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Diverging Diamond Interchange Alternative
The Diverging Diamond Interchange (DDI) design will connect the I-215 and Kendall Drive creating an interchange with fewer conflict points and better sight distance. The DDI will have better flow as the I-215 on/off ramps will have yield control preventing unnecessary stops. This layout will consist of two signal controlled intersections. A median will also be implemented to separate the traffic traveling in the opposite direction, which will, as a result, decrease the number of head-on collisions.
Traffic Analysis
The traffic team performed a traffic analysis for this alternative using Synchro 10 and forecasted traffic volumes from the San Bernardino County Traffic Analysis Model (SBTAM) and the Southern California Association of Governments model (SCAG) and using the Highway Capacity Manual 6th Edition standards for the level of service. For intersection 5, the Level of Service was found to be D and F for AM and PM peak hours, respectively. While other intersections had the Level of Services (LOS) ranging from A to C because Intersection 5 expected to accommodate high traffic on Campus once the alternative is built. Our analysis also showed that the delay times of all intersections have reduced significantly. The key intersection 5 has delay time decreased by half despite having the same LOS for PM peak hour when compared to No-Build alternative.
Geometric Feasibility
The DDI with a signal control consists of a three or four-legged intersection that has traffic lights used to control the traffic in this intersection. The signals will have multiple phases to provide the most efficient way for vehicles to safely proceed through the intersection. These signals will be optimized to provide better flow at the different intersections at different times of the day. A potential conflict exists each time a vehicle, pedestrian, or bicycle crosses or turns across the path of another direction of traffic. Compared to a conventional diamond interchange, the DDI reduces vehicle-to-vehicle conflict points by nearly 50 percent and eliminates many of the most severe crash types.
Impact Evaluation
The complex design of the DDI increases the costs of the right of way that would need to be purchased and the amount of retaining walls that would need to be built compared to other alternatives. The project site is alongside a channel and is located within a 100-year floodplain but to address these issues a NEPA (National Environmental Policy Act) permit is required and the first level must be constructed 2 feet above the flood plain which as been taken into account with the design. In addition, proper drainage will be implemented in the design and a Storm Water Pollution Prevention Plan as outlined by SBCFCD (San Bernardino County Flood Control District) will be created. Due to the project site located parallel to residential homes a sound wall is to be considered due to the noise that may come during and after construction along with a possible sound analysis for the area.
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There are three different particular species (shown above) that have been identified that require proper mitigation techniques. Proper mitigation techniques such as relocation of a species are thought to be the best tactic not only ensure the completion of this project, but also the safety and prosperity of the species. Some possible permits needed would be an incidental take permit for the San Bernardino kangaroo rat or a voucher collection permit for endangered plants for relocation.
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Detailed Design
In this alternative, the Campus Pkwy bridge overcrossing I-215 will have a total of five 12' lanes with through traffic traveling on temporarily on the left side, while bike lanes and walkways will be placed at the center of the bridge. On ramps will consist of two single-lane legs merging into a two-lane ramp, and off ramps will have one lane splitting into two single-lane legs. All ramp lanes are 12' wide with an 8' right shoulder and a 4' left shoulder.
The cross-section of the overcrossing will have a 6’ bike lane in each direction to improve access for all users, two 12’ through lanes, and one 4’ shoulder on each side of the 16’ median with a 6’ sidewalk on the north side.
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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. The DDI has a total of 5 bridge structures: the mainline going over the I-215, two bridge structures of the southbound off ramp going over Devil Creek, and two bridge structures of the northbound on ramp going over Devil Creek. All these structures will be constructed out of cast-in-place concrete. The Bridge Design Aids were used for the design of the new bridges. Please refer to the Geometric Approval Drawing and Advanced Planned Study under the Deliverables tab for geometric details of this alternative.
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Viability
The Diverging Diamond 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.