第二段关键词成句为：Low road area ratio is a direct reason.

　　第三段关键词成句为：A sharp increase in the number of private cars partially causes the road congestion.

　　第四段关键词成句为：The reducing transport capacity of public buses stimulates people to buy private cars to some extent.

　　第五段关键词成句为：Inadequate traffic facilities aggravate the road congestion and raise the chance of traffic accidents.

　　第六段关键词成句为：The traffic network is built without scientific plan and overall consideration.

　　范文

　　The Causes of Traffic Problem in Cities

　　At present, traffic problem is an acute issue for cities, especially metropolises. There are several reasons for it. Firstly, low road area ratio is a direct reason. Secondly, with the reducing transport capacity of public buses stimulating people to buy private cars to some extent, the number of private cars increases sharply, which partially causes the road-congestion. In addition, inadequate traffic facilities aggravate the road congestion and raise the chance of traffic accidents. Finally, another important reason is that the traffic network is built without scientific plan and overall consideration.