NITC Dissertation Fellow Sheida Khademi Uses Modeling to Manage Freeway Capacity
We're proud to announce the publication of a new NITC dissertation: "Modeling Capacity: Multiple Weaving Areas," by Sheida Khademi of the University of Texas at Arlington.
"Traffic congestion on freeway systems is one significant concern in urban areas throughout the U.S.A. In this era, building new freeways to reduce congestion is less feasible due to the high capital and social costs. Thus, the effective management and operation of existing freeway facilities has become a preferred approach to reduce traffic congestion. Using the outcomes of this research, agencies can get an idea of which effective variable they should control to manage freeways' multiple weaving areas more efficiently; obtaining the highest capacity while planning for existing freeways. The results will be presented to DOTs and MPOs. The model would be highly useful and money-saving for these agencies, as they prefer to obtain higher capacity by managing existing freeways rather than buying right of ways," Khademi said.
Systematic, well-designed research provides the most effective approach to the solution of many problems facing highway administrators and engineers. Traffic congestion on freeway systems is one significant concern in urban areas throughout the U.S.A. In this era, building new freeways to reduce congestion is less feasible due to the high capital and social costs. Thus, the effective management and operation of existing freeway facilities has become a preferred approach to reduce traffic congestion.
A weaving section is a common design on major highway facilities that always has been an interest to researchers. Weaving areas are characterized by frequent lane changes, which significantly reduce the capacity of the freeway system. The Highway Capacity Manual (HCM) defined weaving capacity as “any combination of flows that causes the density to reach the LOS E/F boundary condition of 43 pc/m/ln for freeways” based on configuration, number of lanes in the weaving section, free-flow speed, length of the weave, and volume ratio.
Weaving sections are often problematic because of the increase in lane changing. These are common design elements on freeway facilities between an on ramp and off ramp with an auxiliary lane. They are located between merge and diverge points, near ramps where a lane is added or dropped, and at multilane ramps. A weaving section is a freeway segment in which traffic flows cross each other without traffic control.
Traffic demands exceeding segment capacity at weaving areas cause congestion, which affects the operation of the entire freeway section. Traffic operation problems often exist at weaving areas even when traffic demands are less than capacity and may be experienced at lower traffic flows because of the complexity of vehicle interactions, that is, increased lane changing, resulting in a degradation in level of service (LOS) and potential safety problems.
A significant amount of research has been done to estimate quality of service and capacity in weaving sections. However, little has been done to address multiple weaves.
A multiple weaving area is one where two or more weaving areas overlap. No satisfactory means of estimating capacity has been found. Current procedures make assumptions about where weaving occurs in the individual weaving segments. A multiple weaving area is found when “a series of closely spaced merge and diverge areas create overlapping weaving movements (between different merge-diverge points).”
This work examines capacity and quality of service conditions for a specific example of a multiple weave. Capacity is evaluated through micro-simulation by gradually raising flows for a range of geometric and fraction of weaving-traffic conditions. Models are developed to express capacity in terms of lane configuration, flow ratios, traffic mix (heavy traffic percentages), and overall flow rate. This work’s results will be presented to DOTs and MPOs as a guidebook.
ABOUT THE PROJECT
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The National Institute for Transportation and Communities (NITC) is one of seven U.S. Department of Transportation national university transportation centers. NITC is a program of the Transportation Research and Education Center (TREC) at Portland State University. This PSU-led research partnership also includes the Oregon Institute of Technology, University of Arizona, University of Oregon, University of Texas at Arlington and University of Utah. We pursue our theme — improving mobility of people and goods to build strong communities — through research, education and technology transfer.