It is understood within the road race industry that the course is defined as below (IAAF Rule 240.2). If the road is bordered by a curb (kerb for my friends on the other side of the “Pond”), it is no different than a curb on a track – you step on or cross over and you are no longer on the course and therefore subject for disqualification.
Several “posters” have indicated that they don’t see this as a problem, I respectfully disagree. The majority of incidents result in runners running less than the distance thereby giving them an unfair advantage. And unfortunately runners in a race are like lemmings, if one runner does it, others will follow compromising the integrity of the event.
As the race director I would want to know about it and then I would speak to the athlete. Plus, I would evaluate that and other turns to see if additional monitoring is required.
IAAF Rule 240
Course
2. The races shall be run on made-up roads. However, when traffic or similar circumstances make it unsuitable, the course, duly marked, may be on a bicycle path or footpath alongside the road, but not on soft ground such as grass verges or the like. The start and finish may be within an athletic Field of Play. Note (i): It is recommended that, for Road Races staged over standard distances, the start and finish points, measured along a theoretical straight line between them should not be further apart than 50% of the race distance. For approval of Records, see Rule 260.21(b). Note (ii): It is acceptable for the start, finish and other segments of the race to be conducted on grass or other non-paved surfaces. These segments shall be kept to a minimum.
3. The course shall be measured along the shortest possible route that an athlete could follow within the section of the road permitted for use in the race.