Road Management & Engineering Journal
January 1, 1998
Fax: (360) 335-6402
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.
Traffic Circles: Neighborhood Traffic Control Program
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.
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:
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.
[Recorded Correctable Accident Rate
based on past three years]
.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
|500-1100 vehicles per day
1101-1700 vehicles per day
1701-2300 vehicles per day
2301-2700 vehicles per day
[85th % Speed]
|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
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
II. Demonstration Phase