Anchorage Alaska

Introduction

MTJ Engineering reviewed the Dowling Road Roundabout design at the request of Lounsbury and Associates, and Buckhurst Fish and Jacquemart. MTJ’s review included an evaluation of the expected safety, capacity, and operation of the existing design. Based on the findings of this review, the geometric design was revised to remove substandard elements, and to improve safety, capacity, and operation. Changes were in accordance with the recommendations of the FHWA "Roundabouts: An Informational Guide." Geometric changes were limited to the essential minimum, and were constrained to the original utility and ROW constraints to avoid significant cost increases. The revised design was based on the same design-year traffic volumes as the original design.

Safety

The FHWA Guide states that The FHWA Guide states that "the most critical design objective is achieving appropriate vehicular speeds through the roundabout: Roundabout speed is determined by the fastest path allowed by the geometry." Adherence to these recommendations in the FHWA Guide is necessary for controlling vehicle speeds and maintaining safe operation. The original design allowed entry speeds in excess of those recommended by the FHWA Guide, and the revised geometry reduced entry speeds to values close to the FHWA recommendations. The following table compares the original and the revised entry speeds with the FHWA values.

Vehicle Path Overlap

“Vehicle path overlap occurs when the natural path through the roundabout of one traffic stream overlaps the path of another. This can happen to varying degrees. It can reduce capacity, as vehicles will avoid using one or more of the entry lanes. It can also create safety problems, as the potential for crashes is increased.” (FHWA Guide pg. 174) The original design produced vehicle path overlap at entries and exits. The revised design corrects this deficiency.

Lane Balance

For high volume multilane roundabouts to operate well, lanes should be utilized equally by traffic. Lane imbalance leads to the under-use of some lanes while others are overused leading to un-necessary congestion. The original geometry would have produced lane imbalance in several ways.
1. The original arrangement of three entry-lanes into one exit-lane is sub-standard as this would cause traffic streams to merge within the roundabout. To avoid this traffic will tend to avoid using some of the entry lanes and this would lead to lane use imbalance. The revised design matches the number of entry and exit lanes to the traffic turning volumes to ensure that lanes are evenly used and that no weaving or merging is needed within the roundabout.
2. The large left turning volumes from both off ramps cannot be accommodated on a single left turn lane without excessive queues and delays. Consequently, double left turn lanes are needed. The original design did not address this problem. To ensure that the double lanes are fully utilized and that balance lane use is achieved, special signing and striping must be included in the revised design to instruct and guide drivers. In principle this is identical to the measures needed to ensure that double left turns are fully used at signalized intersections.

Summary of Modifications

Partial slip lanes

• Smaller, safer geometry
• Easier large truck movements
• Sufficient capacity
• ‘weave free’ on-ramps.

  Removal of ‘Tear-drops’

• Provides consistency on all approaches
• Achieves slower and safer entry for bridge traffic
• Improves off ramp capacity
• Provides U turns for side streets circulation.

  Capacity Improvements

• Off ramps require designated left and thru-left
• Two receiving lanes under bridge
• Four lanes (two in each direction) between roundabouts

  Balanced Lane Use

• Safer operation
• Lane congestion avoided

  Pavement Marking

• Advance lane use signs
• Matching circulating stripes
• Matching pavement arrows
• No weaving o More intuitive to drive
• Improved capacity
• Improved safety

  Construction Phasing

• Construct ultimate ‘improved’ geometry now
• Avoid cost of future construction required with original design

Summary

Unlike the original design, the revised design adheres to the advice in the FHWA Guide with respect to safety, lane use and vehicle path overlap. The original design did not have the benefit of the FHWA Guide as it was not published until after original design was completed. The revised design makes minimal geometric changes to provide essential improvement to the capacity, safety and operation of the roundabouts without exceeding the constraints imposed on the original design.