Semi-on-Demand Hybrid Transit Route Design with Shared Autonomous Mobility Services

TL;DR

The paper tackles the problem of integrating Shared Autonomous Mobility into public transit by designing semi-on-demand hybrid routes that combine a fixed, high-density segment with a flexible, on-demand segment along corridors. It develops two tractable analytical cost formulations to guide strategic (flexible portion and fleet size) and tactical (headway and vehicle size) planning, and derives closed-form solutions for optimal $x_f$ under uniform and triangular demand, with extensions to variable vehicle sizes. Numerical examples on Chicago routes and a city-scale case study demonstrate that SAVs’ lower operating costs favor more flexible or hybrid routing, especially when demand gradients are strong, while larger vehicles tend to push toward fixed or hybrid configurations. The analytical tool provides transit agencies with a scalable method to assess the trade-offs between fixed, hybrid, and flexible routes and to jointly optimize headway, fleet size, and vehicle size for SAV-enabled feeders, with practical insights for investment decisions and network design.

Abstract

Shared Autonomous Vehicles (SAVs) enable transit agencies to design more agile and responsive services at lower operating costs. This study designs and evaluates a semi-on-demand hybrid route directional service in the public transit network, offering on-demand flexible route service in low-density areas and fixed route service in higher-density areas. We develop analytically tractable cost expressions that capture access, waiting, and riding costs for users, and distance-based operating and time-based vehicle costs for operators. Two formulations are presented for strategic and tactical decisions in flexible route portion, fleet size, headway, and vehicle size optimization, enabling the determination of route types between fixed, hybrid, and flexible routes based on demand, cost, and operational parameters. Analytical results demonstrate that the lower operating costs of SAVs favor more flexible route services. The practical applications and benefits of semi-on-demand feeders are presented with numerical examples and a large-scale case study in the Chicago metropolitan area, USA. Findings reveal scenarios in which flexible route portions serving passengers located further away reduce total costs, particularly user costs, whereas higher demand densities favor more traditional line-based operations. Current cost forecasts suggest smaller vehicles with fully flexible routes are optimal, but operating constraints or higher operating costs would favor larger vehicles with hybrid routes. The study provides an analytical tool to design SAVs as directional services and transit feeders, and tractable continuous approximation formulations for planning and research in transit network design.

Figures (13)

• Figure 1: Illustration of Fixed Route, On-demand Flexible Route, and Semi-on-Demand Hybrid (Fixed/Flexible) Route as a Feeder Service
• Figure 2: Illustration of Optimality Demarcation of Fixed, Hybrid, and Flexible Route from Result \ref{['thm:route_choice']} (Ignoring Additional Vehicle Requirement)
• Figure 3: Maps of Bus Routes CTA126 and CTA84 in Chicago chicago_transit_authority_cta_2022chicago_transit_authority_cta_2022-1
• Figure 4: Total Costs $c(x_f)$ (a), Fleet Size $s(x_f)$ (b), and Average Cost Components (c) of Case CTA126 with respect to Flexible Route Portion $x_f$ under Uniform Demand Distribution
• Figure 5: Total Costs $c(x_f)$ (a), Fleet Size $s(x_f)$ (b), and Average Cost Components (c) of Case CTA126 with respect to Flexible Route Portion $x_f$ under Triangular Demand Distribution

Theorems & Definitions (3)

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