After Roy and Rocky switch cars and are turned down at a motel, they continue their journey. When the merge into the night traffic, it consists of modern cars.
The only way for the Galveston directional sign to point left and Beaumont point right is if they drove from ocean which they did not.
When Roy and Rocky approach a T-Junction the sign says that Galveston is 35 miles away and Beaumont is 39 miles away. In reality Galveston is over 100 miles away from Beaumont rendering the distances between the two places shown on the sign to be completely inaccurate.