روشی ابتکاری برای محاسبه قابلیت اطمینان ظرفیت و دسترسی (اتصال) در شبکه های حمل و نقلی آسیب پذیر درون شهری*

نویسندگان

1 استادیار، دانشکده عمران، دانشگاه علم و صنعت ایران، تهران، ایران

2 دانشیار، دانشکده عمران، دانشگاه علم و صنعت ایران، تهران، ایران

3 دانشجوی دکتری، دانشکده عمران، دانشگاه علم و صنعت ایران، تهران، ایران

چکیده

شبکه های حمل و نقلی ممکن است تحت تأثیر یک سری عوامل خارجی یا داخلی دچار افت عملکرد و ناپایداری جریان ترافیک شوند. عوامل خارجی عمدتاً شامل شرایط آب و هوایی خاص (مانند بارش برف، یخبندان و طوفان)، زلزله، بهمن و برخی عوامل دیگرند. عوامل داخلی آسیب پذیری ناشی از خرابی سیستم های کنترل، تصادفات، تعمیرات و ساخت و سازها هستند. تمامی عوامل یادشده می توانند منجر به افت ظرفیت کمان ها شوند و پاره ای از آنها تقاضای پایه موجود در زوج های مبدأ- مقصد شبکه را تحت تأثیر قرار می دهند. با توجه به اهمیت حفظ عملکرد شبکه در چنین شرایطی در یک سطح مطلوب، در دهه آخر قرن بیستم تعریفی جدید برای ارزیابی عملکرد شبکه های حمل و نقلی ارایه شده است که اعتمادپذیری شبکه را در شرایط عدم قطعیت بیان می کند. از این رو ارزیابی قابلیت اطمینان عملکرد شبکه های حمل و نقل در طراحی و یا ارتقا و بازسازی آنها نقش بسیار پر اهمیتی پیدا کرده است. در این مقاله روشی ابتکاری برای محاسبه قابلیت اطمینان عملکرد اجزای شبکه در سطوح مختلف سرویس دهی ارایه شده است که از آن می توان برای محاسبه قابلیت اطمینان دسترسی (اتصال) و قابلیت اطمینان ظرفیت شبکه در قالب یک رابطه ریاضی استفاده کرد. همچنین نتایج بکارگیری این روش در یک شبکه شماتیک، جهت نمایش کارآمدی آن، ارایه شده است.

کلیدواژه‌ها


عنوان مقاله [English]

A Heuristic Method for Connectivity and Capacity Reliability Evaluation in Degradable Urban Transportation Networks

نویسندگان [English]

  • A. Shariat Mohaymany 1
  • A. Mansourkhaki 2
  • M. Babaei 3
1 Assistant Professor, Department of Civil Engineering, Iran University of Science and Technology, Tehran, Iran.
2 Associate Professor, Department of Civil Engineering, Iran University of Science and Technology, Tehran, Iran.
3 Ph.D. Student, Department of Civil Engineering, Iran University of Science and Technology, Tehran, Iran.
چکیده [English]

A transportation network, subject to some internal or external events, may be involved in performance degradation and traffic flow instability. The external events mainly include weather conditions and natural disasters like snowfall, ice, hurricane, earthquake and avalanche. Internal degradations are due to traffic related factors such as control systems failure, accidents, and maintenance and construction operations. All aforementioned circumstances can severely decrease links’ capacities, and some of them affect the base demand of the origin-destination pairs; hence, the network cannot perform as well as that in normal situation.When a transportation network or a number of its components (links) is vulnerable pertain to the foregoing incidents or disasters, its performance cannot be evaluated through the conventional methods in which we calculate deterministic measures such as average travel speed, average delay per vehicle and vehicle-kilometer traveled. In such situations the capacity of degraded links has not certain values, and they must be referred to as probability distribution functions; we therefore will face with a probabilistic network whose performance index ought to be assessed in reliability terms. Until now different types of performance reliability have been established to evaluate the reliability of degradable transportation networks. The first one is the connectivity reliability which considers the probability that a special origin-destination (OD) pair of the network remains connected. The other measure addressed frequently in previous studies is travel time reliability, which is defined as the probability that a trip performs successfully between a given OD pair within a specified time interval. Recently, another challenging measure has been introduced in order to estimate that at what level of a certain demand the network can perform accurately. This measure is known as capacity reliability and, in other words, is the probability that a network can accommodate a certain traffic demand at a required service level.This paper presents a heuristic method to calculate the connectivity (terminal) reliability and the capacity reliability. The capacity of each link is assumed to have a normal distribution function that is verified by a proposed method in order that a lower bound and an upper bound can be assigned to the function. Using these modified normal distribution functions the performance reliability of each link is obtained based on the integration of its probability density function from to :where is the traffic volume on link that is computed through the application of user equilibrium assignment with respect to the expected link capacities. and are respectively upper bound and probability density function of capacity of link . Notice the link performance reliability will be zero if the volume exceeds the upper bound of link capacity.Having calculated the links performance reliabilities, the capacity and connectivity reliability of the network are calculated by means of the closed formulas presented in this paper for each type of considered performance reliability measures. Another contribution of this paper is that the presented method has the ability of taking into account a required level of service for each link, namely it would be possible to assess the network performance when there exist especial expectations for maximum volume per capacity ratios ( s). Therefore, it might be said that in the proposed method the travel time reliability is considered along with evaluating the two explained performance reliability measures because the travel time of each link is a function of its .Finally, for the sake of comparing the numerical results obtained from proposed method and from the other previous works, we have examined a simple test network employed repeatedly in network reliability analysis, particularly in the case of capacity reliability studies.Keywords: Urban transportation networks, uncertainty, connectivity reliability, capacity reliability

کلیدواژه‌ها [English]

  • Urban transportation networks
  • Uncertainty
  • connectivity reliability
  • capacity reliability