As of 11 pm Tuesday, the ballot measure is at 60% "yes" and 40% "no." It needs 66.7% to pass.
In those more than three decades, it has funded improvements to state highways, county roads and streets in the county's 15 cities.
If approved for an extension in November, the half-cent sales tax would raise $7 billion over 30 years.
About half of the funding would go to road maintenance.
Supporters say this extension will fund bike lanes, street widening, sidewalks, and much-needed repairs countywide.
Measure C extension on the ballot November 8, what it means