University of Twente Student Theses


Effects of incidents on freeway capacity

Siemerink, Martijn (2008) Effects of incidents on freeway capacity.

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Abstract:Congestion on freeways has always been a big problem in the field of transportation. Over the years solutions for congestion problems have shifted from building new, and expanding existing roads to improving traffic operations on the existing freeways. On freeways, traffic operations can be affected by incidents, such as car crashes and disabled vehicles. Statistics show that incidents on freeways have a big impact on traffic operation, making up one-third of traffic delay in urban areas (U.S. Department of Transportation Federal Highway Administration Office of Operations, 2008). With this large influence of incidents on traffic operations, it is very effective to improve traffic operations by focusing on delay caused by freeway incidents. To reduce delays caused by freeway incidents, incident management systems are in place. To make the incident management systems work and to improve them, the consequences that freeway incidents and measures of incident management have on traffic operations must be understood. The contribution of this report on to better understand the effects of incidents and measures to reduce delay is twofold. First there will be a focus on the effects that incidents have on capacity. Secondly, a microscopic traffic simulation tool is assessed on its capabilities of modeling incidents and their effects on traffic on a freeway that can be used to evaluate incident management strategies. Combined these two objectives can aid in the evaluation of incident management strategies. Traffic incident management systems used for the reduction of delay caused by incidents are comprised of distinct activities that form the incident management process. These activities are carried out by personnel from a variety of response agencies and organizations. Although the activities are very distinct, they can however be performed simultaneously. The seven activities of incident management are: Detection Verification Motorist information Response Site Management Traffic Management Clearance Detection is notification of the agency or agencies responsible for maintaining traffic flow and safe operations on the facilities. When the agencies are notified the incident is verified from an independent source to confirm it really happened. After confirmation the necessary agencies respond to the incident and assist in the clearance process. During the clearance motorists are informed of the incident location and best alternatives to avoid the affected roadway by 511 motorist information via the Traffic Management Center. At the site the appropriate agency is in charge or the agencies work coherently together to clear the incident. An important part in seeing if measures against incident related delay can improve traffic operations is to look at how incidents affect freeway capacity. The premier reference in the United States regarding capacity related information is the Highway Capacity Manual 2000 (HCM). The HCM provides an analysis procedure for freeway facilities and in this analysis information is presented on the effects of incidents on capacity. The information on the effects of incidents on freeway consists of adjustments factors for capacity when different lane closures occur during incidents. This information, however, is outdated and somewhat limited. The main limitations are that it does not make a distinction in capacity reduction between minor and major incidents and it does not give 5 information on capacity reduction due to rubbernecking in the opposite direction of travel. The age and limitations of the material make it desirable to update the material on incidents affecting capacity in the HCM. The limitations and age of the material in the HCM make it very desirable for the Highway Capacity and Quality of Service Committee (HCQSC) of the Transportation Research Board (TRB) to update the information on the effects on capacity from freeway incidents. The synthesis however shows that the information on the topic in more recent research is also very limited. There is simply very little research done on capacity reduction due to incidents on freeways. A comparison of the results that were found is shown in the table. The figures in the table show that the reductions in capacity are similar to each other. However it is vital that more research is being performed on the topic, so that results can be generalised. A good addition to the HCM could be the research performed by Masinick & Teng, 2004 on the effects of rubbernecking for the opposite direction of travel. Future research should focus on the same type of incident on different freeways, so that insight can be gained in a uniform reduction factor for certain lane closures. Furthermore future research should consider characteristics of incidents such as time of day (i.e. peak vs. off-peak vs. night time), portion of the freeway affected (e.g. vehicles on the shoulder, single lane closure, two-lane closure, full closure) and duration (1-hour, 2 hours, etc.).With more research on the topic a better update of the HCM can be realised with better and more complete information. To further improve incident management systems, new strategies are planned within various areas of the incident management systems. These strategies need to be evaluated before, during or after implementation. A low-cost manner to evaluate different strategies is the use of a microscopic traffic simulation tool, because no strategy has to be implemented in reality. However the use of the simulation tool is limited to evaluating strategies for certain activities. Because the simulation tool is only useful for activities related to traffic flow, it is most effective when used for the activities of Traffic Management and Motorist Information. The microscopic simulation tool CORSIM was evaluated on its capabilities of modeling incidents and their consequences on a freeway. CORSIM is a useful tool for modeling incidents since it has a special option for modeling incident, so that a lane-blocking (incident) does not have to be created indirectly. Within the special incident option in CORSIM the most important inputs for modeling the incident on the freeway network are provided, such as the location, duration, lanes affected and rubberneck factor. The rubberneck factor is the extra capacity reduction due to the incident besides the physical capacity reduction. The rubberneck factor must be determined by the user and can be estimated using the field data or a reference such as the HCM, which makes it important that the HCM contains reliable information. The program has a limitation in the modeling that for simulating incidents would have been ideal. This limitation is the fact that law enforcement presence cannot be modeled into CORSIM, although it can be attempted indirectly via another parameter. Furthermore a flaw is present in the CORSIM tool. In the current version of CORSIM the merging process behind incidents does not work correctly, so repairing it and maybe including the special merging process for incidents should enable CORSIM to produce better results in simulating incidents. All in all CORSIM could be very useful in evaluating incident management strategies when the merging problem is fixed.
Item Type:Essay (Bachelor)
University of Florida, Gainesville, United States
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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