You're probably aware that, when driving your car, you have two blind spots when looking at the traffic around you - it's the spots to your left and right your rearview mirror and side view mirror (as most people adjust them) don't quite cover (truck drivers have different blind spots, covered in a different diary). It's easy to forget, however, that most other automobile drivers on the road have that same blind spots, and you can be squarely in one of them. And when you're in that blind spot, you're physically very close to that driver's car, which makes for a potentially dangerous situation, particularly if that driver isn't paying attention the way he should be.
For example, if you're in another driver's blind spot, and the lane ahead of you is relatively free, that other driver might think your lane is free enough to merge into. If that driver decides to merge, he'll merge right into you, and you'll only get a second's warning, even if he decides to signal the lane change.
The other guy's blind spot is particularly an issue on multi-lane freeways, for example, freeways three or more lanes wide going through or around cities. On these freeways, it's quite common for two cars to go for the same open lane. If you're in the blind spot of a driver two lanes over, it's quite possible you and he will go for the same lane at the same time.
How to avoid this problem? Get sensitive to where other drivers' blind spots are, and avoid them when possible, either by slowing down or speeding up a bit. Once the other driver has you in one of his mirrors, or can see you next to him, your odds of having him merge into you go way down.
And that's another way to be a better driver.