Abstract: This paper discusses the critical problems in urban freeway traffic systems and the need for traffic control.Some measured traffic curves are presented to explain the characteristics of such traffic control problems.Three control methods are evaluated,and the on ramp control,which is applied widely and discussed in detail.After the application and limit of the on ramp control methods are reviewed,a new research method,which combines multi objective optimization and hierarchical intelligent control theory so as to achieve the best performance of the whole traffic systems in a city,is proposed.