Improving The Network Performance Of Urban Transit Systems

Author : Haoyu Chen
Publisher :
ISBN 13 :
Total Pages : 73 pages
Book Rating : 4.:/5 (957 download)

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Download or read book Improving Public Transit Systems at Intersection and City-wide Scales written by Haoyu Chen and published by . This book was released on 2016 with total page 73 pages. Available in PDF, EPUB and Kindle. Book excerpt: Designing a highly efficient public transit system can help reduce the congestion and pollution in large cities. This dissertation tries to improve public transit systems at both the micro and macro scales. At the microscale, a Transit Signal Priority strategy is developed to integrate a pre- signal scheme. Buses often compete with cars for limited road space and signal times at intersections. Providing signal priority together with exclusive lanes for buses will often diminish car discharge flows into the intersection. This can create huge car delays, which will undermine the benefit brought by the priority measures to buses. To solve this problem, we propose a midblock pre-signal to recover the loss in the cars' discharge capacity caused by the bus priority measures. We show how the pre-signal can be coordinated with the intersection's main signal under a simple signal priority scheme. Analytical models are formulated to examine the effects of our strategy on two performance metrics: the expected bus delay and the car capacity. Pareto frontiers between the two performance metrics are developed. Numerical case studies show that huge benefits for both buses and cars can be achieved by the proposed strategy, as compared to baseline scenarios where a bus signal priority scheme or pre-signal is absent. At the macroscale, two continuum approximation (CA) optimization models are formu- lated to design city-wide transit systems at minimum cost. Transit routes are assumed to lie atop a city's street network. Model 1 assumes that the city streets are laid out in ring-radial fashion. Model 2 assumes that the city streets form a grid. Both models can furnish hybrid designs, which exhibit intersecting routes in a city's central (downtown) district and only radial branching routes in the periphery. Model 1 allows the service frequency and the route spacing at a location to vary arbitrarily with the location's distance from the center. Model 2 also allows such variation but in the periphery only. We show how to solve these CA optimization problems numerically, and how the numeri- cal results can be used to design actual systems. A wide range of scenarios is analyzed in this way. It is found among other things that in all cases and for both models: (i) the optimal2 headways and spacings in the periphery increase with the distance from the center; and (ii) at the boundary between the central district and the periphery both, the optimal service frequency and the line spacing for radial lines decrease abruptly in the outbound direction. On the other hand, Model 1 is distinguished from Model 2 in that the former produces in all cases: (i) a much smaller central district, and (ii) a high frequency circular line on the outer edge of the central district. Parametric tests with all the scenarios further show that Model 1 is consistently more favorable to transit than Model 2. Cost differences between the two designs are typically between 9% and 13%, but can top 21.5%. This is attributed to the manner in which ring-radial networks naturally concentrate passenger's shortest paths, and to the economies of demand concentration that transit exhibits. Thus, it appears that ring-radial street networks are better for transit than grids. To illustrate the robustness of the CA design procedure to irregularities in real street networks, the results for all the test problems were then used to design and evaluate transit systems on networks of the" wrong" type grid networks were outfilled with transit systems designed with Model 1 and ring-radial networks were designed with Model 2. Cost increased on average by a little: only 2.7%. The magnitude of these deviations suggests that the proposed CA procedures can be used to design transit systems over real street networks when they are not too different from the ideal, and that the resulting costs should usually be very close to those predicted.