Well, after reading this, I feel like a [censored] driver.
If I'm on the interstate, and going a constant speed, I just watch what's ahead of me, and ahead on my right for the onramps.
The same, basically, for driving in town on roads with no lights. Unless the conditions require me to change speed, I just monitor in front of me.
If I'm in stop-and-go city traffic,, I check my rear anytime I'm coming to a stop - watching that the driver behind me is stopping, and just bracing for the chance of an impact. Maybe a full surrounding check every 20-30 seconds just to monitor what's behind me in case of a change up the road, I feel like I can reasonably know there are X cars behind me within a few hundred feet with checks of that interval.
If I need to change lanes, I'll scan all 3 + a look behind me over my shoulder, all with my necessary blinker on.
Hopefully I'm not alone, otherwise all of you are significantly better drivers.