一、问题的提出环形交叉路口,也称环岛,具有节约时间、减少事故、节约经费、美化环境等优点。在我国现阶段,环形交叉路口作为一种重要的交通形式,应用广泛。但由于交通流量随城市经济的迅猛发展而急剧增长,一些城市主干道路上 First, the problem raised Roundabout, also known as the island, with the time saving, reduce accidents, save money, beautify the environment and so on. At the present stage of our country, the roundabout is widely used as an important traffic form. However, due to the rapid growth of traffic flow with the rapid development of urban economy, some urban trunk roads