This post comes from my last re-reading of "Stayin' Safe," a book of columns by the late Lawrence Grodsky for "Rider" magazine. I would highly recommend the book, but what follows are my interpretations.
1. What is unfolding 4 seconds ahead of you requires your fullest attention and often immediate action. (12 seconds ahead is what you would like to see and often can't.) So are you aware of everything 4 seconds ahead of and off to the side of your bike? And since you probably don't have a second hand mounted on your faceshield, try counting. (One thousand one, one thousand two... or One Mississipi...) This is a very good reality check.
2. If you are really safety conscious, you want to be able to stop in any "blind" situation, whether that is a curve or a hill. Grodsky's informal tests seemed to indicate 4 seconds was the time needed for good experienced riders to stop in a curve when there were not any road surface problems. Cresting a hill can be a more insidious problem unless you take into account the possibility of a hidden driveway to your right or a vehicle crossing the centerline. Again, count the seconds to what you can see.
There are many riders who violate the second rule - and some of them have been doing it for many miles and years. I don't understand why they are still alive. Maybe they can tell us how Larry and I are wrong. At any rate, I am not telling YOU how to ride. Just offering some thoughts which make sense to me.