A Study on Route Diversion Point and Diversion Flow Rate on Freeway and Expressway Corridors

• Main Authors: Jenq-Yuan Jenq, 鄭正元
• Other Authors: Shou-Ren Hu
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/61194072963091639963

碩士 === 淡江大學 === 運輸管理學系 === 92 === The purposes of this study are to analyze traffic flow patterns in the presence of traffic incident and find appropriate diversion points and diversion rates accordingly. Traffic assignment based models were constructed to carry out the study. We divided the analyzing time horizon into two stages, i.e., “before incident” and “after incident”, and proceeded static traffic assignment analysis individually. Before incident was happened, user equilibrium (UE) analysis was conducted, and relevant path flows and link flows under UE assumption were obtained. Furthermore, when an incident was observed in a specific link, resulting in the decrease of link capacity, motorists traveling at that specific link might be seeking better alternative paths to avoid extra travel times and/or delays. The two-stage analysis was conducted as follows. Firstly, we sort the equilibrium path flow into influenced path flow and non-influenced path flow, which both represent the flow passing and without passing through the link with incident, respectively. Taking the non-influenced flow as the background flow, we reassign the influenced flow to potential paths. Secondly, we conduct both UE and system optimal (SO) analysis, and compare the path and link flows under both equilibrium conditions. The nodes with significant flow changes before and after the incident are appropriate diversion points, and relevant diversion information can be disseminated preferably through VMS in front of these diversion points. Because the two-stage analysis process might generate several desirable diversion points, and each diversion point might better divert different percentage of affected motorists, therefore sensitive analysis was employed to find the optimal diversion rate. If we guide the same proportion in all diversion points, we might find the global optimal diversion rate, meaning that if there are such proportion of motorists at all diversion points following the guidance information, then the system can save the maximum travel time. On the other hand, if we provide different information dissemination strategies at different diversion points, we can obtain optimal individual diversion rate at each diversion point. To demonstrate the proposed framework, a simplified network composing the freeway and expressway corridors in northern Taiwan was employed to conduct the case study under different traffic conditions. Various test scenarios were designed to check if location of incident, severity of incident, and single or multiple incident happened at the same time will affect the analyzing results of diversion point and diversion rate. The numerical results indicated that the appropriate diversion points were usually located at the 2~3 main system interchanges upstream of the incident, irrespective to the severity and location of the incident. Compared to the case of single traffic incident, it has been found that multiple incidents might result in more diversion points so that the affected motorists could be diverted to alternative routes at appropriate decision points. In addition, according to the results of the sensitivity analysis on route diversion rate, there actually exists a global route diversion rate that minimize the total system cost by providing affected motorists with appropriate route diversion information. However, we could not find the general rule and/or relationship between the optimal diversion rate and above affecting factors. If we calculate individual diversion rate at each potential diversion point, and assume that those affected motorists all follow the route diversion instructions, we might further gain benefits of lower total system cost as opposed to the cost associated with the global diversion rate. Moreover, the optimal rate of route diversion is generally inversely proportional to the distance to the incident. Finally, as the severity of incident is increased, the diversion rates of those diversion points closer to the incident are also found to be increased.