You can't just expect drivers to do a head check before changing lanes or be aware of other vehicles around them. That is just completely unreasonable. 
Changing lanes and merging is easy if you think ahead, but many don't based on my observation. I cannot count how many people I have seen on the highway that will run right up behind a slower driver, hit the brakes, then decide to change lanes, then accelerate again. There was either an open lane or room to pass the whole time. A simple lane change would have avoided all that. I see it repeated all the time. I wonder sometimes if it is a complete lack of depth perception with some drivers.
I'm pretty sure it's that most people won't look further ahead than the bumper of the vehicle in front of them. I can't count the number of times I've seen people swerve out of the lane I was in to try and jump ahead via a gap (to either side), with me travelling at the same speed as the lane of traffic ahead of me. If they're lucky, they can swerve right back in behind me when the lane I'm in catches up to and passes them, because we were moving faster than traffic in that other lane all along.
There was one guy several months ago who was BOUND and determined to get ahead of me on the Beltway. Heavy traffic moving under posted speed in the afternoon rush, and he comes on, gets in behind me (at prevailing speed in the far-left lane) and proceeds to tailgate me, I'm pretty sure LITERALLY 6 inches off my bumper, under the delusion that would get him where he was going faster. We were already moving slightly faster than all other lanes, and ain't NO ONE was going anywhere any faster. Over the next several miles, dude proceeded to swerve in and out of the lane trying to get past me, sometimes falling a few car lengths back, sometimes able to recover and get back in behind me, but there literally was NO WAY he was going to be able to pass me - miracle of miracle, the far-left lane actually WAS the fast lane, that day. I watch him go shooting off to the right, making it across the entire width of the Beltway into an exit lane, and figure I'd seen the last of him, as all traffic came to a stop. Probably took about 5 full minutes to get past that exit. As we made it around the curve after that exit, I see... same freakin' idiot swinging all the way over from the off-ramp (having gone over the solid lane lines of the off-ramp to do it) and laboriously working his way back over, visibly frothing at the mouth. To slot in right behind me again, where he could just have stayed all along.
I literally laughed out loud over that. Pretty sure he saw me, and I didn't care one bit.
He shot off to the right again almost immediately thereafter, fell behind (
), and I never saw him again.
Re: blinker use in these parts - I too will make an effort to accommodate someone using their signals, either speeding up to get past or hitting my brakes to open a gap for them. Don't use a signal when trying to break into an existing lane of traffic? Why, that means you want to follow THAT lane, not be in MINE, right?
