network evolution

Dr. Bahman Lahoorpoor

| David Levinson
Bahman Lahoorpoor

Congratulations to Dr. Bahman Lahoorpoor for “satisfying the requirements for the award of the degree of Doctor of Philosophy at the University of Sydney.”

Thesis Title:  “Terraces, Towers, Trams, and Trains : Examining the Growth of Sydney using Empirical Models and Agent-based Simulation“

Lead Supervisor: Professor David Levinson.

Abstract: Transport networks and land use are inter-dependent. This joint co-development process of infrastructure and building location is often theorised to be a positive feedback cycle: transport infrastructure produces accessibility that induces land development, which induces transport demand and increases accessibility, increasing the production of transport networks (i.e. inducing supply) and further intensifying land development. In Chapter 2 we investigate the critical elements of this dual connection between land use and transit. Sydney as a good example of a rapidly developing city, had public railway transport services beginning in the 1850s, which facilitated and responded to the development of suburbs. Chapter 3 explains how the historical railway network, including trams and trains, and historical population data are collected and digitised. A series of network characteristic measures and a metric for characterising the population distribution are also presented. In Chapter 4 we test whether trams expanded accessibility relative to buses by comparing the services provided by historical trams, the replaced bus services, and the remaining train and light rail networks. We compare 1925, when the tram system was at its peak, and 2020. In Chapter 5 we investigate the theory of interaction between land use and the transit network. We investigate the direct and indirect links between land development and transit investments using the concept of accessibility. We develop an empirical model to capture the Greater Sydney area’s historical evolution of land use and public transit networks. In Chapter 6 we develop a simulation framework to replicate the growth of railway networks by given exogenous historical evolution in land use. The framework is an iterative process that includes five consecutive components including environment loading, measuring access, locating stations, connecting stations, and evaluating connections.

Publications by Bahman Lahoorpoor include:

  • Lahoorpoor, B. and Levinson, D. (2022) In Search of Lost Trams: Comparing 1925 and 2020 Transit Isochrones in Sydney. Findings, March. [doi]
  • Lahoorpoor, B., Rayaprolu, H., Wu, H., and Levinson, D. (2022) Access-oriented design? Disentangling the effect of land use and transport network on accessibility. Transportation Research Interdisciplinary Perspectives. [doi]
  • Lahoorpoor, B., Rayaprolu, H., Wu, H., and Levinson, D. (2022) Prioritizing active transport network investment using locational accessibility. TeMA – Journal of Land Use, Mobility and Environment (in press)
  • Rayaprolu, H., Wu, H., Lahoorpoor, B., and Levinson, D. (2022) Maximizing Access in Transit Network Design. Journal of Public Transportation. 24. [doi]
  • Wang, Yingshuo, Lahoorpoor, B. and Levinson, D. (2022) The Spatio-temporal Evolution of Sydney’s Tram Network Using Network Econometrics. Geographical Analysis. [doi]
  • Lahoorpoor, Bahman and Levinson, D. (2020) Catchment if you can: The effect of station entrance and exit locations on accessibility. Journal of Transport Geography. 82, 102556. [doi] [full report]

Creating a Historical Database for Roads in Greater Sydney using Map Digitisation

| David Levinson

Recently published:

Abstract

The aim of this thesis is to create a historical database for roads in Greater Sydney using map digitisation. The database was constructed to label the status of road in Sydney at a certain time by nominating their opening date, and if applicable, their closing date. The project was completed in the QGIS working environment, an open-sourced program that allows for map digitisation to be completed. A method was developed for map digitisation that can be used to extract spatial data from historical maps and place them in a collective vector layer. The method considers extensive georeferencing of the maps, as well as editing and cleaning the maps through raster and vector analysis. Preferred methods for map digitisation used in the project were identified. For a considerable area of Sydney, in which approximately 52000 road links were included, almost half of the links were identified with an open date by the start of the twentieth century. A further half of these links were confined to opening within a thirty-year period. The progressive change in open links, the length of the network, the area of the network surveyed and the number of intersections open was also reviewed with time. The project has established a strong foundation for a historical road database for Sydney. It has also outlined methods and procedures that can be followed to progress the database further.

Growth of the Sydney Street network. Source: Turner, Hamish (2021)

Network Econometrics and the Evolution of Transport Systems

| David Levinson

Recently Published

Abstract

This thesis systematically develops a network correlation matrix that explicitly distinguishes competitive and complementary link pairs in transportation networks. Embedding the matrix in network econometric analysis, this thesis consolidates that incorporating representative spatial information with a network perspective is capable of improving the performance of traffic forecasting models. The method is validated in the context of a real-world transport system rather than within simulated settings adopted by previous research. An Autoregressive-Distributed Lag (ARDL) model is specified, and reveals that the combination of correlation strength and magnitude of lagged flow change on correlated links is an significant predictor of future traffic flow. This thesis innovatively extends network econometric methods, previously exclusively used for traffic flow forecasting, to the domain of network structure prediction by specifying a logit model. It finds that complementary and competitive links play distinct roles in shaping the network structure. If positively correlated, a link is more likely to undergo the same structural change influential links underwent previously where the influence is measured by a combination of correlation strength and link importance, reflected by historical flow level. Additionally, this thesis establishes a digitized database of the Sydney tramway system, providing a complete set of data for more research.

Trams running through Railway Square, 1920s

On Hypo- and Hyper-connectivity in Transport

| David Levinson

Connectivity is good. Is more connectivity better?

During the early stages of a useful technology like roads or transit, adding links generally adds more benefits than costs. However there are limits. A four way intersection is good does not mean a five way intersection (or six or seven) is necessarily better. The more complex intersection adds to the friction of travel and cost of construction over its simpler alternatives.

Muller's Hexagonal Network
Muller’s Hexagonal Network

A grid network, with streets at 90-degree angles to each other might not be as good as a network with streets at 60-degree angles, which reduces travel costs and increases directness (reduces circuity), but it is most assuredly better than a fine mesh with streets at 10-degrees or 1-degree, where almost all is pavement and little is actually buildable land. While 1-degree network would reduce surface travel distance, it does so at many other costs, including a reduction in accessibility because of fewer development opportunities.

Consider the circuity additions based on network angle. If all places are connected via a 90-degree square grid, the circuity at worst is SQRT(2), but on average 1.21.  So travel distance increases by 21% over a straight-line path. With a 60-degree grid, the circuity is lower, at worst 1.22, on average nearer 1.11. (Bus transit networks, which tend not to follow the shortest path, have much worse circuity.)

The optimal level of connectivity depends on what you are trying to optimize.

Hypo and Hyper are antonyms. Wiktionary says:

I would maintain that most developed countries are pretty close to optimal in terms of road connectivity, that there are few missing links whose costs outweigh their benefits. If subsidies for modes were to be eliminated, some large cities might be under-developed in terms of transit connectivity because of a bias towards coverage (and circuity) aims rather than frequency.

Let’s think of this in the context of induced demand. More connectivity in one sense means a faster network, which users exploit by traveling longer distances in the same amount of time. They gain utility by being in a house they prefer. However they use up the capacity gains of the network. But more connectivity increases the friction of connections (junctions, interchanges, transfers) which slows down the network. Induced demand due to connectivity is thus self-limiting.

Braess Paradox is the most famous supply side example of hyper-connectivity. In this situation, removing a link improves travel for road users at large because the additional network link induces travelers to use a link with a lower average cost but higher social marginal cost.

A key point is that whether a network is over or under-connected depends on the technology of travel, as well as the amount. A network which is overconnected for cars may be underconnected for pedestrians who don’t congest so easily. A network which is overconnected for 2000 cars may be underconnected for 1000. This is the challenge in building cities. Networks last for seemingly forever, but technologies that use them change more frequently. How can you design a permanent infrastructure flexible enough to serve future technology?

 

Accessibility and the choice of network investments in the London Underground

| David Levinson

Recent working paper:

Levinson, D., Giacomin, D., and Badsey-Ellis, A. (2014) Accessibility and the choice of network investments in the London Underground. Presented at the World Symposium on Transport and Land Use Research, June 2014, at Delft.

Accessibility in London in 1881
Accessibility in London in 1881

 

 

  • Abstract: In 1863, the Metropolitan Railway of what came to be known as the London Underground successfully opened as the world’s first subway. Its high ridership spawned interest in additional links. Entrepreneurs secured funding and then proposed new lines to Parliament for approval, though only a portion were actually approved. While putative rail barons may have conducted some economic analysis, the final decision lay with Parliament, which did not have available modern transportation economic or geographic analysis tools. How good were the decisions that Parliament made in approving Underground Lines? This paper explores the role accessibility played on the decision to approve or reject proposed early London Tube Schemes. It finds that maximizing accessibility to population (highly correlated with revenue and ridership) largely explains Parliamentary approvals and rejections.
    Keywords: Accessibility, Network Growth, Subways, Public Transport, Travel Behavior, Networks

Path dependence

| David Levinson

If you don’t know where you’re going, any path will get you there.

Path dependence is the idea that where we are today depends critically on where we were yesterday. Some systems are path independent, those that have a single unique equilibrium. Finding the solutions to some math problems is independent of where you start, as long as you follow a particular algorithm.

However, most systems we deal with on a daily basis have some characteristics of path dependence. Where you live might depend on what job you took, which depends on what your previous job was and where you went to school, and a different decision anywhere along the way would change today’s position.

Nowhere is this more true than transportation. On the one hand, it is obvious that certain locations were destined to be important cities because of significant natural advantages. New York has a deep harbor at the confluence of major navigable river. Chicago is at the pivot point between vast agricultural lands to the Northwest and the shortest land path to the East Coast. It was natural railroads would flow through the point on the map we now call Chicago.

On the other hand, many city sites that were selected for natural advantages in one technological era (The Romans selected London and the Dutch and English chose New York in large part for their capability as ports), remain important even after that technology becomes obsolete. With the logistics revolution and the new dominance of container shipping, London’s shipping has moved northeast to Felixstowe as large container ships cannot easily ply the Thames, while New York’s shipping has migrated to the wide open spaces of New Jersey.

The one-time advantages result in a set of complementary investments and inter-related decisions that take on a life of their own. Because of local trading advantages, commodities markets, banks, insurers, and other related organizations located nearby. A critical mass of those institutions felt no need to migrate just because their initial raison d’être vanished. While a building is under construction, temporary framing will often be used until the more permanent structure is erected. Once the final building can stand on its own, the falsework is dismantled. In a sense, everything is falsework for what comes after.

This kind of mutual complementarity happens repeatedly in transportation. Airplanes are the perfect example of mobile capital. If Amalgamated Airlines no longer wants to serve a particular city pair, the airplane can easily be redeployed elsewhere. Yet 80 years into the commercial aviation industry, airlines today serve mostly the same hubs their predecessors did on the Airmail routes of the 1930s. American Airlines is still in Dallas, United in Chicago, Delta (Northwest) in Minneapolis, and so on.

While very few decisions are completely irreversible, transportation decisions come close. Where we place a right-of-way, or an airport will explain where that facility will be decades, or even centuries from now.

A slight deviation from the efficient path to solve a short term problem today will cost travelers time for years to come. It is important to get the design right for the long term. (Undoubtedly this has social costs, see e.g. I-94 through the Rondo in Saint Paul).

But a slight deviation from the path will also change what the long term is. Build a bridge “here” rather than “there”, and then you will adjust all of the roads feeding into the bridge to meet it “here” (instead of “there”). And then land will be developed along the road to “here” to take advantage of the newly created accessibility, properties will be platted, buildings will be built, travel and trade patterns established, and other critical dependencies will come to assume that the bridge is “here”. At some point, say 50 years in the future, the bridge will need to be replaced. Even if “there” was a better location than “here” initially, after five decades of adaptation, it is quite likely that “here” is better now. The whole may have been better were a different initial decision been made, given conditions at the time. Given current realities, that path must now be foregone.

In transportation we say build it right the first time, because there won’t be a second chance. And that is true. But also remember the world will adapt to whatever we do, and we cannot let the perfect be the enemy of the good.

Special Issue on the Evolution of Transportation Network Infrastructure in Networks and Spatial Economics: Volume 9, Issue 3 (2009)

| David Levinson

A special issue of Networks and Spatial Economics on the Evolution of Transportation Network Infrastructure, for which I was the editor, is now out. Many thanks to my co-authors and the journal for making this happen. (I cannot however see the final version, as it is behind a pay-wall and my university does not yet subscribe to the journal. I have read all of the articles though, and it is well worth reading if you do have access).
Introduction to the Special Issue on the Evolution of Transportation Network Infrastructure
David Levinson
289-290
Modeling the Growth of Transportation Networks: A Comprehensive Review
Feng Xie and David Levinson
291-307
Inter-Modal Network Externalities and Transport Development: Evidence from Roads, Canals, and Ports During the English Industrial Revolution
Dan Bogart
309-338
The Efficiency of the Victorian British Railway Network: A Counterfactual Analysis
Mark Casson
339-378
Graph-Theoretical Analysis of the Swiss Road and Railway Networks Over Time
Alexander Erath, Michael Löchl and Kay W. Axhausen
379-400
Co-evolution of Density and Topology in a Simple Model of City Formation
Marc Barthélemy and Alessandro Flammini
401-425
The Topology of Transportation Networks: A Comparison Between Different Economies
Efrat Blumenfeld-Lieberthal
427-458
Jurisdictional Control and Network Growth
Feng Xie and David Levinson
459-483