You sign up for a marathon hoping to get a Boston Qualifier or PR. Everything is going on track. Suddenly, with only 10km to go, you face a wall of traffic and slow to a crawl. Not car traffic, but walkers from a half marathon that the organizers jammed in to increase their revenue. This is the dreaded "half" traffic. Do you really want to zig zag all over the road after 20 miles? Not to mention if you are racing you can no longer see your competition.
It is hard to find a marathon without a half or other event anymore. The standalone marathon is a dying breed. Some marathons, however, manage multiple races better than others. The worst marathons will even put a short fun walk that overlaps with the end of the marathon course. Outrageous. When rating marathons on this site, if there is minimal or no "half" traffic, click on the checkbox for this option.