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ROUNDABOUTS

ROUNDABOUTS. What Is A Roundabout?. A specific type of traffic circle Not all traffic circles are roundabouts. Defining Elements. Yield controlled at entry Priority to circulating vehicles Traffic rotates counterclockwise Low speeds Parking Pedestrian access. Yield Controlled at Entry.

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ROUNDABOUTS

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  1. ROUNDABOUTS

  2. What Is A Roundabout? • A specific type of traffic circle • Not all traffic circles are roundabouts

  3. Defining Elements • Yield controlled at entry • Priority to circulating vehicles • Traffic rotates counterclockwise • Low speeds • Parking • Pedestrian access

  4. Yield Controlled at Entry Roundabouts Other Traffic Circles

  5. Priority to Circulating Vehicles Roundabouts Other Traffic Circles

  6. Traffic Rotates Counterclockwise Roundabouts Other Traffic Circles

  7. Low Speeds

  8. Parking Roundabouts Other Traffic Circles

  9. Pedestrian Access Roundabouts Other Traffic Circles

  10. Key Roundabout Features • Central island • Splitter island • Inscribed circle diameter

  11. Pros and Cons • Pros • Reduces delay • Shorter queues • Improves safety • Aesthetics • Cons • Space requirements • Driver unfamiliarity • Grade requirements • Pedestrians

  12. Planning • Consider the context (context sensitive design) • Determine design year traffic volumes • Determine roundabout category • Estimate space requirements • Determine space feasibility • Perform operational analysis

  13. Context Sensitive Design • Regional policy constraints • Safety • Aesthetics • Pedestrians and other users • Land Use Type • Site constraints • Roadway speeds • Right-of-way

  14. Design Year Traffic and Roundabout Category Exhibit 3-1 From the FHWA Roundabouts: An Informational Guide) Two-lane roundabouts One-lane roundabouts • Design AADT = 38,000 • Left Turn % =30% • % Minor street volume = 50% •  Use 2 lane roundabout

  15. Space Requirements Exhibit 6-19 (FHWA Roundabouts: An Informational Guide) WB-15 (WB-50) = General Semi / Tractor Trailer

  16. Example: State Hwy 8 and Mt View Rd • Objective With the available information and the general design parameters discussed in class, determine whether or not a roundabout is a feasible alternative for this intersection.

  17. State Hwy 8 and Mountain View Rd

  18. Consider the Context • Entrance to Moscow and potential developments to the south • Encourages speed reduction • Very few pedestrians • Urban setting • Major through street with truck traffic

  19. N Predict Design Year Volumes 2025 Design AADT Entering Volumes • Total AADT for intersection • 12905 + 6210 + 8605 + 6070 = 33790 6210 • Percent minor street volume • 100*(6210 + 6070) / 33790 = 36% State Hwy 8 • Left-turn Percentage • Assume 30% 12905 8605 6070 Mountain View Rd

  20. Determine Category Exhibit 3-1 From the FHWA Roundabouts: An Informational Guide) • Design AADT = 33,790 • Left Turn % =30% • % Minor street vol = 36% •  Use 2 lane roundabout

  21. Determine Size of Roundabout Exhibit 6-19 (FHWA Roundabouts: An Informational Guide) • Urban Area • Truck Traffic (WB-15) • Double lane •  Use an Inscribed Circle Diameter 150 ft – 180 ft

  22. State Hwy 8 and Mountain View Rd

  23. Perimeter Drive and Sixth Street

  24. Sixth and Line Street

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