Do you stop at the scene of an accident?

If the emergency services haven't arrived yet, but it looks like some people are assisting? I passed a bad accident today on my way home from work; there were at least two totalled cars, there were around 6 cars stopped, it was hard to tell if they were involved, or were assisting, there was a lady lying on the median (being assisted by another woman), there were people stumbling around and crying, and I saw someone on their cell phone, likely calling 911. I was probably one of the first cars to pass by, and ran over a lot of glass. I just felt kind of bad after, wondering if I should have stopped to try to assist. I just figured that they might not want cars stopping, and blocking any emergency vehicles.