"Traffic safety is extremely important to ensuring livability for people who live, work, and play in our city. Pedestrians and motorists alike must feel confident that traffic is engineered in such a way that risks of accidents and injuries are minimized. Traffic circles are an effective traffic calming measure, and when designed well, can be an aesthetic addition to neighborhood intersections." -Mayor Belton

Minneapolis sets aside $20 million per year to improve the quality of life in its neighborhoods under the city’s Neighborhood Revitalization Program (NRP). Each of the city’s eighty-one residential neighborhoods may request funds to spend on their improvements, but these requests must be accompanied by an action plan that has broad neighborhood involvement and support. NRP plans, in general, have a three- to five-year implementation horizon.

Sixty-six neighborhoods have submitted action plans to date, with forty-eight requesting funds to improve transportation and/or pedestrian safety. Traffic circles and speed humps are measures frequently requested by the neighborhoods concerned with traffic safety.

Process for Neighborhood Input and City Action

Once the NRP Policy Board and the City Council approve a neighborhood’s action plan, the Public Works staff meets with elected/appointed neighborhood representatives to identify transportation- and safety-related problems. Staff solicits suggestions from the neighborhood representative for solutions to identified problems. Public Works staff offers comment and suggests one or more solutions that address the identified transportation problem, stressing the city’s experience with each. These discussions are documented.

Where indicated, Public Works staff conducts engineering studies to substantiate the nature and severity of identified problems. After review and comment, Public Works staff recommends specific solutions, giving reasons for the recommendations. When the neighborhood and Public Works reach a consensus on the problem and a solution is agreed upon, Public Works staff prepares a petition for neighborhood volunteers to circulate to affected residents and property owners. The petition asks the city to test the proposed solution. In most cases before-and-after studies document the test solution’s effectiveness. If the test is successful, neighborhood volunteers circulate a second staff-prepared petition, asking the city to construct a permanent measure.

On completion of the construction, the Public Works Finance Section bills the neighborhood account - within the annual $20 million set aside for NRP - for payment of the testing and construction costs incurred.

Traffic Circles

For the last seventy-five years, the base-posted speed limit in urbanized areas of Minnesota has been thirty miles per hour. Over the years, as handling qualities of the fleet improved, the average and 85th percentile speed on city streets began to creep upward. By 1982, Minneapolis began implementing a basket-weave stop sign plan to control traffic on local streets and reduce the number and severity of accidents at uncontrolled intersections. Today the pattern of placing alternating two-way stop signs at every other block is virtually complete.

By the mid-1990s, however, city residents expressed growing concern over the volume and speed of vehicles going through their neighborhoods Ñ in spite of the growing number of stop signs. At the same time, a citizen movement to reclaim ownership of city streets began to gain momentum. City residents became increasingly inclined to redefine collector and arterial streets as neighborhood ("local") streets. They began pressing the city’s elected officials and staff to implement physical measures that would limit vehicle speed and volumes not only on local streets, but on collector and arterial streets as well. Traffic circles moved to the fore as one of the key measures to accomplish these objectives, and Minneapolis began experimenting with the concept in 1994.

Geometrics: Design and Signing of a Minneapolis Traffic Circle

A traffic circle in Minneapolis is a 3.6 meter (twelve-foot) to 7.3 meter (twenty-four-foot) diameter circle placed at the crossing of the centerlines of two intersecting local streets. Except in rare circumstances, the intersection has four approach legs. Mountable by emergency vehicles, the traffic circle’s curb is ten centimeters (four inches) high at the outside and fifteen centimeters (six inches) high at the inside edge Ñ as measured from street level. The width of the curb is 60.9 centimeters (twenty-four inches), and the circle has no gutter.

Landscaped in the center, the circle is subject to a 0.9 meter (three-foot) clear zone from the inside edge of its curb, in which nothing growing higher than 45 centimeters (eighteen inches) may be planted. A tree may be planted in the center of traffic circles whose diameter is 4.9 meters (sixteen feet) or greater.

Care is taken that the intersection is well lit. Facing each approach, a nine-point diamond sign is mounted 0.9 meters (three feet) inside the inner edge of the circle’s curb. Warning signs are posted forty-five to sixty meters (150 to 200 feet) on each approach in advance of every intersection containing a traffic circle. Where volumes warrant, obstruction approach markings are painted on the pavement.

Minneapolis prohibits on-street parking for a distance of twelve meters (forty feet) Ñ measured from the nearest curb face of the intersecting street Ñ on both sides of each approach street. This measure is taken to accommodate the turn characteristics of Metro Transit vehicles, fire emergency vehicles, school buses, and city garbage trucks. Advisory signs describing acceptable turning behavior are also posted at the traffic circle. (See diagram with traffic calming signs.)

The Public Works Department requires a twenty-two foot clearance between the outside edge of a traffic circle and an intersection’s curved curb sections to allow for snow accumulation, safe vehicle movement past the circle, and safe conditions for pedestrians in the crosswalks.

Geometrics: (T) Intersections

Though Minneapolis’s first two permanent traffic circles were installed at (T) intersections, such installations are no longer permitted, except under very controlled circumstances. In the city’s experience, the effectiveness of a traffic circle depends on the forced, lateral deflection of a vehicle from its straight-line path. Where on-street parking is present virtually twenty-four hours a day, a vehicle approaching a traffic circle cannot run the gutter for a distance greater than forty feet. The driver must slow his/her vehicle to comfortably negotiate around the traffic circle. This condition of omnipresent on-street parking is the rare exception in Minneapolis, but was the condition that prevailed at the locations of the first traffic circle installations. Where traffic circles were tested at the more ordinary (T) intersections, it was found that gutter-running speeds actually increased over pre-test speeds. Now the Public Works Department will permit traffic circles at (T) intersections only if permanent bump-outs (throats) are constructed on the approaches to prevent gutter-running behavior.

Costs

Construction costs range from $3,500 to $4,000 for a traffic circle in the center of a four-legged intersection, depending on the size of the circle. These costs include associated signage, pavement marking, soil replacement, and sodding. They do not include the cost of landscaping beyond sodding. The construction of throats at (T) intersections interrupts curbside drainage patterns and requires addition or relocation of catch basins and associated storm sewer lines. Costs in these situations can range from $8,500 to almost $40,000.

Lessons Learned about Traffic Circles

Minneapolis has learned the following from testing and constructing traffic circles:

1. Traffic circles reduce vehicle speeds. Properly designed and installed, traffic circles reduce average and 85th percentile vehicle speeds to the twenty-two to twenty-seven mile per hour range (mph).
2. The traffic circle must be well lit.
3. Under certain circumstances, traffic circles may cause a diversion of traffic. In one instance, more than 700 vehicles per day were redirected from a three-block-long segment of an impacted local street back to a paralleling arterial street one block (200 meters or 660 feet) distant. The sequence of measures on the local street was: traffic circle stop sign.
4. The number of fixed-object accidents may increase when a traffic circle is installed on a collector or arterial street.
5. It is no longer recommended that a tree be planted in a traffic circle with a diameter less than 4.9 meters (16 feet).
6. Stop signs are used to define a right-of-way at an intersection containing a traffic circle as opposed to the former practice of using yield signs.
7. Traffic circles are no longer tested or constructed at (T) intersections except under very controlled circumstances, as noted above.
8. Traffic circles are no longer tested or constructed on collector or arterial streets.
9. All traffic circles now tested or constructed must conform to the geometrics noted above.

Speed Humps

Stop signs and traffic circles modify driver behavior at intersections. Speed humps modify driver behavior between intersections. Prior to installing speed humps, Public Works staff verifies vehicle speed conditions on the street segment in question and determines the appropriateness of a speed hump application. If a favorable finding is made, staff prepares a petition that neighborhood volunteers circulate to all affected residents and property owners. The petition asks the city to install a pair of speed humps and indicates a willingness to pay for the humps whether by assessment or out of neighborhood NRP funds. Since the humps cannot be tested for a short time as can traffic circles, the neighborhood must make provision for the humps’ removal in the event the petitioners are not satisfied with the humps or their results.

Geometrics

Speed humps are between seven and eight centimeters (three inches) high, 3.6 meters (twelve feet) wide and stretch across the traveled roadway from edge of gutter pan to edge of gutter pan. Properly designed and installed, speed humps will not interfere with normal curbside drainage. Speed humps are installed in pairs, spaced about 250 feet apart. However, the city will not install more than four speed humps per half-mile. Elected officials approved criteria for speed hump use in August 1994. (See the diagram for speed humps signing.)

Costs

Minneapolis’s cost for installing, striping and signing a pair of speed humps is $7,000.

Lessons Learned about Speed Humps

The experience with speed humps in Minneapolis follows:

1. They work. When used in pairs, they reduce average and 85th percentile mid-block speeds to the fifteen to twenty mph range.
2. The city does not install speed humps on collector or arterial streets, public transit bus routes, designated truck routes, or primary emergency vehicle access routes.
3. Speed humps are not installed on local streets with volumes less than 500 or more than 2,000 vehicles per day.
4. Neither are they installed on streets with adverse geometry, such as steep grades, sharp vertical or horizontal curves, or streets whose stopping distance sight lines are less than 200 feet.
5. Residents on streets with speed humps have generally been pleased.

Available Materials

The Minneapolis Public Works Department has handout materials on the NRP and traffic improvements, including:

• "Partners for a New Transportation Future," a two-page description of the process for Public Works Department and neighborhood interaction;
• "Criteria for Speed Hump Use," a one-page description;
• a one-page flyer on traffic circles;
• a one-page diagram of typical signing and marking for speed humps;
• a one-page diagram of pavement marking detail; and
• a one-page diagram of typical cross-sections for traffic calming.

Contact: Michael J. Monahan, Assistant Director of Public Works, Director of Transportation, Minneapolis, telephone: 612/673-2411; FAX: 612/673-2149.

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The United States Conference of Mayors

J. Thomas Cochran, Executive Director
1620 Eye Street, NW, Washington, DC 20006
Telephone (202) 293-7330, FAX (202) 293-2352

Copyright ©1996, U.S. Conference of Mayors, All rights reserved.