TRANSPORTATION

The purpose of this Interchange Operational Analysis Report (IOAR) is to evaluate the operational and safety beneit from the interim improvements proposed at the I-10 and SR 121 interchange. The interim improvements are expected to alleviate the traffic operational and safety deficiencies of the study area before the ultimate build improvements, as detailed in an approved Interchange Modification Report (IMR), are implemented at this location.

The following deficiencies have been identified under the Existing Year (2015) conditions that are anticipated to improve as part of this project.

• The I-10 and SR 121 interchange is a partial cloverleaf configuration with nonstandard loops in the southeast and northwest quadrants. Under existing conditions, these loop ramps that were built with lower design speeds, hinder normal traffic operations, especially in the westbound I-10 direction. The westbound I-10 off-ramp is currently a three center radii loop ramp that terminates at a stop-controlled intersection with SR 121. This configuration results in interrupted flow and traffic backups, specifically during the AM and PM peak hours.

• Southbound SR 121 commuters encounter poor sight distance due to the vertical curve over I-10. The I-10 westbound ramp terminal intersection is unsignalized. Hesitation to perform the turning movement to head north on SR 121 due to poor sight distance over the vertical curve leads to high delays for motorists exiting the freeway. Drivers of heavy trucks making this movement have been observed making risky decisions. These conditions result
in unsafe travel conditions and an increase in queue length that backs into mainline I-10 impeding its operations.

• The study area possesses a high volume of heavy truck traffic, which accounts for more than 11 percent of peak hour traffic volumes. The grade differentials and curves of the loop ramps paired with the high truck volumes generate speed differentials that deteriorate operating conditions and safety.

The deficiencies found in the Existing Year (2015) analysis are anticipated to worsen with increased traffic volumes by Design Year (2040). Without adequate improvements to this interchange, the traffic operation and safety within the study area will continue to deteriorate, the queue lengths and delays will increase, and the number of crashes will go up.

This IOAR compares the operational and safety performance for a No-Build Alternative and two Build Alternatives. Two interim improvements: Build Alternative 1 and Build Alternative 2 are analyzed. Build Alternative 1 has the following improvements:

• Add directional ramp for traffic from westbound I-10 off-ramp to northbound SR 121
• Improve SR 121 in the northbound direction
• Install a new traffic signal to control the northbound SR 121 and westbound I-10 off-ramp movements

The Build Alternative 2 is designed to utilize the existing geometry and install a new traffic signal at the intersection of westbound I-10 ramp and SR 121 which is currently stop-controlled.

The design traffic volumes developed through the approved IMR for the ultimate SR 121 interchange improvements are used for this IOAR. The future traffic forecast was performed utilizing the NERPM Version 4.2 with the Base and Design Years of 2005 and 2035, respectively. Additionally, historical growth rates of the study area were used. For the purposes of this study, the analysis years included Existing Year (2015), Opening Year (2020), and Design Year (2040).