Alternative 4: Roundabout Interchange
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The roundabout interchange that is proposed consists of a spread diamond configuration, however instead of a typical intersection, where the ramps meet the local street, the intersection are in a roundabout configuration. Due to the design and environmental constraints of the north sections of the interchange, the intersection in the north side cannot be configured to a roundabout. The purpose of this interchange is to provide better traffic flow and reduce traffic delays especially for trucks and industrial vehicles.
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Traffic Analysis
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The traffic analysis for the Roundabout Alternative was performed identical to the L2 and L9 Diamond Alternatives, which was analyzed using Synchro 10 and the Highway Capacity Manual 2010 (HCM 2010) standards to determine the level of service. The only difference is that the proposed roundabout (which was located at the Jack Rabbit/EB ramps) was analyzed using the program Sidra, which is a program that can effectively analyze the level of of service of roundabouts. In this alternative, initially 2 roundabouts were to be designed with one at the westbound ramp intersection and one at the eastbound ramp intersection. However, the traffic analysis performed in Sidra and Synchro showed that there would be an excessive buildup of vehicles on Jack Rabbit approaching both roundabouts. Due to this, the traffic team attempted the analysis again by replacing the northern roundabout with a signalized intersection, and the analysis showed that this modification would increase the circulation for the roundabout alternative. The roundabout had the overall best level of service compared to all other alternatives, but was not chosen due non-traffic related reasons.
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Geometric Feasibility
The roundabout interchange that is proposed consists of a traffic circle at the south and a four-ways controlled intersection at the north. By designing a controlled intersection at the north side, this promotes a remarkable Level of Service by improving traffic delay that would have caused by a standard traffic circle. Traffic circle on the south portion yielded passing Level of Service and did not impede trucks maneuvers based on Synchro, Sidra, and Rodel analysis. Therefore, we may proceed to the next step in selection process.
Result: PASS
Result: PASS
Impact Evaluation
The possible negative impacts of the Roundabout Interchange to the surrounding area were evaluated in this step. The design goes through an 100-year flood hazard zone; therefore, a flood hazard analysis will need to be conducted. A portion of land with a high potential for paleontology resources goes through the Roundabout design boundary. This means that a Paleontology Identification Report needs to be done in the northwest portion of the site. The drainage at the site needs to accommodate additional storm water runoff since pavement will be added at the site. Sound walls will also need to be constructed as the design is located near a noise sensitive receptors. This alternative is still considered viable at this point and will move on to the next step in the selection process.
Detailed Design
The Roundabout Interchange consists of a roundabout on the south side of State Route 60 and a T-intersection on the north side. This interchange does not have a roundabout in the north because a traffic analysis was conducted with a double roundabout showing that this would cause a buildup on the ramps and Jack Rabbit Trail. There were a variety of standards that were used to design the roundabout. This includes the Wisconsin Roundabout Standards, The U.S Department of Transportation Design manual, the National Cooperative Highway Research Program 672 Roundabouts, and Caltrans Highway Design Manual. Since there is no official design manual on roundabouts for California, these standards were cross-referenced in order to meet their guidelines.
The cross section of Jack Rabbit Road is based on the County of Riverside Secondary Highway Standards. It has been modified to include a 6’ bike lane in each direction to improve access for all users. This bike lane included a 2’ buffer to increase comfort and security for the users. An 8’ sidewalk is included on the right side of the cross section. County standards suggest a 5’ sidewalk be included on both sides of the highway but this standard was modified because no development exists on the west side of the highway and conservation land in this area makes development unlikely. Instead, an 8’ wide sidewalk as included to meet ADA requirements and serve development on the eastern side of the highway. The Jack Rabbit Cross section is shown below:
For our project study report, an Advance Planning Study (APS) was done to define the scope of structure work and develop a reasonable cost estimate for budget purposes. APS consists of a plan sheet for each structure with basic dimensions and minimal detailing including: the vertical clearance, span length, and structure type. Each plan sheet contains the plan, elevation, typical section view, and cost estimate of the structure. All these structures will be constructed out of precast concrete. The Bridge Design Aids were used for the design of the new bridges.
The roundabout design is similar to the spread diamond since the span length is the same. However, Jack Rabbit Trail will have a higher vertical profile due to the roundabout intersection being flat grade instead of down grade which means that the bridge will have a higher clearance of 22’. The APS of the roundabout bridge can be seen below:
The roundabout design is similar to the spread diamond since the span length is the same. However, Jack Rabbit Trail will have a higher vertical profile due to the roundabout intersection being flat grade instead of down grade which means that the bridge will have a higher clearance of 22’. The APS of the roundabout bridge can be seen below:
Viability
The Roundabout Interchange went through all steps done while reviewing possible alternatives - Alternative Scoping, Traffic Analysis, and Geometric Design. It was considered as one of the three viable alternative design through all of the aspects mentioned in detail above. It also passed the traffic analysis, geometric feasibility, and impact evaluation steps and was considered viable for detailed design. All documents and deliverables pertaining to this design can be found under the Project Deliverables Tab.
