This paper aims to investigate the threshold at which the ramp control be opened in the freeway/on-ramp merged by analyzing the factors affecting the efficiency of merge. Firstly the fuzzy C-means clustering algorithm（FCM）is used to determine the membership of an non-congested traffic with unobstructed flow. This membership can well reflect the proportion of free part in the total traffic, so the M3 headway model can be accurately calibrated. In the meantime, a probability model is established to define the acceptable input volume of expressway in the merge. Through the analysis of simulation, the downstream occupancy of main line is introduced as the component of the critical threshold. Finally, a comparison is conducted to validate the rationality of the proposed threshold. The results show that the control initiated at the threshold can better balance the operation of the main line and the ramp traffic.

CICTP