Road Management & Engineering Journal
Road Management & Engineering Journal
January 1, 1998
TranSafety, Inc.
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Traffic Circle Design Criteria

(Reproduced with permission from the City of Seattle, Engineering Department, Transportation Division, Neighborhood Traffic Engineering. The design criteria below are Exhibit 8 of "Neighborhood Traffic Control Program--Citizen Requested Traffic Circle (City Funded)," City of Seattle, Engineering Department, Transportation Division, Division Operating Instructions, effective April 22, 1986.)

The purpose of this design criteria is to provide, at the lowest cost possible, for a traffic circle that will reduce traffic speeds and accidents while allowing for the movement of large trucks through the intersection. This design criteria will also provide for the largest possible traffic circle, and thereby allow maximum landscaping for beautification, and to visually warn drivers of the obstruction.

There will be cases where this design criteria cannot be totally followed and/or where one or more curb turns have to be reconstructed. Some of these cases are, where intersecting streets are of different widths, and where one or more of the intersecting streets are off-set or angled. There will also be situations requiring that special attention be given to landscape and aesthetic considerations. In these cases, engineering judgment will be used in following the design criteria as closely as possible, with traffic safety and operation of prime concern.

Design Criteria:

  1. The distance between a traffic circle and the street curb projection (off-set distance) shall be a maximum of 5 1/2 feet. See Intersection Diagram on page 2.

  2. The width between a traffic circle and a curb return (opening width) shall be a minimum of 16 feet and a maximum of 20 feet. See Intersection Diagram on page 2.

  3. As the off-set distance decreases from the maximum 5 1/2 feet, the opening width shall increase from the minimum of 16 feet. See Intersection Diagram on page 2 and Dimension Chart on page 3.

  4. The outside 2 feet of the traffic circle will be constructed with a mountable monolithic cement concrete curb and pavement surface doweled to the existing pavement. See Exhibit 9.

  5. Traffic circles will be landscaped when agreement with neighborhood is made for them to maintain.

  6. Traffic circles less than 15 feet in diameter will have one tree centered along with other plantings. See Exhibit 9.

  7. Traffic circles at least 15 feet in diameter will have three trees equally spaced and set back 4 feet from the curb face along with other plantings. See Exhibit 9.


Traffic Circles: Neighborhood Traffic Control Program

(Reproduced with permission of the City of Seattle, Engineering Department, Transportation Division. The source brochure for this information was produced by Seattle Engineering Department, Transportation Division, Community Affairs Graphics. The brochure is part of the "Neighborhood Traffic Control Program--Citizen Requested Traffic Circle (City Funded)" described above.)

For Your Information

In recent years, citizens of many of Seattle's communities, in partnership with the City, have been involved in the installation of raised traffic control devices such as traffic circles on neighborhood streets. The purpose of the Neighborhood Traffic Control Program (NTCP) has been to reduce accidents and/or speeds on residential streets, thereby creating safer, more pleasant neighborhoods for Seattle's citizens. The projects which have been completed to date have shown substantial reduction in such traffic problems with minimal inconvenience to automobile drivers. They have also attracted the attention of many other neighborhoods in the City with similar concerns and goals.

Based on a high demand and identified need, a traffic control program was established in 1978 as part of the City's annual Capital Improvement Program (CIP). The CIP has appropriated money each year for approximately twenty traffic control projects.

Criteria for Participation in the Program

Because of the high demand and limited funding, an evaluation process for determining projects to be included in the NTCP has been established. This process involves the following two steps, both of which must be completed prior to a location being considered for funding.

Step 1

Community Support -- No project will be considered without support from the citizens of the affected area. Responsibility for completion of this step rests with the community and involves the following:

  1. An individual or group must make a request for inclusion in the current NTCP by calling the Transportation Division of the Seattle Engineering Department at [206] 684-5087. Calls will be directed to the NTCP staff. All requests should be made prior to July 1; any received after this date will not be considered for funding until the following June.

  2. If the neighborhood decides to proceed with the project, petition forms will be given to the applicants and signatures must be gathered from at least 60% of the households (owners or renters) and businesses (property or business owners) within the affected area. Only one signature per household is needed. Signed petitions must be submitted to SED by July 15, to be considered for the following year's construction. Completed petitions can be mailed or delivered to: Transportation Division, Room 708 Municipal Building, Seattle, WA 98104, ATTN: NTCP PROGRAM.

Step 2

Traffic Safety Analysis -- During this step the Engineering Department will evaluate the safety record of each location using the collision history and may also conduct speed studies and volume counts. The data will be used to rank the locations based on the point criteria listed on the back of this brochure. The point total or rank will determine each location's priority for construction of a traffic circle.

A location must accumulate at least 3 points to compete for funds. Locations with a minimum of 3 points will then be prioritized. If, at this point, the location does not qualify, the project will be dropped from the list of potential traffic circle locations and the contact person listed on the petition will be notified by mail.

If a location ranks sufficiently high on our prioritization list for NTCP funding and the physical characteristics of the area allow for a traffic circle installation, then the contact person will be notified and a community meeting will be arranged.

Point Criteria

Accident History
[Recorded Correctable Accident Rate
based on past three years]
Points
1
2
3
4
5
6
1/2

1/2


.5-.875 Accidents annually
.876-1.250 Accidents annually
1.251-1.625 Accidents annually
1.626-2.000 Accidents annually
2.001-2.375 Accidents annually
2.376-2.750 Accidents annually
If "non-correctable" intersection accidents exceed an average of 2 per year over the last three years
If accidents on a midblock section of street exceed 2 per year over the last three years, average

Traffic Volumes
[Weekday Average]
1/2
1
1 1/2
2
500-1100 vehicles per day
1101-1700 vehicles per day
1701-2300 vehicles per day
2301-2700 vehicles per day

Traffic Speeds
[85th % Speed]
1/2
1
1 1/2
2
2 1/2
3
26-29 miles per hour
29.1-32 miles per hour
32.1-35 miles per hour
35.1-38 miles per hour
38.1-41 miles per hour
41.1-44 miles per hour

Project Implementation

Based on past experience, legal requirements and City policy, a process for project implementation has been developed involving a strong commitment on the part of both citizens and City staff. The process has two components: plan development and demonstration. These phases are described below:

I.     Preliminary Plan Development Phase

A meeting will be held in the neighborhood to discuss the traffic problems, the analysis to date, the design and appearance of the traffic circle, and the procedures which will be followed to arrive at the construction of the final product. This phase may take up to three months to complete for all program locations.

II.    Demonstration Phase

The second phase of the project involves the demonstration of the desirability of a traffic circle.

  1. The circle will be installed and monitored for approximately six months, at which time a survey of the residents and any business will be conducted. Where landscaping is feasible, the circle will be landscaped if a local resident will commit to maintaining the landscaping. The landscaping will be installed after the trial period.

  2. The Engineering Department will prepare a final Project report based on information collected during the demonstration. This information will include surveys of residents/business people within the area, community meetings, review by the Police and Fire Departments, and further traffic analysis.

  3. The Engineering Department will make the final decision regarding the project on the basis of the final report. However, if more than 40 percent of the residents oppose retaining the circle, a public meeting will be held to discuss the project with the residents before any decision is made.



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