All posts by yni957

Recent research

Fall and rise of taxi travel during COVID

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This is our second COVID19 related study, completed in 2020 and published in Transportation Research Part A in 2021. You may read  the other one here, which is about optimally adapting transit design and operations in a pandemic.

We examined taxi trajectory data collected in four weeks that cover the onset of COVID19, the shutdown, and phased reopening in the city of Shenzhen. Our analysis revealed how the pandemic and the travel restriction policies affected both the supply and the demand of the taxi market in the city.   One of the more interesting findings is that the city’s stimulus policy, designed to boost taxi supply and help taxi drivers, might have led to oversupply, by inducing taxi drivers to spend more time on the road than what the prevailing market condition would justify.  We uncovered direct evidence from data to support this finding through a clustering analysis.

A preprint is available here.

Abstract: This paper traces the plunge and rebound of the taxi market in Shenzhen, China through the COVID-19 lockdown. A four-week taxi GPS trajectory data set is collected in the first quarter of 2020, which covers the period of lockdown and phased reopening in the city. We conduct a spatiotemporal analysis of taxi demand using the data, and then select taxis that continued to operate through the analysis period to examine whether and how they adjusted operational strategies. We find, among other things: (i) the taxi demand in Shenzhen shrank more than 85% in the lockdown phase and barely recovered from that bottom even after the city began to reopen; (ii) the recovery of taxi travel fell far behind that of the overall vehicle travel in the city; (iii) most taxis significantly cut back work hours in response to the lockdown, and many adjusted work schedule to focus on serving peak-time demand after it was lifted; (iv) taxi drivers demonstrate distinct behavioral adaptations to the pandemic that can be identified by a clustering analysis; and (v) while the level of taxi service dropped precipitately at the beginning, it quickly rebounded to exceed the pre-pandemic level, thanks to the government’s incentive policy. These empirical findings suggest (i) incentives aiming at boosting supply should more precisely target where the boost is most needed; (ii) the taxi market conditions should be closely monitored to support and adjust policies; and (iii) when the demand is severely depressed by lockdown orders or when the market is oversupplied, taxi drivers should be encouraged and aided to use more centralized dispatching modes.

Previous research

How can the taxi industry survive the tide of ridesourcing?

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This  paper makes two empirical findings and one prediction. First, it reveals the intensity and scope of the impact of ridesourcing on the conventional taxi industry. Second, it uncovers evidence that taxis may be competitive in densely populated areas.  The second finding leads to a follow-up study you can read here.

I predict that the taxi industry is here to stay in the foreseeable future.    Here is what I wrote in the conclusion:

“Beyond e-hailing, economy of scale and aggressive pricing, ridesourcing does not seem to have other means at present to drive its expansion in the market. E-hailing is no longer the secret weapon that once glorifies the cause of TNCs – it can be easily picked up by a taxi dispatcher that owns and operates its own fleet. Aggressive pricing, on the other hand, has proven at best a double-edged sword, as Uber’s recent bitter defeat in China has vividly demonstrated. The scale of TNCs, which gives outside visitors a brand to stick to, is indeed an important competitive advantage. Even this lead is not that difficult to catch up, however, if a mobile platform, presumably operated by a third party, can unify taxi dispatchers around the world. Such a platform can easily work within cities’ existing regulatory structure, rather than against it, because it utilizes a dedicated and existing fleet. It can also improve the experience of street-hailing, a decisive advantage it holds against ridesourcing, by offering customers the amenities considered only available to e-hailing users, such as paying the fare on-line and rating drivers, all in real-time. An obvious solution may be allowing customers, as they board the taxi hailed off street, to open up an electronic transaction session similar to those seen on e-haling platforms, by e.g. scanning a QR code attached to the taxis or the driver’s smart phone.”… therefore, “The revolution of ridesourcing is unlikely to eliminate the necessity of a dedicated service fleet, and for years to come we will continue to live in a world with both ridesourcing and (upgraded) taxis.”.

The Journal of Transportation Research Part C selected this paper to receive the Best Paper Award in 2018. You may download a preprint here.

Abstract:  This paper aims to examine the impact of ridesourcing on the taxi industry and explore where, when and how taxis can compete more effectively. To this end a large taxi GPS trajectory data set collected in Shenzhen, China is mined and more than 2,700 taxis (or about 18% of all registered in the city) are tracked in a period of three years, from January 2013 to November 2015, when both e-hailing and ridesourcing were rapidly spreading in the city. The long sequence of GPS data points is first broken into separate “trips”, each corresponding to a unique passenger state, an origin/destination zone, and a starting/ending time. By examining the trip statistics, we found that: (1) the taxi industry in Shenzhen has experienced a significant loss in its ridership that can be indisputably credited to the competition from ridesourcing. Yet, the evidence is also strong that the shock was relatively short and that the loss of the taxi industry had begun to stabilize since the second half of 2015; (2) taxis are found to compete more effectively with ridesourcing in peak period (6-10 AM, 5-8 PM) and in areas with high population density. (3) e-hailing helps lift the capacity utilization rate of taxis. Yet, the gains are generally modest except for the off-peak period, and excessive competition can lead to severely under-utilized capacities; and (4) ridesourcing worsens congestion for taxis in the city, but the impact was relatively mild. We conclude that a dedicated service fleet with exclusive street-hailing access will continue to co-exist with ridesourcing and that regulations are needed to ensure this market operate properly.

Book review, English writing

Winners take all

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Winners take all is about the dream of “doing well by doing good”, the idea that there is always a win-win solution to every social problem, and the belief that elites equipped with technology and market tools should be entrusted to lead social changes, preferably independent of the democratic processes, or politics. The author argues these ideas are largely an illusion, if not a deception, because the overlap between individual and collective interest is limited.  There is nothing wrong about wanting to do well by doing good; but we are kidding ourselves if we believe these do-gooders are saviors of our collective future.  The book is very harsh on the intellectuals who promote these ideas.  These so-called “thought leaders” are described as the cheerleaders employed by the “idea industry” to project positive energy, presumably at the expense of our collective good.

Overall, a good and easy read, and the main argument is fair and well-reasoned, if not entirely neutral.

Recent research

To pool or not to pool

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To Pool or Not to Pool: Equilibrium,  Pricing and Regulation

This paper was the first published based on  Kenan’s PhD research. It introduces ride-pooling into the equilibrium analysis of the ride-hail market and analyzes the effect of pricing strategies and various regulations on pooling.

After the first draft is completed in the Spring of 2019,  it took almost two years  to move the paper through various stages of the review process, first at Management Science, then at Transportation Research Part B (three rounds). While the long waiting was no doubt frustrating, the quality of the paper might have benefited from intensive scrutiny and repeated revisions.  For a preprint, please check here; the link to the final version is here.

Abstract: We study a monopoly transportation network company (TNC) in an aggregate market that offers on-demand solo and pooling e-hail services, while competing with transit for passengers. The market equilibrium is established based on a spatial driver-passenger matching model that characterizes the passenger wait time for both solo and pooling rides. We prove, under mild conditions, this system always has an equilibrium solution. Built on the market equilibrium, three variants of pricing problems are analyzed and compared, namely, (i) profit maximization, (ii) profit maximization subject to regulatory constraints, and (iii) social welfare maximization subject to a revenue-neutral constraint. A comprehensive case study is constructed using TNC data collected in the city of Chicago. We found pooling is desirable when demand is high, but supply is scarce. However, its benefit diminishes quickly as the average en-route detour time (i.e., the difference between the average duration of solo and pooling trips) increases. Without regulations, a mixed strategy—providing both solo and pooling rides—not only achieves the highest profit and trip production in most scenarios, but also gains higher social welfare. The minimum wage policy can improve social welfare in the short term. However, in the long run, the TNC could react by limiting the size of the driver pool, and consequently, render the policy counterproductive, even pushing social welfare below the unregulated level. Moreover, by maintaining the supply and demand of ride-hail at an artificially high level, the minimum wage policy tends to exacerbate traffic congestion by depressing the use of collective modes (transit and pooling). A congestion tax policy that penalizes solo rides promotes pooling, but consistently harms social welfare. However, it promises to increase both social welfare and pooling ratio, when jointly implemented with the minimum wage policy.

Recent research

Travel demand management practice in China

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This article stemmed from a research report commissioned by World Bank back in 2019.   I’ve never tried to publish it in a journal, though my friend Daizong Liu had helped translate it into Chinese and published it online through his very successful Wechat public platform (一览众山小).  The article reviews the travel demand management practice in China and attempts to draw some useful lessons from it.  You may read the abstract below and download the Egnlish version at ChinaTDM.

Lessons Learned from China’s Travel demand management practice

China’s car ownership has been expanding at a staggering pace in the past two decades. The rapid motorization brought unprecedented level of traffic to its densely populated cities
unprepared to accommodate it, causing severe congestion and air pollution problems. Chinese cities have responded to these challenges with sweeping travel demand management (TDM) measures. The practice of TDM in China is unique not only because it is large in scale and broad in scope, but also because it occurs against the backdrop of a fast and historical transition of the most populous country on earth. The objective of this note is to review and document this practice, discuss its outcomes and lessons, and examine what the rest of the world could learn from it.

Book review, English writing

Antifragile

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This short review was originally written in April of 2021.

Overall,  Antifragile is a disappointment.  This is not to say it offers no interesting and useful ideas. It does.  What strikes me the most are the oversized impact of tail events (black swans) and their utter unpredictability, our ruinous obsession with optimization and intervention, and the agent problem ubiquitous in modern societies.  However, Taleb could have explained these ideas in 40 pages. Instead, he wrote 400, filling many of them with impulsive bragging, as well as his signature rant against the entire intellectual establishment.  In the end, I felt these self-inflicted distractions severely undermine the narrative and the logic flow.    I was looking forward to reading Black Swan, but after this experience, I wonder whether it would be worth my time.

A special note for my fellow academics who might be interested in Taleb’s work: he absolutely hates professors and minces no words berating them, so if you take the challenge, buckle up for a bumpy ride.

Recent research

Hyperbush

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Hyperbush Algorithm for Strategy-based Equilibrium Traffic Assignment Problems

This recent paper extends the concept of bush to hyperbush and uses it devise a new class of algorithms for strategy-based traffic equilibrium assignment problem, of which frequency-based transit assignment is an archetype.  One of the longest papers that I’ve ever written (nearly 50 pages), the paper just appeared online in Transportation Science.    Here is a preprint  Hyperbush.

Strategy-based equilibrium traffic assignment (SETA) problems define travel choice broadly as a strategy rather than a simple path. Travelers navigating through a network based on a strategy end up following a hyperpath. SETA is well suited to represent a rich set of travel choices that take place en-route at nodes, such as transit passengers’ transfer decision, truckers’ bidding decision and taxi drivers’ re-position decision. This paper recognizes and highlights the commonalities among classical and emerging SETA problems and proposes to unify them within a same modeling framework, built on the concept of hypergragh. A generic hyperbush algorithm (HBA) is developed by decomposing a hypergraph into destination-based hyperbushes. By constructing hyperbushes and limiting traffic assignment to them, HBA promises to obtain more precise solutions to larger instances of SETA problems at a lower computational cost, both in terms of CPU time and memory consumption. To demonstrate its generality and efficiency, we tailor HBA to solve two SETA problems. The results confirm HBA consistently outperforms the benchmark algorithms in the literature, including two state-of-the-art {hyperpath-based} algorithms. To obtain high-quality equilibrium solutions for SETA instances of practical size, HBA runs up to five times faster than the best competitor with a fraction of its memory consumption.

Recent research

Transit Design in Response to a Global Pandemic

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Optimizing Operational Strategies for Mass Transit Systems in Response to a Global Pandemic

This is one of my COVID inspired research projects that was started in 2020.  The idea is that, in order to operate safety during a pandemic, transit agencies might have to adjust their operational strategies, in terms of service frequency and capacity.  The underlying tradeoff we are trying to explore here is that between the benefit of frequently testing drivers (as it reduces the transmission risk) and the cost of lowering the number of passengers allowed in buses, subject to the need to maintain certain safety standard, measured by infection risks. A novelty of the work is a physical model aiming to estimate infection risks based on vehicle size/type, service capacity and a few external risk factors.

The paper is currently under revision at Journal of Transportation Research Part A.   Please download a preprint here.

Abstract              This study analyzes the risk involved in riding various transit modes during and after a global pandemic. The goal is to identify which factors are related to this risk, how such a relationship can be represented in a manner amenable to analysis, and what a transit operator can do to mitigate the risk while running its service as efficiently as possible. The resulted infection risk model is sensitive to such factors as prevalence of infection, baseline transmission probability, social distance, and expected number of human contacts. Built on this model, we formulate, analyze and test three versions of a transit operator’s design problem. In the first, the operator seeks to jointly optimize vehicle capacity and staff testing frequency while keeping the original service schedule and satisfying the infection risk requirement. The second model assumes the operator is obligated to meet the returning demand after the peak of the pandemic. The third allows the operator to run more than one transit line and to allocate limited resources between the lines, subject to the penalty of unserved passengers. We find: (i) The optimal profit, as well as the testing frequency and the vehicle capacity, decreases when passengers expect to come in close contact with more fellow riders in a trip; (ii) Using a larger bus and/or reducing the testing cost enables the operator to both test drivers more frequently and allow more passengers in each bus; (iii) If passengers weigh the risk of riding bus relative to taxi, a higher prevalence of infection has a negative effect on transit operation, whereas a higher basic transmission probability has a positive effect; (iv) The benefit of improving service capacity and/or testing more frequently is limited given the safety requirement imposed. When the demand rises beyond the range of the capacity needed to maintain sufficient social distancing, the operator has no choice but to increase the service frequency; and (v) In the multi-line case, the lines that have a larger pre-pandemic demand, a higher penalty for each unserved passenger, or a greater exposure risk should be prioritized.

Book review, English writing

Post War

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One of the good things that came out of COVID19 pandemic is I suddenly discovered (or rediscovered?) a new hobby: reading.  Post War is among the first books I read after the pandemic starts. This short review was written in December 2020.

Tony Judt’s Post War is a great read for anyone who is curious about Europe since WWII.  You may be disappointed if you expect a completely objective narrative based on data and stories. Don’t get me wrong—Judt is a good storyteller and he tells a wide range of stories, in fact, so broad he even commented on David Beckham, describing him as “an English player of moderate technical gifts but an unsurpassed talent for self-promotion”….He does, however, insistently make you feel his presence, preference, and emotions in these stories. I love his style, but I realize some may prefer historians without strong opinions.

Judt never hides his love for the “European Social Model”, which recognizes the state has the duty to “shield citizens from the hazards of misfortune or the market”, and “social responsibility and economic advantage should not be mutually exclusive”.  At the end of the book, he passionately compares Europe with America and China, writing, “America would have the biggest army and China would make more, and cheaper, goods. But neither America nor China had a serviceable model to propose for universal emulation. In spite of the horrors of their recent past—and in large measure because of them—it was Europeans who were now uniquely placed to offer the world some modest advice on how to avoid repeating their own mistakes. Few would have predicted it sixty years before, but the twenty-first century might yet belong to Europe.”

Recent research

An Efficiency Paradox of Uberization

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The main finding is that e-hail (e.g., Uber and Lyft) may not scale as well as street-hailing taxi.  In other words, a thicker market may help improve the performance of taxi more than that of e-hail. We develop a physical model to describe the matching process of both modes. Using the model, we then derive the production function and measure the returns to scale in the matching process.  It indicates that taxi has returns to scale of  2 (implying increasing returns to scale) whereas e-hail has returns to scale of 1 (constant returns to scale).   Empirical data collected in Shenzhen, China largely confirm the theory.

While this paper has yet to be published, I must have spent more time writing and revising it than any of my other papers.  Hongyu convinced me to write it  for a general journal, which, with the benefit of hindsight, might have caused us to oversell an otherwise fine idea.   That being said, I did learn a lot from the process, and I still think it is one of my better works.   The preprint is here, and the abstract follows.

Uberization promises to transform society based on an intuitive proposition: Advanced peer-to-peer matching guarantees greater overall efficiency. Here we show a paradox challenging this proposition in uberized ride-hail service, known as e-hail. By analyzing hundreds of local markets in Shenzhen, China, we discover e-hail is outperformed—in terms of wait time and trip production—by taxis hailed off street in areas with high densities of passengers and drivers. This paradox arises because a quicker match does not always expedite and enhance a service. On the contrary, it can induce competition that undermines the network effect, making a passenger less likely to benefit from more drivers, and vice versa, in e-hail than in taxi service. Consequently, simply attracting more users may not improve e-hail’s efficiency (defined as trip production at a given density of passengers and drivers), because its competitive edge diminishes with scale. The finding implies uberization has a limited impact on efficiency and is unlikely to create a “winner-take-all” in transportation