(photo from flickr)
Picture this situation: You are driving down a two-lane road when you come upon the inevitable semi-truck going *gasp* under the speed limit. This is, of course, unacceptable. This truck should be passed at the first opportunity, but any driver who has been in this situation before knows that this is risky business, even under ideal conditions. Semis are wide and long, preventing visibility, and taking more time to pass. Typically, the way a driver in America would look for an opportunity is to sneak out slightly into the oncoming lane and either go for it, or jerk back into their lane to avoid getting hit by an oncoming vehicle. Sounds dangerous! But there is another way...
In Namibia, almost all roads are only two lanes, but they have developed a useful way to get the cars passing the trucks efficiently. It's simple: when the truck driver sees that the road ahead is clear, he puts on his right flasher (they drive on the left) to signal any cars behind him that the coast is clear. I'm guessing it's safer too, since the truck has a clear view of the oncoming lane, and I'm guessing a professional truck driver can make a safer estimate of whether you're going to make it past him.
I'm guessing this might be something that happens in other countries besides Namibia, and maybe even other parts of the states, but it sure doesn't happen in the northeast.
