Throughout the movie a series of flashbacks show young Oliver, who Petulia and David had brought to San Francisco from Tijuana, being treated in hospital. Another flashback finally explains why - Oliver had suffered a serious accident.
... from 1965 ... the vintage photo below shows the accident site (arrow) as it was when the scene was filmed, alongside the elevated Embarcadero freeway in front of the Ferry Building close to where Market Street joins (map). That's the Clay Street on-ramp in the right foreground and the Washington Street off-ramp in the left foreground. (The ugly freeway was destined to be demolished after it was damaged in the 1989 Loma Prieta earthquake).
Petulia buys Oliver a Greyhound ticket to Tijuana - this booth was presumably set up under the freeway especially for the movie - the freeway passes overhead to the north past the signs pointing to Market and Washington. But the boy doesn't want to leave and runs away...
Then ... He darts between the passing traffic. Note the YMCA sign down the road on the building with the flagpole.
... and Now, the same view looking south down the Embarcadero, now a wide-open bustling boulevard after the demise of the elevated freeway. The Y, near the corner of Howard Street, is still there and its flag is still flying.
Then ... We are facing north again as the cars desperately try to avoid the boy but one of them hits him. In the background are the two freeway ramps seen in the accident site photo at the top of this page.
... and Now, no elevated freeway, no ramps. The large office tower on the left is part of the Embarcadero Center on Clay Street.