I normally don't stop for people in cities or on heavily traveled roads. I figure in areas like that there are plenty of LEOs, cell phones, roadside assistance, etc which are all much better equipped to help out than I would be. Of coarse this excludes the situation where I see the incident happen and am the first on the scene.

However, I also spend a fair amount of time on less traveled back roads where there is no cell coverage, and have winched a fair number of vehicles out of ditches. I figure I'll at least check things out because who knows when the next car will go by? I normally approach the scene slowly, flip on my light rack, roll down my windows and ask if the stranded vehicle needs assistance.