It forms part of University Avenue in San Mateo County between U.S. Route 101 in East Palo Alto and State Route 84 in Menlo Park just west of the Dumbarton Bridge.

[2] The California State Route 109 designation was first defined in 1963, running from Sunset Cliffs Boulevard to Interstate 5 in San Diego.

The segment in East Palo Alto between US 101 and Notre Dame Avenue remains in local control.

Except where prefixed with a letter, postmiles were measured on the road as it was when the route was established, based on the alignment that existed at the time, and do not necessarily reflect current mileage.

[2] Segments that remain unconstructed or have been relinquished to local control may be omitted.