Optimization of Consecutive On-Ramp Control for Urban Freeways: An Application of the Store-and-Forward Approach

Highway congestion is a common problem worldwide, and different management strategies have been proposed and implemented. In Taiwan, commuter trips in urban areas tend to use freeways for daily trips. One particular problem in Taiwan is consecutive on-ramps within metropolitan areas, such as Taipei, Taichung, and Kaohsiung. Three to four on-ramps bring more vehicles to the freeways, and coordinating with an integrated ramp control strategy is critical to avoiding congestion in metropolitan areas. Based on the concept of the store-and-forward method, this study proposes a linear optimal control model to optimize integrated on-ramp control strategies. The store-and-forward method is chosen because freeway congestion is usually observed in these areas, and freeway efficiency depends on avoiding queue length. The developed mathematical program can be solved efficiently through mathematical optimization packages. The objective of the formulation is to minimize the total queue length within considered areas. In the Taipei metropolitan area, numerical experiments were conducted based on three consecutive on-ramps, including Yuanshan, Taipei, and Sanchun ramps. Several scenarios were designed to illustrate the possible benefits of the proposed model.