Jeff,
I think you might be surprised at how far off most people will run from the SPR, even in small races. Now that I am old and slow I end up in the middle of the pack in the rare race that I run. I have noticed that people tend to just follow the people in front of them. If a person runs next to the left edge of the road around a right-hand turn, everyone behind will follow. I think people do this partly because they don't pay that much attention, but also because taking a shorter route than the person in front of you would feel like cheating.