Most lanes on modern streets are 12 feet wide, most cars are in the 6-7 foot range. If you're all the way over on one side of the lane, you're going to be ok, even if the cyclist isn't completely in the bike lane. If they're right on the edge, you've got about a foot of bike, 3 feet of buffer, and 7 feet of car.

Another way to look at it is that if there's no bike lane, the car is probably going to have to go outside the lane to be safe and legal.