SR 55 was built in 1931 and originally numbered Route 43. It was built from the southern terminus of SR 1 (the Pacific Coast Highway, or "PCH") and continued northbound on roughly the same route it follows today, following Newport Road (today Newport Boulevard) northeast to Tustin, and then Tustin Avenue north to near its current terminus at SR 91. From here, Route 43 continued east on what is now SR 91 towards Riverside. In 1959, the highway was renumbered as Route 55, and its route was shortened from Route 1 to the also-renumbered Route 91. The freeway portion from Chapman Avenue to SR 91 opened on January 18, 1962, at a cost of $4.6 million (equivalent to $ in ). The segment between SR 73 and Chapman Avenue opened in 1966.