在英国，车辆为什么要“靠左行”呢？据记载，罗马教皇卜尼法斯八世（Boniface VⅢ） 在1300年举行第一个基督教庆典时，命令所有赴罗马的朝圣者均须“靠左行”，在当时，“靠左行”是有道理的，因为一般人都是右手便利，如果靠左行，右手就永远处于外侧，以便做其他事情，另外，当时人们的交通工具是马，而上下马必须在左边，左边的马路边上造些马石（ platform）．人们踩石上马便可疾奔而去。
Why does traffic keep to the left in Britain? According to the records concerned, when the first Christian celebration was held in 1300 by the Pope Boniface Ⅷ, all the pilgrims traveling to Rome were ordered to keep to the left. At that time, it was reasonable to keep to the left, because people generally used their right hand. If they kept to the left, the right hand was always on the outside to do other things. Besides, people used horses for transportation at that time, and they got on and off their horses from the left. Therefore, platforms were built on the left side of the road for people’s convenience. And they could get on the horse from the platform and ride off immediately.