In 1987, San Diego County residents approved the 20-year TransNet program, a half-cent sales tax to fund a variety of transportation projects throughout San Diego County. This program, which became operative in 1988 and expired in 2008, is one of the largest transportation improvement programs in California.

In November 2004, voters approved an extension ordinance and expenditure plan (Proposition A) that extends the TransNet program to 2048. Funds are expected to be generated among highway, transit, and local road projects to reduce traffic congestion in San Diego County. The San Diego Association of Governments (SANDAG) administers the funds generated by TransNet. SANDAG celebrated the 25th anniversary of TransNet in 2013, reflecting on the transportation improvements and environmental conservation projects made possible by TransNet funds.

Proposition A mandated the formation of an Independent Taxpayer Oversight Committee (ITOC) for TransNet. This committee provides oversight for the expenditure of TransNet funds and ensure that voter mandates are carried out. The ITOC also makes recommendations to improve the program’s financial integrity and performance.

Partnering with the California Department of Transportation (Caltrans), Metropolitan Transit System (MTS), and North County Transit District (NCTD), SANDAG has shifted into high gear to accelerate a variety of high-priority, TransNet-funded transportation improvements throughout the region. These major construction and infrastructure projects, called Early Action Projects, include Interstates 5, 15, and 805; State Routes 52 and 76; the Mid-Coast Corridor Transit Project to extend the Trolley from Old Town to University City; I-15 Bus Rapid Transit (BRT) from Escondido to Downtown San Diego; the Mid-City Rapid bus line between San Diego State University and Downtown San Diego; the South Bay BRT from the Otay Mesa Port of Entry to Downtown San Diego via I-805; and SuperLoop circulator transit system in the University City area. Several of the major interstate projects, such as I-15, also include the incorporation of managed lane systems.