1) People who drive in constant fear of merging/changing lanes. These are the people who get all panicky when they see a sign that says exit in 1 mile. As if it is hard to change lanes twice in 1 mile. These people also block traffic when they need to turn right and then a mile later turn left. They refuse to turn right until they have a clear path to the far left lane.
2) People who are afraid to look rude, so they get in line instead of making efficient use of the road.