That's one thing that I've noticed with the RNR and some of the more casual runner friendly races. Thanksgiving turkey trots come to mind as well. I ran a 10k thanksgiving, I was up towards the front with most of the faster runners and it was still about 5 minutes before I even started to move my feet.
As for the increased injuries at crowded races, I make sure I hydrate very well and carry a belt with a couple bottles of water/electrolytes so I can run past all of the early water tables as they tend to crowd up before the herd thins out. I have a buddy that runs for RNR as a promoter and he was injured when someone ran into him and dug her foot into his Achilles at a water table. Things can happen anywhere though, and I don't want to sound all negative. If you're worried about crowding, edge your way to the front before start and then stay to one side of the road/trail so the faster runners aren't having to run around you.
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