The weigh station at the start of their trip is on northbound Interstate 5 near the Magic Mountain amusement park
This movie and the TV show He Who Digs a Grave (1973) ran two weeks apart back in 1973 and in both, Lee Purcell played David Janssen's love interest.
In addition to this, Keenan Wynn was in two other movies where David Janssen was the lead: King of the Roaring 20's: The Story of Arnold Rothstein (1961) and Warning Shot (1966).