I think most measurers would draw the finish line perpendicular to the SPR. But your average Joe could draw it in just about any direction, parallel to the road is just as likely as perpendicular to the SPR. In this case I think you have to give more description of the finish than just a single point.
For the case where the finish is on a road, your average measurer and your average Joe would both draw the finish line across the road, perpendicular to the direction of the road.