Lyman: We use segments mostly for marathons and half marathons, whether measured on one day or several days. Deciding when to use a segment is guess work at best. Usually the guess is based on what the race director may want, or what I think of the chance that the course will be changed to accommodate traffic or less police or runners' complaints. 5Ks and 10Ks don't usually warrant segments since they are easy to remeasure.