South Bay Expressway (SBX) toll road is a 9.3-mile privately-funded southern extension of SR 125 extending from SR 905 near the International Border to SR 54 near Sweetwater Reservoir in San Diego, California. SBX connects the only commercial port of entry in San Diego to the regional freeway network. This project, made possible through an innovative public-private partnership, completes the missing link in San Diego's third north-south freeway corridor. The project also included a publicly-funded 3.2-mile contiguous segment to the north, including a new SR 125/SR 54 interchange, known as the Gap & Connector.

SBX connects Otay Mesa, the largest area of industrial-zoned land remaining in San Diego County, with Eastern Chula Vista and points north and east, drastically reducing commute times and providing convenient access to Downtown San Diego, Sorrento Valley, Santee, I-8 and I-15, and Mexico.

The toll road was developed under California's AB 680 legislation passed in 1989. A limited partnership, South Bay Expressway, L.P., holds a franchise with the State of California under which it financed and built the highway, then transferred ownership to the State. The limited partnership then leases back, operates, and maintains the facility for 35 years. In 2042, control goes back to the State at no cost.

South Bay Expressway was privately financed and operates as a toll road, offering both cash payment and the FasTrak electronic toll collection system. The northern 3.2-mile segment of the South Bay Expressway including the SR 54 interchange was financed publicly using a mix of federal funds and local sales tax proceeds. Both sections were built by the same contractor under two separate design-build contracts.

Southern toll section

• Investor equity, bank debt - $495 million
• TIFIA loan - $140 million (backed by toll receipts)

Northern Gap & Connector section

• Federal funds (Federal aid receipts)
• Local sales tax receipts