Using PET-Measured Traffic Conflicts to Analyze Safety Performance of Merging Areas of Freeways

The merging area is an important part of a freeway system, and its safety performance is usually of great concern. This study investigated the safety performance of merging areas by using traffic conflict techniques, and post encroachment time (PET) was employed as the conflict measure. To measure the PET of merging events, a video-based method was proposed. Approach to identify thresholds which distinguish serious conflicts from other events was also developed. Combining the conflict number and conflict rate, the safety performance of merging areas was analyzed. Based on the video data collected from seven merging areas of two freeways in Liaoning province of China, a total of 898 PETs were measured, and the identified threshold for serious conflicts was 1.54 s. The analysis results of conflict number and rate combination method showed that Gaosheng, Huludao, and Suizhong merging areas were the most hazardous among the seven merging areas while Xingcheng, Wulongbei, and Tongyuanpu merging areas were the safest. A further analysis also indicated that both the conflicting volume and the length of acceleration lanes had significant impact on the safety performance of merging areas.