SD5830 : Britain's First MotorwayI believe this commemorative plinth was originally located next to the motorway, in the triangle formed by the northbound entry and exit sliproads (SD581297). It was therefore not accessible legally unless you happened to have an emergency on that very short section.
When the motorway was widened to 4 lanes, the northbound entry and southbound exit sliproads were moved to the north side of the A59 by building extra bridges over the River Ribble. The plinth was moved to its present location.