An Investigation Of Centroid Connector Placement For
! An Investigation Of Centroid Connector Placement For Advanced Traffic Assignment Models With Added Network Detail Ehsan Jafari; Mason D. Gemar; Natalia Ruiz Juri; Jennifer Duthie ! ! ! ! ! ! ! ! ! ! ! ! The University of Texas at Austin ! ! ! f ! ! f ! ! ! INTRODUCTION Closest Node CASE STUDY Bi-level ! ! ! ! ! ! Research Motivation ! ! ! β’ Advanced traffic assignment models typically incorporate more detailed network representations. ! ! ! ! ! ! ! β’ Changing network topology by adding detail to the network requires carefully editing the connector structure. ! ! ! ! ! ! ! β’ The number and placement of centroid connectors can also significantly impact the congestion pattern in capacitated models. Research Objective ! 1 πΎπ = ππ π ! β’ Two strategies are proposed for centroid connector placement. β’ A new metric has been introduced, the locality factor, to describe the use of minor streets by local traffic. APPROACHES 1- Closest Node approach: ! πΉπ πΏππ πππ ! ! πΎπ ππ πΉπ πππ πΏππ ! link π locality factor total flow on link π path π flow distance between link π and path π 1 if link π is on path π, 0 otherwise ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Closest Node! Selection Layout ! ! ! ! ! ! RESULTS Bi-Level Selection Layout ! ! ! ! ! ! ! ! ! Contributions Complete Network Layout ! An alternative to measure the share of local traffic on each link: β’ The primary purpose of this study it to determine the importance of modeling local streets and to address issues related to including them in a dynamic framework. Base Network Layout ! ! Locality Factor (LF) β’ ! ! ! ! ! ! ! ! ! ! ! ! ! β’ The bi-level strategy increases the share of local traffic on local streets because some portion of demand is required to start/end their trip within the inner subzones β’ TSTT decreases as the number of connectors increases because: β’ more alternative routes with lower costs are available in a network with more connector links, β’ also less concentration of trip ends at any particular connector location β’ LF decreases which suggests a trend toward avoiding minor roads when alternatives are present ! ! β’ The effect of inner-centroid fraction on locality factor (left) and TSTT (right) using the bi-level selection approach ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Total Locality Factor (TLF) Network Complete Closest Bi-Level Total Locality Factor 0.087 0.092 0.14 ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! β’ The first approach radially distributes the connectors to link centroids to the nearest nodes in order to account for added network detail, and is intended to exemplify some of the limitations of the most common techniques in practice. 2- Bi-level approach: β’ The second strategy involves dividing the centroid and subsequent demand into two parts and distributing the demand across one subcentroid linked to nearby nodes and one linked to the periphery, effectively establishing a bi-level distribution. β’ Evaluation with field measurements β Williamson County, TX, β’ 1,281 nodes, 158 zones, 2,617 links β’ Travel times along 9 corridors covering 365 links were collected β’ The best performance is achieved using the bi-level approach by assigning 90% of demand to the inner subzones Network Layout Travel Time Error (min) Base Network Bi-level Network Closest Network min max avg min max avg min max avg .03 .03 .02 5.4 1.2 5.4 1.3 4.2 1.1 Sensitivity to the Number of Centroid Connectors SUMMARY ! ! ! ! ! ! ! ! ! Number of Connectors per SubZone β’ Incorporation of minor roadways without any changes to the centroid connector layout may lead to inaccurate usage patterns β’ Two strategies were proposed to redistribute the connectors β’ A new performance metric was also introduced, the locality factor, to quantify the utilization of roadway segments by demand generated from (or attracted to) nearby centroids β’ The bi-level method and selection process results in the highest locality factor and generally achieves better model performance metrics ! ! ! ! ! Demand # of Centroid Split Connectors TLF TSTT (hr) ! ! ! ! ! ! ! ! ! ! ! ! ! 1 2 3 4 5 6 7 8 50-50 50-50 50-50 50-50 50-50 50-50 50-50 50-50 239 371 471 571 653 691 773 811 0.55 0.25 0.20 0.15 0.14 0.12 0.10 0.11 8748.87 8310.6 8318.21 8317.08 8153.54 8071.98 8057.52 8010.96 ! ! ! ! ! ! ! ! ! ! ! collaborate. innovate. educate.
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