The roundabout interchange that is proposed consists of a partial cloverleaf setup on the west side of the I-215 to reduce the number of left turns from the southbound off-ramp, then we added a bypass lane to divert the people now turning right away from the roundabout to decrease delay. The east side of the I-215 is a tight diamond setup to minimize right-of-way impacts.

Roundabout Interchange

Traffic Analysis

Initially, Synchro 9 was used to conduct the traffic analysis on the roundabout alternative but the alternative failed after multiple trials. We researched more about roundabouts and found that Synchro 9 is not a recommended software for roundabout analysis. Rodel and SIDRA are the industry standard software for roundabout analysis.

We then used Rodel to analyze the roundabout alternative using the HCM 2010 Method. Our initial layout was a compact diamond with roundabout intersections, however the alternative obtained inadequate Level of Service at the southbound off-ramps due to the high number of left turns. The HCM 2010 Method analyzes only the vehicles circulating in the roundabout, so we came up with the idea of changing the southbound off ramp to a loop ramp to convert the left-turn movements to right-turns and creating a bypass to divert the vehicles from the roundabout. This resulted in adequate Level of Service at both ramp intersections.

Result: Pass

D/C

A/B

Geometric Feasibility

This step involved a rough design for the roundabout interchange. By designing a hybrid configuration with a partial cloverleaf setup (namely, a southbound loop ramp) the west of the freeway and compact diamond ramps east of the freeway, we determined that the interchange could be laid out within the existing right-of-way and undeveloped land, while still providing the passing levels of capacity as determined in the Synchro analysis. As this alternative is still considered viable, we may proceed to the next step in the selection process.

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 undetermined flood hazard zone; therefore, a flood hazard analysis will need to be conducted. A geological impact report will also be needed for this alternative because the design is located in an area that is susceptible to landslides. 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 northeast portion of the site. The drainage at the site needs to accommodate additional stormwater runoff since pavement will be added at the site. Soundwalls will need to be constructed as well as the design is located near two noise sensitive receptors. This alternative is still considered viable at this point and will move on to the next step in the selection process.

Result: Pass

Detailed Design

Merge/Diverge Analysis

A Merge/Diverge Analysis is an examination of the impact a ramp is expected to have at freeway-ramp junctions. The methodology provided in the HCM 2010 was followed for this analysis. The analysis procedure first involves computing the demand flow rate immediately upstream of the merge (on-ramp) area or diverge (off-ramp) area. The capacity of merge/diverge area was compared to the demand flow rate; if the demand is greater than capacity, then the LOS for the ramp-junction is F. If not, the density was then computed and the LOS was determined accordingly. Failing LOS indicates that the length of the merge/diverge ramp is inadequate. However, since the nearest adjacent interchange next to this project location is only 1 mile north of Keller Rd, auxiliary lanes must be added as per the Caltrans Highway Design manual between the Scott Road and Keller Road Interchanges. Shown below is the LOS obtained from the analysis.

Geometric Design

For the roundabout, FHWA standards and a variety of standards from different states in the US, namely Massachusetts and Wisconsin, were followed as there are no official standards for roundabouts in the state of California. This is an example of design flexibility as well as innovation. The roundabout is an intersection design that does not use signaling or stop signage. Vehicles approaching a roundabout yield to the cars already in the roundabout and cars move in a counterclockwise direction into whichever leg of the intersection they choose.

For basic technical aspects of the roundabout, the roundabout is designed to slow down traffic to around 30 mph from the Keller Rd design speed of 45 mph. The lanes in the circle are wide enough in order to allow the STAA Long design vehicle to get by the circles. The sidewalk and bike lane are exclusively on the north side of Keller to make it easier for the on and off ramps on the west side of the interchange. There are bypass lanes to limit the traffic going into the circles. The interchange is in a diamond configuration on the east side and a partial-clover configuration on the west side. This provides an innovative aspect of this design. Both the on-ramps and off-ramps on the west side of the interchange are on one side of the circle. This was done after analyzing the traffic flow through Rodel and finding that the best traffic conditions for the roundabout would occur with this configuration.

B

C

D

APS, or Advance Planning Studies, are required in Project Study Reports and Project Reports to document the scope and cost of structure work in projects. They consist of one plan sheet for each structure showing the basic layout details and cost estimate. Typically APS must be prepared for all structural work including bridges, culverts, tunnels, retaining walls, and any non-standard or special design structures. It helps decide on a feasible structure type, requirements, constraints, and an associated cost estimate summary for the structure work involved. For this project, the only major structure within the scope of the project is the bridge which lies at beneath the I-215 freeway at the intersection with Keller Road. From current conditions, the existing bridge at this location is below the minimum 15 foot vertical clearance from Caltrans bridge design aid requirements.

For the Roundabout, due to its unique layout, a new design for a new bridge needs to be put in place. The length is increased to 150 feet to accommodate for the two sections of the local road that come into contact with the bridge. However, the cantilever walls and extended width remain the same. Finally, we changed the structural section type to a pre-cast California wide flange girder due to its relatively low cost as well as its time savings in construction compared to the other possibilities considered.

Advance Planning Study

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.