Don't assume other vehicles are going to behave in a way that is intelligent, predictable, or which takes you into account.
It is not possible to overestimate how bad a car driver can be. What follows are things I have personally experienced riding with one young woman (who I refuse to ride with any more) On every trip she takes, she:
a) runs all stop signs [claims she does not see them];
b) pulls out right in front of oncoming traffic when leaving a parking lot [believes she always has the right of way];
c) speeds....she drives a BMW 335xi coupe at crazy speeds: 60-65 through a small town downtown, 60 on residential streets with 30-35mph speed limits;
d) believes that "right on red" means that you don't have to stop and look before you turn, that you can just zoom through it and other drivers have to make way for you;
e) refuses to wear glasses even though she is supposed to wear them when driving [purely vanity...and is willing to risk her own and everyone else's life because of it].
She has been ticketed, and had one accident so far but nothing's changed. The cops were unfair. The accident was the other guy's fault, etc etc. She will never change.....unless, maybe, she kills someone.
She is the worst driver I've ever been with, and I'm sure she is not alone. I've ridden with two others who were pretty good but who had a bad habit of looking at you when talking....and so we would veer into the right hand lane or onto the shoulder. At speed. Anywhere. From the left lane of the freeway to the shoulder in a couple of seconds...I would be lunging for the wheel.
These people are out there. They are a menace and they will kill you without even trying, given half a chance. I recommend training yourself to never assume a car driver is a good driver or to use your own driving skill as the basis for your estimate of others' driving skill....unless you are an horrific driver...then go right ahead.