I agree about the letting one car in rule. however, on part of my commute, you can only do this if you're coming to stop at a light. The speed limit on the road I take for most of my commute is 50, but many do 60 and sometimes more. To slow down and let someone in would cause an accident. However, If I'm coming up to a light that's turning yellow or is red, I'm slowing down anyway, as are others, and will let someone in. But in normal traffic, just not happening since I'm not willing to be rear-ended simply by being polite.