A three-stage process was implemented in order to develop and evaluate the project concepts and provide recommendations for the project.

Transcription

1 MEMORANDUM TO: FROM: Highway 36/Rice Street PMT Mark Benson DATE: March 23, 2009 RE: Selection of a Preferred Alternative SEH No The purpose of this memorandum is to document the process for selecting the preferred interchange concept as part of the Highway 36/Rice Street Interchange Improvement Project. A. PREFERRED ALTERNATIVE Based on the detailed traffic analysis and open house input, the Project Management Team (PMT) selected the Offset Single Point - Option 6 (Figure 1) as the preferred alternative to be carried forward for further review, along with the No-Build Alternative, in the Project Memorandum/Categorical Exclusion (PM). The reason for this decision is identified below. Provides the best long term traffic operations Does the best job of accommodating bicycles and pedestrians (reduces one intersection and provides standard geometry and signalized protection for all the crossings) Allows for maintaining traffic during construction Is most compatible with plans for a future park and ride in the SE quadrant of the interchange The preferred alternative also includes improvements to the Rice Street corridor from CR B to CR B2 as indicated below: The roadway would be reconstructed to 4-lanes and median divided Turn lanes would be provided at the major intersections where traffic volumes dictate Stormwater and drainage improvements would be included in order to achieve watershed standards B. ALTERNATIVES DEVELOPMENT AND EVALUATION PROCESS A three-stage process was implemented in order to develop and evaluate the project concepts and provide recommendations for the project. The first stage of the process identified possible geometric concepts for improving the interchange configuration and signal spacing at the ramps and County Road B. A brainstorming session by Mn/DOT and consulting engineers resulted in seven reasonable concepts for further evaluation.

2

3 Preferred Alternative Memo March 23, 2009 Page 2 The second stage of the process reviewed the feasibility of the concept alternatives at a fairly high level to determine if there were any fatal flaws associated with the alternative. The third stage involved a detailed evaluation of the alternatives not eliminated through the second stage. This involved a planning level examination of the alternatives in which items such as traffic operations, construction cost, right-of-way impacts and other items were determined and compared. During this final stage, there was considerable interaction between the Project Management Team and the public in which variations and refinements to the alternatives were made and analyzed. The screening process ultimately resulted in one build alternative being recommended for further review in the Project Memorandum (PM). The three stages are discussed in greater detail in the following sections. 1. Stage 1: CONCEPTUAL ALTERNATIVES DEVELOPMENT & EVALUATION Mn/DOT initiated a brainstorming and internal screening effort to determine the range of feasible interchange concepts that would be reviewed by the PMT and consultant team. The Mn/DOT process resulted in five unique concepts. The consultant team developed two additional concepts. This resulted in a total of seven interchange concepts that were considered during the initial screening process. A brief description of the conceptual alternatives is provided below. Refer to the Figures in Appendix A for a graphical representation of the seven interchange concepts. Option 2A Single Point: This interchange configuration would convert the existing diamond interchange to a single point which would eliminate one ramp signal and improve the intersection spacing along Rice Street. Option 2B Single Point with Roundabout at CR B: Same as Option 2A but would include a roundabout at CR B instead of a signal. Option 3A Diamond with Two Roundabouts: This interchange configuration would utilize roundabouts at both ramp terminals. The south ramps and CR B would be combined into a single roundabout eliminating an intersection along Rice Street. Option 3F Buttonhook with Roundabouts: This interchange configuration would consist of roundabouts at the north ramp and at CR B. The south ramps would become a button hook west of Rice Street and tie into CR B. Option 4A Diverging Diamond: A diverging diamond interchange is a rare form of diamond interchange in which the two directions of traffic on the non-freeway road cross to the opposite side on both sides of the bridge at the freeway.

4 Preferred Alternative Memo March 23, 2009 Page 3 Option 5 Tight Diamond with Signalized Intersections: This interchange configuration would be similar to the existing but would move the south ramps north to improve the intersection spacing with CR B. Option 6 Offset Single Point: This interchange configuration would be similar to Option 2A but would shift the intersection north to improve the intersection spacing with CR B. 2. Stage 2: Fatal Flaw Screening Fatal flaw analysis identifies alternatives that may be removed from consideration early in the evaluation process by identifying easily determined deficiencies which cannot be overcome through project design or other mitigation efforts. The fatal flaw screening was based on feedback received through the public outreach efforts, input from the Project Management Team, and a generalized planning-level feasibility assessment that focused on traffic operational issues. In association with Ramsey County s CSAH 49 Improvement Project, the project team held two public open house meetings on November 18 and 19, The meetings provided an opportunity for residents to learn about the Rice Street/Highway 36 project, submit comments, and ask questions. During the initial screening process, the following performance measures were used to evaluate how well each alternative achieved the project purpose and need and satisfied the project goals and objectives. These were considered the minimum criteria that needed to be met for an alternative to move forward for additional evaluation. Minimize property impacts (right-of-way needs/relocations) Improve overall area traffic operations and safety Meet driver expectancy (geometric designs that are easy to navigate) Incorporate pedestrian friendly access A level of service (LOS) analysis was conducted for the signalized intersection and roundabout options using 2033 AM and PM peak hour traffic forecasts. Based on this analysis, the traditional interchange concepts, such as a single point or diamond, appear to accommodate the forecast traffic volumes and operate at a high level of service. Four of the alternatives did not. The following conceptual alternatives were screened from further consideration and their respective fatal flaws are identified below: Option 2B Single Point with Roundabout at CR B: This concept was dismissed from further consideration since it creates additional land use impacts and does not provide a substantial benefit over other single point concepts from a traffic operations perspective.

5 Preferred Alternative Memo March 23, 2009 Page 4 Option 3F Buttonhook with Roundabouts: This concept was dismissed from further consideration since it would introduce non-traditional geometry which did not meet the driver expectancy criteria. In addition it would place a major ramp intersection across from a neighborhood street. Option 4A Diverging Diamond with Underpass: This concept was dismissed from further consideration since it would introduce non-traditional geometry which did not meet the driver expectancy criteria. Option 3A Diamond with Two Roundabouts: This alternative was dismissed from further consideration due to traffic operational failures associated with the 6-legged roundabout at the south ramps. Given the results of the initial screening process, alternatives 2A, 5, and 6 were recommended by the Project Management Team for detailed evaluation. Additionally, a variation of Option 3A was added (Option 7) to offset potential operational issues with the six-legged roundabout in Option 3A. 3. Stage 3: DETAILED EVALUATION OF SCREENED ALTERNATIVES The final stage screening included full traffic simulation modeling for the four alternatives. The operational modeling results were used to ensure that each of the four alternatives was viable, and if an appropriate Level of Service (LOS), generally defined as LOS D or above, could be maintained. If not, the alternatives were screened from further consideration. The results from the simulation modeling were presented to the Project Management Team and an alternative ranking process was used to further screen the project alternatives. The alternatives that passed the more detailed operational analysis moved forward for a qualitative screening that included such factors as construction cost, right-of-way impacts, pedestrian friendliness, etc. The alternatives screening process was based on technical evaluation criteria established by the Project Management Team. A public open house meeting was held on February 4, The purpose of this meeting was to provide an opportunity for residents to view the project alternatives, submit comments, and ask questions. Those who stated an interchange preference supported the offset single point interchange (Option 6). Many comments also expressed the importance of accommodating bikes and pedestrians with the design. Based on the evaluation criteria and the PMT screening process, one build alternative (Option 6) was recommended as the preferred alternative. The following alternatives were screened from further consideration and their respective flaws are identified below: Option 2A Single Point: This alternative modeled well with all intersections operating at acceptable LOS. However, four of the movements were LOS E. Fewer movements experienced queuing that extended beyond storage capacity than with Option 5 (Tight Diamond with Signalized Intersections). This alternative was dismissed from further consideration for the following reasons:

6 Preferred Alternative Memo March 23, 2009 Page 5 o it did not perform as well from a traffic operations perspective as the other single point alternative (Option 6) o it did not accommodate pedestrians as well as the other single point alternative o it would be more difficult to construct than the other single point alternative Option 5 Tight Diamond with Signalized Intersections: This alternative modeled well with all intersections operating at acceptable LOS. However, two movements were LOS E and F. Some queuing extended past available storage lengths which led to concerns regarding sensitivity to traffic growth beyond the 2033 forecast. This alternative was dismissed from further consideration since it does not provide adequate reserve capacity beyond the 20 year forecast. Option 7 Diamond with Three Roundabouts: This alternative underwent extensive refinement and traffic operational modeling. In order to achieve acceptable traffic operations a very complex roundabout design was proposed. Upon further review this option was dismissed due to concerns with the driver expectancy/understanding. Furthermore, the option did not accommodate pedestrian movements as well as the offset single point. S:\PT\R\Ramsp\105803\Screening process\alternatives Selection Memo for PMT.doc

7 Appendix A Alternatives Considered

8 Option 2A Single Point Option 2B Single Point with Roundabout at CR B

9 Option 3A Diamond with Roundabouts Option 3F Buttonhook with Roundabouts

10 Option 4A Diverging Diamond SEH also developed several interchange concepts and two of these options (shown below) were included in the initial screening process. Option 5 Tight Diamond with Signalized Intersections

11 Option 6 Offset Single Point Option 7 Diamond with 3 Roundabouts. This option was added during the second stage screening to mitigate the operational problems with Option 3A Diamond with 2 Roundabouts.