I learned driving taxi cab and tractor trailer YOU CAN'T TRUST YOUR MIRRORS! They leave huge amounts of space uncovered...
I also learned EVERYONE has a blind spot. In your eyes, not your mirrors. If you don't believe me, stare straight ahead and without moving your eyes hold up your hand and wiggle one finger. Move the hand to the upper outside quadrant of your vision and fish around whilst wiggling yer digits...you will find your LITERAL blind spot. A point in your peripheral vision where you simply cannot see your finger no matter how much you wiggle it...even though it seems you SHOULD be able to see it.
That's how that semi appeared out of thin air even though you swear you looked; it was in your eye's blind spot.
I always keep a constant check in my mirrors, and "running inventory" of who's where around me, try to maintain a bubble (no one next to me or real close if possible) and I always head look TWICE! More than once the 2nd look revealed a car/truck/bike I did NOT see the first time (blind spot.) Just recently I looked once (driving car - just came off entrance ramp) saw nothing, looked twice and a BMW bike was on my left rear quarter panel. I would've changed lanes right into him had I not looked twice.
I always assume the cagers are half asleep, or that the other person will do the worst possible thing (90% of the time they do) I always plan an escape route, and if I think there is the slightest possibility someone might turn across my path I hit the 88's (emergency flashers.) And I NEVER assume that because the light is red or there is a stop sign people are actually gonna stop.
As long as I'm rambling, I always wanted a bumper sticker, "Blowing the horn does NOT throw a magic shield around your car." I have an air horn on my Harley and will use it if I absolutely must, but most of the time evasive action is the safter option.
All of the above will keep you outta a heap of trouble.
And yes I've been hit on my bike and had the cager say, "I swear I never saw him..."
PS for new bikers, practice head checking at slow speeds on empty roads. If you turn your head as much as you can looking behind you and then look ahead again, you just might find yourself wandering into the next lane. It DOES take practice.
Moe.