Adsense safe traffic for me is .traffic that doesnt break against adsense rules and get your adsense account suspended. for example multiple hits/klicks from same ip number could get your account canceled
Natural traffic is always nice (search engine rankings or organic backlinks). Bots are bad. Click trades are bad. A little common sense should answer your question or do as seo-roof said and refer to the Program Policies.
...was this really necessary? Anyway, I'm not asking this question because I'm looking for shady ways around the adsense TOS, but because I'm seeing alot of threads offering "Adsense Safe Traffiic"