But courtesy is important -- not only in terms of interpersonal conflicts with other drivers and the problems those cause, but also in terms of keeping traffic moving. Here's an example: you're on a freeway, with three lanes of traffic that are narrowing to two lanes just ahead. Do you encounter drivers who stay as close as they can to the rear bumper of the vehicle ahead, so the folks in the right lane cannot merge? Have you done the same thing to others? I know there are times when my little green attitude "devil" takes over. The result is a sure bottleneck and backed-up traffic. Yet if even one driver allows someone to merge in front of them, it often sets the example for others to follow -- and traffic keeps flowing. Is there some big harm in letting someone merge into your lane in front of you? The only real cost is a split second of driving time -- and you lose more than that if traffic chokes to a stop.
It's important to be courteous -- but extending courtesy to others must be done with conscious thought for unintended consequences. Here's a real world example: You are sitting in traffic, waiting for a signal light. You are in the middle of three lanes. A vehicle on your right wants to cut across traffic from a driveway and make a turn into traffic going the opposite direction as you. The driver in the right lane stops and leaves room for them to come across -- and you do the same, waving the person across in front of you. YOUR thought in waving them across your lane is to let them know YOU will allow them to cross in front of you, but in their mind, you are telling them it is safe to cross. They do not see another vehicle coming down the left lane at 45 mph -- and perhaps that driver doesn't see them either -- since they are hidden from view by your vehicle and the other traffic.
In the past fifteen years, I have seen two serious collisions happen EXACTLY this way (and one near miss). In one of them, I was driving the vehicle that was oncoming in the lane and got hit by the crossing traffic (and yes, I should have seen it coming). The other occurred three cars in front of me when others motioned a crossing driver through traffic. The near miss was a high school boy, jaywalking between cars instead of using the crosswalk at the intersection nearby, and a motorist stopped short and waved him across -- where he was almost hit by another vehicle flying down the turn lane approaching the intersection. He was so shaken he collapsed on the sidewalk. Another example is when a driver unnecessarily and abruptly stops for a jaywalker or an animal, and forces someone following behind (who isn't expecting the sudden stop) to take dangerous evasive action.
When being courteous, think about the big picture, and make sure that what you do doesn't contribute to a dangerous situation for the other guy.
Keep the shiny side up!