporate public input.
Traffic-calming and management measures should fit into, and preferably enhance, the street environment .
Traffic-calming designs should be predictable and easy to understand by drivers and other users.
Devices that meet multiple goals are usually more acceptable. For example a raised crosswalk may be more understandable to motorists than a speed hump. The former has a clear goal whereas the latter may be perceived as a nuisance.
Treatments need to be well designed and based on current available information on their applications and effects. Information on U.S. experiences with various traffic-calming measures can be found in ITE’s Traffic Calming: State of the Practice.
Devices should accommodate emergency vehicles . Emergency response times shall be considered.
Traffic-calming areas or facilities should be adequately signed, marked, and lit to be visible to motorists.
Treatments need to be spaced appropriately to have the desired effect on speed —too far apart and they will have a limited effect, too close and they will be an unnecessary cost and annoyance. Devices usually need to be spaced about 300 to 500 feet apart. If they are spaced too far apart, motorists may speed up between them.
Whole street designs are usually able to create an environment that supports slower speeds for the entire length.
Facilities should not be under-designed or they will not work . Keeping the slopes too gradual for a speed table or curves too gentle for a chicane will not solve the problem and will appear as a waste of money and may ruin chances for future projects.
Traffic-calming measures should accommodate bicyclists, pedestrians and people with disabilities .
If a measure is likely to divert traffic onto another local street, the area-wide street system should be considered so as not to shift the problem from one place to another.
Devices should be thought of as elements of a traffic calming system and be placed to improve pedestrian conditions throughout an area.
Additional Considerations for Traffic Calming on Arterial Streets
Seattle has streets within the arterial network that are primarily residential and are optimally used as routes to “collect” local traffic to move it to higher capacity arterial streets. Since Seattle’s street grid is largely built and congestion continues to increase, collector arterials are being used as cut-through routes by motorists trying to avoid congestion. Communities are concerned about higher traffic speeds and volumes on the collector arterials in their residential neighborhoods and are requesting traffic calming solutions. In order to balance the demands placed on the arterial network, including use by large vehicles such as buses, trucks and emergency responders, with neighborhood concerns, the City needs clear policy direction about traffic calming practices appropriate on arterial roadways.
Traffic calming on arterials is most successful when applied on arterial streets where adjacent land uses are primarily residential.
SDOT will attempt to resolve the issue using the following approaches: 1) education; 2) enforcement, and if education and enforcement do not solve the problem; and 3) engineering methods. If traffic calming devices are an appropriate solution, they shall be planned and designed in keeping with sound engineering and planning practices appropriate to the particular functions of the arterial street.
Through traffic should be encouraged to use higher-classification arterials (principal and minor arterials), as designated in Seattle’s Street Classifications.
Emergency vehicle access shall be maintained and traffic calming devices should not unreasonably degrade emergency vehicle response times.
Arterial traffic calming projects should not significantly impact transit service access, safety, and scheduling.
Pedestrian and bicycle movement should be given equal consideration with vehicle movement in the design and implementation of arterial traffic calming projects.
Parking issues should be considered on a project-by-project basis. Parking needs of residents should be balanced with the equally important functions of traffic, emergency vehicle access, transit, bicycle, and pedestrian movement.
Traffic calming on arterials should not divert traffic to non-arterial roadways through the use of traffic diversion devices.
When arterial traffic calming is being considered, certain procedures should be followed by SDOT in processing traffic management proposals in accordance with applicable codes and related policies and within the limits of available resources. At a minimum, the procedures should provide for project selection and evaluation; citizen participation in plan development and evaluation; public and life safety review, and communication of any test results and specific findings to area residents and affected neighborhood organizations.
Typical Traffic Calming Devices in Seattle
The City of Seattle has used the following traffic calming devices in locations deemed appropriate by the SDOT in consultation with neighborhoods. Refer to the City of Seattle Neighborhood Traffic Calming guide.
Not all traffic calming devices are appropriate for use on every type of street or in every location. Traffic Calming Device and Applications (table below) describes the name of the device, they type of issue it is typically used to resolve, and the street classification(s) where the device could be applied. The “typical use” category describes, in general, what kind of change each device can affect: managing traffic, conditions along streets, or pedestrian crossing conditions:
Managing traffic : Concerns about traffic speed and volume can be addressed through effective traffic management. The following devices are used to help manage traffic. Many of these devices restrict the movement of traffic on streets. In most cases the least restrictive method of solving a traffic management problem is the most cost effective, and the easiest for all to agree on.
Conditions along streets : Conditions along streets affect pedestrian travel, comfort, orientation, safety, and affect the aesthetic quality of our streets. This group of traffic calming devices includes on-street parking, lighting, street furniture, and plantings and trees.
Pedestrian crossing conditions : Crossing a street shouldn’t be unreasonably difficult, and there are devices that can help improve pedestrian safety, including pedestrian crossings.
Traffic Calming Devices and Applications
Pedestrian Crossing Conditions
On-street parking (parallel and angle)
Conditions Along Streets
Streetscape improvements (street trees, lighting, street furniture, special paving treatments)
Conditions Along Streets
Crossing islands or short medians
Pedestrian Crossing Conditions
“Road Diets” (reducing number of travel lanes)
Speed cushions (for 25 mph or below)
Pedestrian Crossing Conditions
Neighborhood speed watch program
Pedestrian Crossing Conditions
Speed limit reduction
Partial street closure
Pedestrian districts (woonerfs)
Pedestrian Crossing Conditions
Legend Appropriate for Consideration (●)
May be Applicable (●●)
Process for Installing Traffic Calming
SDOT has an evaluation process for all traffic calming proposals. Refer to Figure 6-29: Traffic Calming Evaluation Process. This process is necessary due to the high demand and limited resources available for traffic calming projects. This section outlines the steps that must be completed before a location may be considered eligible for traffic calming. Note: locations with physical characteristics that do not allow the feasible placement of traffic calming will not be considered.
Step 1: Community Contacts SDOT with Traffic Calming Proposal SDOT requires support from the residents of the affected area before constructing traffic calming devices Responsibility for demonstrating community support for a traffic calming completion of this step lies with the community. To initiate the NTCP process, a community group must contact SDOT NTCP to request inclusion in the annual NTCP evaluation period. Refer to Section 6.6 Contact Information for more information. All requests should be made prior to July 15; any request received after this date will be considered for funding in June of the following year.
Step 2: SDOT Evaluation of Community Request and Initial Action SDOT staff will work with NTCP applicants to determine conditions on the street that the community believes need to change. After initial evaluation, the SDOT Traffic Engineer may authorize actions be taken within the scope of his or her authority to address the applicant’s concern and solve the problem. If problem persists, SDOT may initiate education and enforcement activities to resolve the problem. The SDOT NTCP staff, in consultation with the Fire Department and King County/Metro Transit, may also deem the project to be infeasible at this point and recommend another course of action that does not involve traffic calming.
SDOT will evaluate the safety record of each location which can include collision history, speed data emergency response implications, and traffic volume counts. The data will be used to prioritize the locations for construction on residential streets using the point criteria described in Section 6.5.8. Point Criteria for Traffic Calming Project Ranking.
Step 3: Petition Process If the problem is not successfully resolved in Step 2, SDOT will request a demonstration of community support for traffic calming. SDOT will provide the applicant with an NTCP petition and signatures must be gathered as follows:
Traffic Calming on a Residential Street : Signatures are required from at least 60% of the households (owners or renters) and businesses (property or business owner) typically within one block of the proposed traffic calming device. For more restrictive traffic calming devices (e.g., diverters or partial closures) SDOT may specify a petition area beyond one block.
Traffic Calming on an Arterial Street : Arterial roadways play a major role in moving people and goods within neighborhoods as well as throughout the city. For this reason, any traffic calming proposal on an arterial street must be supported by adjacent neighborhoods. In addition to the petition process defined above, additional letters of support will be requested from the community councils adjacent to arterial locations being considered for traffic calming, as well as the appropriate district council.
Only one signature per household or business is needed. Signed petitions must be submitted by July 15 each year, to be considered for the following year's construction. Completed petitions can be mailed or delivered to the Seattle Department of Transportation - Traffic Circle Program. Projects that meet the required support rate will be considered for funding through an SDOT annual program.
Step 4: SDOT and Community Seek Funding for Project If the location does not qualify for funding through SDOT’s NTCP program, the contact person listed on the petition will be notified by mail. Information about other potential funding sources may be provided. If the location ranks sufficiently high on SDOT’s prioritization list for Neighborhood Traffic Control Program (NTCP) funding, the contact person will be notified.
At this point in the process, SDOT staff will determine if it is necessary to hold a meeting in the neighborhood to discuss the project, including results of the traffic analysis, the design concept, and the procedures leading up to construction. Maintenance of landscaping in traffic calming devices, an important component of this project, and identification of a landscape volunteer will also be discussed during the meeting. For traffic calming on arterial streets, meeting notices will be sent to the community councils in adjacent neighborhoods as well as the appropriate district council.
Step 5: Project Funded Project funding is identified and SDOT proceeds with the design and construction process.
Step 6: Design & Construction Overview
Traffic calming devices are designed according to the existing geometry of each intersection and sized to accommodate the passage of emergency vehicles. The Fire Department, Metro/King County Transit and other agencies review locations for new calming and may conduct a field test to check for maneuverability. Then, final plans are made and sent to construction crews with specifications. If a volunteer signs up to maintain plantings, soil and plants will be supplied; otherwise, the device will be covered in asphalt.
Step 7: Construction The first visible evidence that traffic calming devices will be built is typically an outline of the device drawn in on the street. Construction will follow, including any landscaping and signs (e.g., reflector or directional signs).
Step 8: Post Construction Monitoring After construction of speed humps, speed cushions and chicanes, the traffic calming device will be monitored for a period of six months to one year. During this time, traffic speeds and volumes are measured to help determine the effectiveness of the device.
6.5.8 Point Criteria for Traffic Calming Project Ranking
SDOT ranks locations for traffic calming based on a number of criteria. For traffic circles, a point criteria is used to assign points to an intersection for accident history, traffic volumes and traffic speeds refer to point values in tables below include in Section 6.5.8a Accident History, 6.5.8b Traffic Volumes and 6.5.8c Traffic Speeds . The points assigned for accident history, traffic speeds and traffic volumes are then added together to prioritize the location based on need.
To address traffic calming requests at mid-block locations, SDOT evaluates each corridor based on number and type of mid-block collisions, speeds and volumes.
6.5.8a Accident History
Accident history is determined based on the average number of accidents per year over the most recent 3 year period. The annual accident rate is determined by the number of accidents/number of years over which they occurred. For example if a location has had 6 collisions in the last 3 years, the average annual rate per year is 2.000. As a result, 4 points would be assigned to this location for accident history.
Annual Accident Rate (accidents/year)
0.5 - 0.875
0.876 - 1.250
1.251 - 1.625
1.626 - 2.000
2.001 - 2.375
2.376 - 2.750
For midblock locations, a score of .5 is assigned if the accidents on the midblock section of street exceed two accidents per year over the last three years.
6.5.8b Traffic Volumes
Points for traffic volumes are assigned according to the number of vehicles per day on an average weekday.
Traffic Volumes (Vehicles per Day—Average Weekday Traffic)
500 - 1100
500 - 1500
2000 - 4000
1101 - 1700
1501 - 3000
4000 - 8000
1701 - 2300
3001 - 4000
8000 – 12,000
6.5.8c Traffic Speeds
Points for traffic speeds are allocated based on the 85th percentile speed in miles per hour. The 85th percentile speed is the speed at which 85% of the vehicles are traveling at or below. Speed limits for residential streets are 25 miles per hour, unless otherwise marked. Speed limits for arterial streets are 30 miles per hour unless otherwise marked.
Traffic Speeds (85th Percentile Speed—miles per hour)
26 - 29
31 - 33
29.1 - 32
33.1 - 36
32.1 - 35
36.1 - 39
6.5.9 Trials and Temporary Installations for Traffic Calming
In neighborhoods trying traffic calming for the first time, it may be useful to lay out a new design with cones or temporary markings to test it. This provides emergency vehicle drivers, residents, and others with an opportunity to test the design. Some communities have constructed elaborate temporary devices with concrete or plastic (“jersey”) barriers. These can instill a negative reaction in the community due to their unaesthetic appearance and they do not generally have any significant benefits over the simpler test devices.
6.5.10 Landscaping for Traffic Calming Devices
Neighbors are responsible for the planting and maintenance of traffic calming devices after they are built. Landscaping is installed during the planting season (spring or fall). SDOT’s Urban Forestry Division staff, takes input from residents about plant material selection during the months of January and June, depending on the season when construction is completed. Plants are chosen based on their drought tolerance, resistance to occasional car traffic and street right-of-way landscaping guidelines, which promote visibility. The recommended street right-of-way plant list contains a variety of suitable plants that have performed well in the past. Plants are delivered to the home of the landscape coordinator in the spring or fall and a planting party usually follows.
6.5.10a Plants Provided at the Time of Installation
Each year SDOT provides plants for between 60 to 120 new traffic circles, chicanes, triangles and medians. Purchases are made in bulk to keep costs down, so this requires selecting a common plant pallet that accommodates the needs of everyone. Although every effort is made to get the plants neighborhood groups request we sometimes need to make substitutions. As always, purchasing depends upon plant availability. So, we ask for flexibility and patience as we are serving a considerable number of neighborhood groups.
Groups may purchase their own plant material if they would like to plant sooner or have specialty plants in mind that we might not be able to get. SDOT can not reimburse groups for these expenses, but your planting plan should be sent to SDOT Urban Forestry staff for permitting before plant purchases are made. Flower bulbs are not provided, but they are a lovely addition and we encourage you to plant them! Daffodils, in particular, grow really well. They need little care and will increase in number as they bloom each spring!
A traffic circle must be at least 14 feet in diameter to qualify for a tree. For a typical 16 foot diameter device we usually provide:
Type of Landscaping
Shrubs (no more than two different types)
Perennials (no more than three different types)
Flats of groundcover (one type)
All plant material and trees must be maintained consistent with SMC 15.42.050: Planting Trees and Shrubs. Multi-stemmed trees and shrubs that can grow tall, such as roses and lilacs are not acceptable. It is also advisable not to plant shrubs that require shearing. Usually the plants remain long after the person willing to consistently maintain them has left.