One key thing at the time I believe was the grade separation/right of way stuff, which I think had only occurred on toll roads for any distance before.

Previously almost all highways ended up having to stop or intersect with other roads (and hence stop signs, or traffic lights, or whatever). Most highways are still that way, outside of major arterials.

Most major arterial highways are the same as freeways now everywhere of course, but I think that’s more about economic factors. California I think was just the first mover on it (and got exposed to those same economic factors earlier than most).

At the time (and still does, though it gets less press now), California was a huge oil state, and was at the forefront of designing and building refineries too.