Laurie, I ran the Royal Victoria Marathon, albeit a long way after you and the lead female runner. I was very surprised that the route that I assumed was the measured route (often marked with a blue line) was not being followed, expecially in the last 10 or 15 km. At times the course marshals and even the aid stations had moved to the wrong side of the road and where the runners should have been on the right, but had moved to the left. Pete raised a good point. In this case it was an out and back portion of the course with runners generally restricted to the right side in both directions through the use of an occasional traffic cone, but the road was completely closed to traffic so when there were no opposing runners there was nothing to prevent runners from following the SPR. More cones, signs and properly instructed course marshals could have solved the problem.
I agree with Pete that we should assume the full width is going to be used unless it is clear that runners will be restricted to one side. On busy streets where the road remains open to traffic it would be difficult and not very safe to either run or measure a shorter route so I have followed the race directors commitment that runners will be restricted. I also add a note on the map stating that the certification is not valid if the course is not laid out in accordace with the map and if runner restrictions are not inforced.