The Omen
Based on the current location of the marker and the details given by the police officer, another possible scenario is that Richardson was brought to a hospital in Zwolle and crossed the junction of the N348 with the N35. There he may have noticed kilometre marker 66.6, and immediately after that a road sign stating the direction towards Ommen. This may have freaked him out to the point that it became part of the legend of the curse.