On April 4, 2013, claimant Marshall Roberts, 25, a laborer, was driving a GMC Sierra 1500 Truck for his employer, Triton Construction, on eastbound State Route 24, exiting one of the bores of the Caldecott Tunnel, in Oakland, when his vehicle was rear-ended by a vehicle operated by Gerard Ayala. Roberts claimed he sustained to his neck, back, and right knee. Roberts’ damages were paid by his workers’ compensation carrier, Zurich North America, which had provided benefits equaling $82,561.36. American Claims Management Inc., a claims management company acting on behalf of Zurich North America, sued Gerard Ayala and the co-owner of Gerard Ayala’s vehicle, Teresa Ayala. It was bringing a subrogation claim against the Ayala’s, seeking recovery of the amount paid to Roberts and for any benefits that Roberts may still need. This case was filed in Alameda County Superior Court (Docket No. RG14744485). Mr. Ayala, who was traveling behind Roberts, initially claimed that Roberts slammed on his brakes and came to a sudden stop in the tunnel. Mr. Ayala claimed that as a result, he attempted to stop his vehicle, but it slid forward because of the wet and rainy conditions. Zurich ultimately waived its subrogation claim and resolved the civil lawsuit against the Ayalas for the full amount of the Ayalas’ $100,000 policy limit, which was administered by Esurance Insurance Services Inc. Roberts sought further recovery via the supplementary-underinsured-motorist provision of his own insurance policy, which was administered by AMCO Insurance Co., a subsidiary of Nationwide Mutual Insurance Co. He also sought further recovery via the supplementary-underinsured-motorist provision of the insurance police that covered the work truck that Roberts was driving, Travelers Indemnity Co. of Connecticut, a subsidiary of Travelers Property Casualty Corp. The insurance carriers did not dispute liability., Roberts claimed that collision aggravated his pre-existing knee condition, causing medial and lateral meniscus tears, synovitis, and a contusion to the right knee. Prior to the subject accident, Roberts was involved in a motorcycle accident in 2006. He suffered a dislocation of the right knee with tears of the anterior cruciate ligament and posterior cruciate ligament. He subsequently required a reconstruction of both the ACL and PCL ligaments. He then underwent postoperative physical therapy and care. Roberts claimed he achieved a period of stabilization, but that a number of years later, he was still experiencing residual problems with the development of a right foot drop secondary to a probable peroneal nerve palsy. As a result, Roberts was getting medical treatment prior to the 2013 accident. After the 2013 collision, a co-worker drove Roberts to an emergency room, where Roberts reported pain to his neck, upper and lower back, and some knee tenderness in the area of the prior reconstruction. Imaging showed post-surgical degenerative changes, but no acute injury or effusion. Thus, he was diagnosed with a cervical strain and right knee contusion. However, Roberts claimed the subject accident aggravated his prior knee injury. As a result, after follow-up treatment and imaging, Roberts was diagnosed with medial and lateral meniscus tears, and synovitis of the right knee. He was also diagnosed with a cervical and thoracic sprain, but in his workers’ compensation claim, he received a zero percent impairment rating on both body parts. Six months after the 2013 accident, Roberts underwent an arthroscopic synovectomy of the anterior compartment of the right knee, and a partial medial and lateral meniscectomy. Roberts worked as a laborer for Triton Construction, where his duties were to demolish and clean up job sites. However, after the 2013 collision, he was taken off of work and advised that he could no longer perform his construction job, as he could no longer perform any manual labor. As a result, Roberts had to find employment elsewhere and ultimately ended up getting a job working for a luxury vehicle rental and travel planning company. Roberts claimed that he will require a complete knee replacement. The respondents’ counsel contended that at the time of the subject 2013 collision, Roberts was already experiencing pain and receiving treatment due to his pre-existing knee condition. Thus, counsel asserted that the knee operation was due to Roberts’ pre-existing knee injury.