On the road I use on my journey to work, there is a narrow bridge over a river, not wide enough to allow two vehicles to pass. Although the road is quite heavily used, the highway authority has not followed the common practice of putting up signs to give priority to traffic traveling in a particular direction. But there is no need for any signs: Everyone who uses the bridge seems to follow a clear set of rules. Once one vehicle is crossing the bridge, any number of others behind it will follow, and those going the other way have to wait until there is a gap in the traffic flowing against them. So at busy times there are long periods in which all the traffic flows one way. What is more interesting is what happens when traffic is lighter, and two vehicles approach the bridge from opposite directions, with clear road in between. Then the one which is nearer to the bridge maintains speed and the other slows down and gives way. I shall call this convention the first-on rule. How is it enforced? Principally, I think, by the fact that if two vehicles meet in the middle of the bridge, both drivers suffer, whichever one of them eventually backs up. So if, approaching the bridge, you expect the other driver to maintain speed, you do best by giving way, while if you expect the other to give way, you do best to maintain speed.