Simon Oh | Singapore-MIT Alliance for Research and Technology (original) (raw)

Papers by Simon Oh

Computer-aided Civil and Infrastructure Engineering, May 25, 2016

Travel time prediction is one of the most important components in Intelligent Transportation Syst... more Travel time prediction is one of the most important components in Intelligent Transportation Systems implementation. Various related techniques have been developed, but the efforts for improving the applicability of long-term prediction in a real-time manner have been lacking. Existing methods do not fully utilize the advantages of the state-of-the-art cloud system and large amount of data due to computation issues. We propose a new prediction framework for real-time travel time services in the cloud system. A distinctive feature is that the prediction is done with the entire data of a road section to stably and accurately produce the longterm (at least 6-hour prediction horizon) predicted value. Another distinctive feature is that the framework uses a hierarchical pattern matching called Multilevel k-nearest neighbor (Mk-NN) method which is compared with the conventional k-NN method and Nearest Historical average method. The results show that the method can more accurately and robustly predict the long-term travel time with shorter computation time.

To investigate the impact of traffic state transition and oscillation on highway performance, thi... more To investigate the impact of traffic state transition and oscillation on highway performance, this paper develops a section-based identification methodology to classify traffic state into stationary (FF, BN, CT, and BQ), transitional and oscillatory traffic using point measurement data. Firstly, the section-based perspective presents the asymmetry in the intensity of two different transition paths (breakdown and recovery) resulting inequality in the capacity and recovery rate (section-hysteresis). It is found that there exists decreasing tendency of transition intensity according to the number of lanes and the positive relationship between the intensity of transition and the amount of capacity loss. Moreover, accompanied by traffic oscillations, a series of capacity loss suggests the negative impact of oscillations on traffic dynamics triggering subsequent performance degradations. In addition, we observe the shrinkage of incoming flow to the queue. The findings emphasize the adverse impact of traffic state transition and guide us to settle the management strategy by providing statistical estimations for main highway performance.

Transportation Research Board 91st Annual MeetingTransportation Research Board, 2012

Capacity drop significantly damages highway efficiency. Yet, its mechanism has not been fully rev... more Capacity drop significantly damages highway efficiency. Yet, its mechanism has not been fully revealed. Especially, the casual factors of capacity drop are left unknown to traffic theorists, and there are still many controversial issues on the topic. Accordingly, many researchers observed the capacity drop phenomenon and approached it with a macroscopic view based on LWR theory. However, approach in microscopic level is necessary as the capacity drop is ultimately related to the driver's behavior in bottleneck area. Therefore, in this research we analyzed microscopic data for individual vehicle to explain the mechanism of capacity drop. Microscopic analysis using NGSIM data revealed the impact of disturbances such as lane-changing events and effect of stop-and-go waves reducing discharge flow. The results concluded that the capacity drop is caused by the impact of stop-and-go waves, while a lane-changing event increases the flow by aggressive driving pattern during a lane-changing action, which is a strong counter-evidence for Laval and Daganzo's explanation on the capacity drop, in which they assumed a "void" made by slow lane changers.

Transportation Research Part A: Policy and Practice

In most cities, transit consists of fixed-route transportation only, whence the inherent limited ... more In most cities, transit consists of fixed-route transportation only, whence the inherent limited Quality of Service for travellers in suburban areas and during off-peak periods. On the other hand, completely replacing fixedroute with demand-responsive (DR) transit would imply huge operational cost. It is still unclear how to ingrate DR transportation into current transit systems to take full advantage of it. We propose a Continuous Approximation model of a transit system that gets the best from fixed-route and DR transit. Our model allows to decide, in each area and time of day, whether to deploy a fixed-route or a DR feeder, and to redesign line frequencies and stop spacing of the main trunk service accordingly. Since such a transit design can adapt to the spatial and temporal variation of the demand, we call it Adaptive Transit. Our numerical results show that Adaptive Transit significantly improves user cost, particularly in suburban areas, where access time is remarkably reduced, with only a limited increase of agency cost. We believe our methodology can assist in planning future-generation transit systems, able to improve urban mobility by appropriately combining fixed and DR transportation.

arXiv (Cornell University), Nov 15, 2020

The emergence of ride-sourcing platforms has brought an innovative alternative in transportation,... more The emergence of ride-sourcing platforms has brought an innovative alternative in transportation, radically changed travel behaviors, and suggested new directions for transportation planners and operators. This paper provides an exploratory analysis on the operations of a ride-sourcing service using large-scale data on service performance. Observations over multiple days in Singapore suggest reproducible demand patterns and provide empirical estimates of fleet operations over time and space. During peak periods, we observe significant increases in the service rate along with surge price multipliers. We perform an in-depth analysis of fleet utilization rates and are able to explain daily patterns based on drivers' behavior by involving the number of shifts, shift duration, and shift start and end time choices. We also evaluate metrics of user experience, namely waiting and travel time distribution, and explain our empirical findings with distance metrics from driver trajectory analysis and congestion patterns. Our results of empirical observations on actual service in Singapore can help to understand the spatiotemporal characteristics of ride-sourcing services and provide important insights for transportation planning and operations.

Research in Transportation Business & Management

91st TRB Annual Meeting, Jan 23, 2012

2019 IEEE Intelligent Transportation Systems Conference (ITSC), 2019

In this paper, we simulate the impacts of Automated Mobility-on-Demand (AMoD) using SimMobility, ... more In this paper, we simulate the impacts of Automated Mobility-on-Demand (AMoD) using SimMobility, an integrated land-use transport micro-simulation platform. Using data for Singapore, we examined the future scenarios in which AMoD is added as an additional mode on top of existing modes, or as an exclusive one. We found that AMoD may lead to changes in accessibility, which will consequently affect people's behaviours. Our results show that a scenario in which AMoD is an additional mode is a preferred option, whereas the exclusion of AMoD will lead to lower accessibility for people in the study area.

ArXiv, 2020

Technological advancements have brought increasing attention to Automated Mobility on Demand (AMO... more Technological advancements have brought increasing attention to Automated Mobility on Demand (AMOD) as a promising solution that may improve future urban mobility. During the last decade, extensive research has been conducted on the design and evaluation of AMOD systems using simulation models. This paper adds to this growing body of literature by investigating the network impacts of AMOD through high-fidelity activity- and agent-based traffic simulation, including detailed models of AMOD fleet operations. Through scenario simulations of the entire island of Singapore, we explore network traffic dynamics by employing the concept of the Macroscopic Fundamental Diagram (MFD). Taking into account the spatial variability of density, we are able to capture the hysteresis loops, which inevitably form in a network of this size. Model estimation results at both the vehicle and passenger flow level are documented. Environmental impacts including energy and emissions are also discussed. Findi...

In cities around the world, transit is currently provided with fixed route transportation only, w... more In cities around the world, transit is currently provided with fixed route transportation only, whence the inherent limited Quality of Service (QoS) for travelers in sub-urban areas and during off-peak. On the other hand, it has been shown that completely replacing fixed-route with demand-responsive transit fails to serve the high transportation demand during peak hours. Therefore, it is still unclear how we can maximize the potential of demand-responsive transit by satisfying the complicated demand pattern varying with time and space. In this paper we propose \emph{Flexible Transit}, a transit system design which gets the best from fixed-route and demand-responsive transit, depending on the demand observed in each sub-region of the urban conurbation and time-of-day. The goal is to provide high transportation capacity while guaranteeing high QoS, two objectives that are instead conflicting with classic fixed-schedule transportation. To this end, we first resort to microsimulation to...

Simulation Modelling Practice and Theory, 2021

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Future Transportation, 2021

Time-sensitive parcel deliveries—shipments requested for delivery in a day or less—are an increas... more Time-sensitive parcel deliveries—shipments requested for delivery in a day or less—are an increasingly important aspect of urban logistics. It is challenging to deal with these deliveries from a carrier perspective. These require additional planning constraints, preventing the efficient consolidation of deliveries that is possible when demand is well known in advance. Furthermore, such time-sensitive deliveries are requested to a wider spatial scope than retail centers, including homes and offices. Therefore, an increase in such deliveries is considered to exacerbate negative externalities, such as congestion and emissions. One of the solutions is to leverage spare capacity in passenger transport modes. This concept is often denominated as cargo hitching. While there are various system designs, it is crucial that such a solution does not deteriorate the quality of service of passenger trips. This research aims to evaluate the use of mobility-on-demand (MOD) services that perform sam...

Transportation Research Part A: Policy and Practice, 2020

 Users may download and print one copy of any publication from the public portal for the purpose... more  Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

IEEE Access, 2020

Recent advancements in automated vehicle technology and the concurrent emergence of ride-hailing ... more Recent advancements in automated vehicle technology and the concurrent emergence of ride-hailing services have focused increasing attention on Automated Mobility-on-Demand (AMOD; a system of shared driverless taxis) as a potential solution for sustainable future urban mobility. However, the impacts of an unrestricted deployment of AMOD are as yet uncertain and likely to be contextspecific; evidence with existing on-demand services suggests that they may lead to the cannibalization of mass-transit and increased traffic congestion. In this context, automated demand-responsive transit (also termed microtransit), which provides similar on-demand services (stop-to-stop or curbside) through higher capacity vehicles, may prove to be a promising substitute and/or complement. In this study, we evaluate the performance of such an automated demand response transit system (hereafter AMOD minibus) through agent-based simulations of the Singapore network. Towards this end, we extend SimMobility (an agentand activity-based microsimulation laboratory) with the capability of modeling an AMOD minibus service including demand, supply and their interactions. On the demand side, we use an activity-based model system that draws on data from a stated-preferences survey conducted in Singapore. On the supply side, an insertion heuristic is applied to dynamically perform both the assignment of requests to vehicles and vehicle routing. Scenario simulations on the Singapore network (with an area-wide deployment of the AMOD services) indicate the potential benefits of an automated demand responsive transit service for local circulation, which can result in a reduction of Vehicle Kilometres Traveled of up to 50% (compared to the AMOD shared taxis) whilst satisfying the same demand, with a modest increase in average travel times. INDEX TERMS Agent-based simulation, automated mobility-on-demand (AMOD), high-capacity ridesharing, shareability.

Transportation Research Record: Journal of the Transportation Research Board, 2019

This paper presents a stochastic approximation framework to solve a generalized problem of off-li... more This paper presents a stochastic approximation framework to solve a generalized problem of off-line calibration of demand for a multimodal microscopic (or mesoscopic) network simulation using aggregated sensor data. A key feature of this problem is that demand, although typically treated as a continuous variable is in fact discrete, particularly in the context of agent-based simulation. To address this, we first use a discrete version of the weighted simultaneous perturbation stochastic approximation (W-DSPSA) algorithm for minimizing a generalized least squares (GLS) objective (that measures the distance between simulated and observed measurements), defined over discrete sets. The algorithm computes the gradient at each iteration using a symmetric discrete perturbation of the calibration parameters and a multimodal weight matrix to improve the accuracy of the gradient estimate. The W-DSPSA algorithm is then applied to the large-scale calibration of multimodal origin–destination (OD...

Transportation Research Record: Journal of the Transportation Research Board, 2017

This paper presents the development of an integrated microscopic mobility simulator, SimMobility ... more This paper presents the development of an integrated microscopic mobility simulator, SimMobility Short-Term (ST). The simulator is integrated because its models, inputs and outputs, simulated components, and code base are integrated within a multiscale agent- and activity-based simulation platform capable of simulating different spatiotemporal resolutions and accounting for different levels of travelers’ decision making. The simulator is microscopic because both the demand (agents and its trips) and the supply (trip realization and movements on the network) are microscopic (i.e., modeled individually). Finally, the simulator has mobility because it copes with the multimodal nature of urban networks and the need for the flexible simulation of innovative transportation services, such as on-demand and smart mobility solutions. This paper follows previous publications that describe SimMobility’s overall framework and models. SimMobility is an open-source, multiscale platform that consid...

Transportation Research Part B: Methodological, 2015

In an effort to uncover traffic conditions that trigger discharge rate reductions near active bot... more In an effort to uncover traffic conditions that trigger discharge rate reductions near active bottlenecks, this paper analyzed individual vehicle trajectories at a microscopic level and documented the findings. Based on an investigation of traffic flow involving diverse traffic situations, a driver's tendency to take a significant headway after passing stop-and-go waves was identified as one of the influencing factors for discharge rate reduction. Conversely, the pattern of lane changers caused a transient increase in the discharge rate until the situation was relaxed after completing the lane-changing event. Although we observed a high flow from the incoming lane changers, the events ultimately caused adverse impacts on the traffic such that the disturbances generated stop-and-go waves. Based on this observation, we regard upstream lane changes and stop-and-go waves as the responsible factors for the decreased capacity at downstream of active bottlenecks. This empirical investigation also supports the resignation effect, the regressive effect, and the asymmetric behavioral models in differentiating acceleration and deceleration behaviors.