PR Referencing Standards for Roundabouts
Revised: May 30, 2007

This standards document has been issued by the MDOT referencing specialist in accordance with the basic PR Referencing Standards as a directive for the framework editing and for use by State Police Training.

I. BOULEVARDS
Each Boulevard uses two PR's, one for each side. Both PR's go in the same direction.
The forward side (usually eastbound or northbound) traffic moves with the PR.
The reverse side (usually westbound or southbound) traffic moves against the PR.

Diagram 1

When two boulevards cross, the four centerline PR's form four physical intersections at one logical intersection. The intersection square contains four different PR's and four corner points. When the officer measures the distance and direction of the crash, he picks one of the four points as his physical intersection starting point. Besides the direction of travel and the direction of the accident from the intersection, the UD10 contains three directions on the road-on and three directions on the crossroad:

1. Directional prefix
2. NB (northbound) may be included as an initial part of the street name for matching.
3. The 4 circled compass direction of the boulevard side: E, N, W, S

When crashes are located (have PR and mile point assigned) the location programs use all of the directional information that the UD10 contains. Especially for boulevards, the location success depends on the completion of adequate directional input.

Diagram 2

II. ROUNDABOUTS
A roundabout is a circular intersection formed by a continuous counterclockwise one-way circular drive with no impediments or stoplights or signals. All intersections approach the circle from the outside of the circle and enter and exit the circle at an oblique angle. With roundabouts, yield controls on all entries give vehicles already in the circle the right-of-way. Entry and exit points are designed with splitter islands and curved roadway that deflect and slow traffic to less than 30 mph. Additional design and pavement markings, such as directional arrows, help guide drivers through the intersection. There is no stopping in a roundabout and no left turns across opposing traffic lanes. Roundabouts possess special features that distinguish them from other forms of traffic circles. In comparison, a traffic circle may contain some, but not all of the features of a roundabout. A traffic circle is designed to allow traffic through as fast as possible, meaning that approaches and exits are kept as straight lines. Some traffic circles use stop control, or no control, on one or more entries. They may also require that circulating traffic yield to entering traffic. A traffic circle has no special provisions to force right-of-way. In some traffic circles parking is allowed and left turns are permitted.

Diagram 3
Like the boulevard-boulevard intersection, the roundabout is modeled with four PR's and four physical intersections. However, the shape of the intersection and the angles of the approaches are different. Diagram 3 shows the representation of a roundabout intersection. Actual roundabouts may have additional traffic controls and traffic lanes, which may be omitted from the standard centerline model for simplification. The same logic that works to locate crashes on boulevard intersections from the UD10 works to locate crashes on roundabouts.

In the centerline model, a roundabout will only be represented as such if the centerline diameter is greater than 50ft. Any roundabout with a diameter of less than 50ft will NOT be represented. This complies with existing PR standards applying to divided roadways. The location logic will work from the UD10 regardless of whether the roundabout is large or small or modeled as a standard roundabout or not. Roundabout models should be validated with the Referencing Specialist for resolution of questions.

Diagram 4a & b
The circular four-point model is used for a variety of roundabout variants. Diagram 4a shows a roundabout. The roads approaching the roundabout are not boulevards. In addition to the four physical intersection points on the circle, there are four divide points on traffic triangles where the traffic is separated by splitter islands as the roads approach the roundabout. These travel paths exist in real roundabouts whether the traffic control triangles are physical or not. Diagram 4b show a variation where the roundabout has a mixture of boulevards and triangles. In some cases a pair of ramps may replace the boulevard. Limited access ramps have the PR in travel direction.

Diagram 5
The standard four-point model serves roundabout variants with fewer approaches. Diagram 5 shows a roundabout that serves a basic T function such as the one at Marsh and Hamilton in Lansing.

Diagram 6
Diagram 6 shows a roundabout that has two ramps on opposite sides. Although the number of approaches to the circle exterior may vary, each approach must be one-way. Even though Diagram 6 has five PR's and four points, notice two peculiarities. The two ramps that exit and merge into the southbound freeway both go south in travel direction and have their own PR's. Second, the U-turn PR that forms the east quarter of the circle goes in travel direction because it is not paired with any reverse side.

III. Street names
Normally the PR's should follow street names. In the diagrams above, the streets and boulevards pass through the roundabout. Sometimes the streets terminate at the roundabout. In case of options, the street name on the roundabout should be the dominant one in terms of volume, length or trunkline status. In such cases, more than four different PR's may approach the roundabout. In addition to the primary road names, a circle name may be posted on the circle itself if there is a sign for the circle or if the circle name itself is customarily used. This provides the location software with the ability to accept either the circle name or the through road name. Although it has not happened in Michigan yet, it is conceivable that a roundabout could be constructed that required more than four intersection points. Not all the PR's on such a roundabout could be through PR's.

*See County PDF files below for examples of roundabouts modeled in the Michigan Geographic Framework version 7.

Calhoun
Intersection of Kalamazoo and Michigan (I94 BL)
Intersection of Hillsdale (M60) and Main (M99)

Ingham
Intersection of Bennett and Hulett
Intersection of Bogue and Shaw
Intersection of Hamilton and March
Intersection of Moores River, Boston, and Pattengill
Intersection of Wilson and Red Cedar

Jackson
Intersection of West and 4th

Kalamazoo
Intersection of Michigan, Knollwood, and Rankin

Livingston
Intersection of Kensington and Jacoby
Intersection of N 3rd and Main
Intersection of US23 NB Ramp, Lee, Fieldcrest, and Village Place
Intersection of US23 SB Ramp, Lee, and Whitmore Lake

Macomb
Intersection of 25 Mile and Hayes
Intersection of Utica Dodge Park
Intersection of Van Dyke 18 ½ Mile, and M53 SB Ramp

Manistee
Intersection of Cottage, Lakeshore, Monroe, and Oakwood

Muskegon
Intersection of Chesapeake and Walker
Intersection of Western and 3rd

Oakland
Intersection of Baldwin, Coats, and Indianwood
Intersection of Loop and Commerce Crossing
Intersection of Runyon, Tienken, and Washington
Intersection of Cemetery, Sheldon, and Tienken

Saginaw
Intersection of Washington, I75 NB Ramp, and I75 SB Ramp

Washtenaw
Intersection of Campus and Community
Intersection of Campus and Suncrest