Speed, okay... and sometimes it's safer going faster than the posted limit (IE with the flow of traffic). However, if you're missing stop signs, you're definitely doing something wrong - either driving too fast or not paying attention.
Lots of people will slow down significantly (say to <10mph), but not stop completely, if no one seems to be there.
It's one of the many things that make me wild about the idea of letting computers do the driving. Zipper-merging, slowing to time lights instead of stopping, and no rubbernecking are wonders of the future I can't wait to enjoy.
Which must be why "stop means STOP!!!" was repeated very forcefully by the driving instructor and the friendly police officer multiple times during my driving course.
I guess they couldn't call it a "California stop" / "California roll" if that wasn't the law.
We all had that case where a stop sign that was hidden behind a tree (not everyone lives in the US where they paint them on the road ;)) or misses the school/children crossing sign or any other temporary sign that was put there due to works or w/e.