I-15/Limonite Ave Interchange
Improvement Project
Traffic Analysis
Diverging Diamond Interchange
A Diverging Diamond Interchange is a non-conventional interchange type where there are no signalized turning movements to get onto the freeway. This is done by having the roadway cross to the opposite side to allow for free-lefts both onto and off of the freeway before the roadway crosses back. There are also free rights both before and after this crossing to allowing vehicles to get on and off the freeway.
A Synchro Analysis was conducted on the Diverging Diamond Interchange to determine whether or not it was capable of handling the predicted 2040 traffic volumes. Initially the roadway was designed to determine whether or not the configuration was capable of improving the level of service. After it was determined that it was the lane configuration was altered until an acceptable level of service was obtained. In the end the final configuration used for design was produced with a level of service of C for both intersections.
The reason this interchange was able to handle the predicted 2040 traffic volumes was its complete removal of left-turns. Because of this flow through the bridge was interrupted only by the crossing flow of traffic. However, it should be noted that the main traffic concerns of a Diverging Diamond Interchange is not just the flow through the intersections. Depending on the design an urban weaving segment might be created which could cause turbulence that would impact traffic negatively. Unfortunately at this time there is no commonly accepted way of analyzing these segments or standards for what an acceptable level of service for them would be.
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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 1: Merge Junction
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 Diverging Diamond Interchange can be seen below:
The geometric design of the Diverging Diamond Interchange was based on the lane configuration results from the Synchro analysis. The design was completed using AutoCAD Civil3D based off of multiple standards including Federal, Caltrans, and Utah DOT. The reason for the wide variety is that this design is non-conventional and as of writing does not exist in California. As such there are no specific design standards for it in California. An overview of the interchange can be seen below. Extra consideration was given to have the angle at which the roads cross each other at the intersections be the recommended45 degrees. We also spaced the intersections out as far as possible to maximize the weaving length cars have to get on and off the ramps.
In addition to the lanes a shared pedestrian and bicycle sidewalk was created to allow for non-vehicular ease of movement across the interchange. Due to the way traffic flows across the interchange the sidewalk needs to be placed running down the center of the interchange and not parallel to the road as you’d typically see. A cross-section shown below shows the typical lane widths and location of the sidewalk on the bridge.
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.
To determine the economic viability of the Diverging Diamond 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:
Geometric Feasibility
Impact Evaluation
Viability
The Diverging Diamond 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, but the Partial Cloverleaf was ultimately chosen. Details on how this final decision was made can be found in the Deliverables under Weighted Decision Matrix.
This step involved making a rough design for the Diverging Diamond 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
The possible negative impacts of the Diverging Diamond Interchange 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 Diverging Diamond 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.
The geometric design along with the Synchro results were combined to produce a Geometric Approval Drawing (GAD) shown below.
Diverging Diamond
GAD
C
C