I-15/Limonite Ave Interchange
Improvement Project
Traffic Analysis
Partial Cloverleaf Interchange
The Partial Cloverleaf Interchange is a conventional interchange that is in essence an expansion of a typical diamond design. In addition to the standard four ramps, two on-ramps and two off-ramps, there are two extra loop ramps, both of which are on-ramps. The purpose of the additional loop ramps is to divert left-turn traffic and turn them into right-turns.
A Synchro Analysis was conducted on the Partial Cloverleaf Interchange to determine whether or not it was capable of handling the predicted 2040 traffic volumes. Initially the ramp configuration was made to determine if any significant improvements could be seen in the level of service. When it was determined there was an improvement the lane configurations were adjusted to achieve a desired level of service. In the end the lane configuration used produced a level of service of C for both intersections which is within the acceptable range.
The Partial Cloverleaf configuration is able to handle the predicted 2040 traffic volumes because of how it diverts the left-turn movements. In the No-Build alternative the highest volumes were the through movements across the bridge. Major delays were created when these movements were being stopped by high left-turn volumes. Although the Partial Cloverleaf does not stop these left-turns completely it does divert a large number of those that would have been trying to get onto the freeway. This reduction along with the increased capacity from the number of lanes allows for an improvement in circulation. This alternative is still considered viable at this point and will move on to the next step in the selection process.
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Result: Passed
A Merge/Diverge Analysis is an examination of the impact a ramp is expected to have on freeway flow. This is done by estimating the vehicle densities around either an off-ramp or diverge (Figure 1) or an on-ramp or merge (Figure 2). This analysis was first conducted examining the merge or diverge lengths created normally by the ramp-freeway intersection. If this length produced a level of service below D the analysis was conducted in reverse to determine the length required to achieve that acceptable level of service. As you can see from the results the interchange on- and off-ramps are designed for an acceptable level of service for the 2040 design year.
Figure 1: Diverge Junction
Figure 2: Merge Junction
The geometric design of the Partial Cloverleaf Interchange was based on the lane configuration results from the Synchro analysis. The design was completed using AutoCAD Civil3D based off of the Highway Design Manual (HDM). An overview of the interchange can be seen blow. Many layouts were considered for the final design but we chose to have four standard ramps and two loop on ramps. This means drivers can make a right turn to get on either side of the freeway, improving traffic.
In addition to the lanes a 10 ft. shared pedestrian and bicycle sidewalk was created to allow for non-vehicular ease of movement across the interchange running parallel to Limonite Avenue. The ramps meet the local road at 90 degrees rather than free turning movements to better accommodate pedestrians. A cross section of the intersection at the bridge can be seen below.
The geometric design along with the Synchro results were combined to produce a Geometric Approval Drawings (GAD) shown below.
In addition to running a traffic analysis with Synchro a simulation of traffic through the interchange was created using PTV VISSIM. The purpose of this simulation is to prove the validity of the Synchro results as well as spot any kind of special traffic concerns that might not be detectible through mathematical analysis. The VISSIM simulation of the Partial Cloverleaf Interchange can be seen below:
To determine the economic viability of the Partial Cloverleaf Interchange a Benefit/Cost Analysis was conducted. First the benefits were calculated based on the reduction in delays and emissions relative to the No-Build alternative. This means the amount of money that would have been lost if no improvements in circulation were made. The value of the reduction in delays were calculated using the Federal DOT’s Hourly Values of Travel Time Savings. The value of the reduction in emissions were calculated using the Caltrans emission costs for California Urban Areas. Those values can be seen below:
Second the costs were first calculated based on the cost of construction. This was done by computing the cost of materials plus a labor factor. Next the construction support costs were calculated by taking 20% of the construction costs. This represents the cost to design the interchange as well as for designer support during construction. These values were added together with the cost of additional right of way for the interchange. The results were shown below:
Finally, the benefits were divided by the costs to get a benefit/cost ratio or B/C ratio. This value tells you whether or not the project is economically viable and a relative idea as to what the project’s value is. These results were shown below:
The AutoCAD Civil3D corridors for the Partial Cloverleaf Interchange were converted from a data set into a 3D digital model. This was done by first plugging the corridor into the AutoDesk Software InfraWorks 360 and then into Lumion. This 3D visual can be seen below:
In order to accurately estimate costs for the project an Advance Planning Study must be conducted. The purpose of the study is to develop typical sections of any structures required for the project. In this case the major structure is the bridge. Due to clearance issues the bridge needed to be re-designed completely. The results of this design can be shown below.
Geometric Feasibility
This step involved making a rough design for the Partial Cloverleaf Interchange. We determined that the interchange could be laid out within the existing right of way and undeveloped land, while still providing the capacity determined in the synchro analysis. This alternative is still considered viable at this point and will move on to the next step in the selection process.
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Result: Passed
Impact Evaluation
The possible negative impacts of the Partial Cloverleaf to the surrounding area were evaluated in this step. The design does go through the nearby park and ride facility, which will need to be relocated. This is further discussed in the Park and Ride Design tab under Deliverables. This alternative is still considered viable at this point and will move on to the next step in the selection process.
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Result: Passed
Detailed Design
The Partial Cloverleaf Interchange was able to pass the previous steps in the selection process so it moves on to the detailed design. The detailed design makes a number of in depth considerations, analyses, and estimates to help us ultimately select one preferred alternative.
Viability
The Partial Cloverleaf Interchange passed the traffic analysis, geometric feasibility, and impact evaluation steps and was considered viable for detailed design. It was one of the two possibilities for recommendation for our site, and was ultimately chosen as the recommended design. Details on how this final decision was made can be found in the Deliverables under Weighted Decision Matrix.
Partial
Cloverleaf
GAD
C
C