Toru Seo

Toru Seo

Optimization using network traffic models requires computing gradients of objective functions with respect to model parameters. However, derivation of gradients of network traffic models has been considered very difficult or impractical due to their complexity and size. Conventional approaches rely on numerical differentiation or derivative-free methods that do not scale well with the parameter dimension, or on adjoint methods that require manual derivation for each specific model. This study proposes a novel end-to-end differentiable network traffic flow simulator based on the Link Transmission Model (LTM) and a dynamic user optimum (DUO) route choice model. We observe that the LTM operates on continuous aggregate state variables (cumulative vehicle counts) through piecewise-linear min/max operations, which admit subgradients almost everywhere and appropriate for automatic differentiation (AD). We incorporate the DUO route choice model and its logit extension to explicitly consider endogenous dynamic route choice of travelers while preserving differentiability, by leveraging the fact that the diverge ratios are continuous functions of per-destination vehicle counts. The resulting simulator is differentiable almost everywhere and computes exact gradients via reverse-mode AD in a single backward pass regardless of the parameter dimension. In order to demonstrate the capability of the proposed model, we solved a dynamic congestion toll optimization problem on the Chicago-Sketch dataset with around 2500 links, 1 million vehicles, a 3-hour duration, and 15000 decision variables. The proposed model successfully derived a high-quality solution in 3000 iterations in about 40 minutes. On average, one simulation run and gradient derivation took 0.8 seconds. The simulator, implemented in Python and JAX, is released as open-source software named UNsim (https://github.com/toruseo/UNsim).

Toru Seo, Kentaro Wada, Daisuke Fukuda

Urban rail transit often operates with high service frequencies to serve heavy passenger demand during rush hours. Such operations can be delayed by two types of congestion: train congestion and passenger congestion, both of which interact with each other. This delay is problematic for many transit systems, since it can be amplified due to the interaction. However, there are no tractable models describing them; and it makes difficult to analyze management strategies of congested transit systems in general and tractable ways. To fill this gap, this article proposes simple yet physical and dynamic model of urban rail transit. First, a fundamental diagram of transit system (i.e., theoretical relation among train-flow, train-density, and passenger-flow) is analytically derived considering the aforementioned physical interaction. Then, a macroscopic model of transit system for dynamic transit assignment is developed based on the fundamental diagram. Finally, accuracy of the macroscopic model is investigated by comparing to microscopic simulation. The proposed models would be useful for mathematical analysis on management strategies of urban rail transit systems, in a similar way that the macroscopic fundamental diagram of urban traffic did.

Hengyi Zhong, Toru Seo

In transportation network analysis, various types of road network data can be used even when focusing on the same region. Since different road network datasets can make different performance in analyses, it is necessary to compare them and make appropriate selections in a qualitative manner. However, many of the existing methods for comparing road network datasets are limited to specific topological evaluations and do not consider transportation. This study proposes a method for quantitative comparison of different road network datasets with explicit consideration for traffic flows on them. The method first conducts a static traffic assignment with hypothetical demand for each dataset, and then compare the results using Wasserstein distance on two dimensional plane. Case study on different sources of road network datasets and their simplifications suggests the potential use of the proposed method in evaluating and selecting road network datasets.

Kimihiro Sato, Toru Seo, Takashi Fuse

Traffic congestion is a serious problem in urban areas. Dynamic congestion pricing is one of the useful schemes to eliminate traffic congestion in strategic scale. However, in the reality, an optimal dynamic congestion pricing is very difficult or impossible to determine theoretically, because road networks are usually large and complicated, and behavior of road users is uncertain. To account for this challenge, this work proposes a dynamic congestion pricing method using deep reinforcement learning (DRL). It is designed to eliminate traffic congestion based on observable data in general large-scale road networks, by leveraging the data-driven nature of deep reinforcement learning. One of the novel elements of the proposed method is the distributed and cooperative learning scheme. Specifically, the DRL is implemented by a spatial-temporally distributed manner, and cooperation among DRL agents is established by novel techniques we call spatially shared reward and temporally switching learning. It enables fast and computationally efficient learning in large-scale networks. The numerical experiments using Sioux Falls Network showed that the proposed method works well thanks to the novel learning scheme.

Yuxun Ma, Toru Seo

Route choice models are one of the most important foundations for transportation research. Traditionally, theory-based models have been utilized for their great interpretability, such as logit models and Recursive logit models. More recently, machine learning approaches have gained attentions for their better prediction accuracy. In this study, we propose novel hybrid models that integrate the Recursive logit model with Graph Neural Networks (GNNs) to enhance both predictive performance and model interpretability. To the authors' knowldedge, GNNs have not been utilized for route choice modeling, despite their proven effectiveness in capturing road network features and their widespread use in other transportation research areas. We mathematically show that our use of GNN is not only beneficial for enhancing the prediction performance, but also relaxing the Independence of Irrelevant Alternatives property without relying on strong assumptions. This is due to the fact that a specific type of GNN can efficiently capture multiple cross-effect patterns on networks from data. By applying the proposed models to one-day travel trajectory data in Tokyo, we confirmed their higher prediction accuracy compared to the existing models.