La ville est insensible aux formes idéales

Conference given outside the academic context (2015)

L’urbanisme, c’est bien plus qu’une simple affaire de dessin en plan. Un dessin en plan, si régulier soit-il sur le plan géométrique/géométral, ne donnera en fait jamais dans la réalité deux villes ... [more ▼]

L’urbanisme, c’est bien plus qu’une simple affaire de dessin en plan. Un dessin en plan, si régulier soit-il sur le plan géométrique/géométral, ne donnera en fait jamais dans la réalité deux villes identiques quant à leur fonctionnement. La vie d’une ville se passe dans le temps, ses occupants et ses activités produisent des strates qui se cumulent et produisent un espace hétérogène. Cette « dépendance de chemin » contraint fortement les activités et les flux, au point de les rendre insensibles à des impositions, même fortes, mais elle crée aussi des opportunités de reconversion, des quartiers attractifs, uniques voire sympathiques. Les villes wallonnes n’y échappent pas, jouons-en ! [less ▲]

Monocentric urban simulation models: getting closer to fractal properties and landscape representation

Scientific Conference (2015, November)

Urban growth generates spatial patterns that in many cases demonstrate fractal properties. Geocomputational models, particularly cellular automata and spatial agent-based simulation models have been used ... [more ▼]

Urban growth generates spatial patterns that in many cases demonstrate fractal properties. Geocomputational models, particularly cellular automata and spatial agent-based simulation models have been used over the last 20 years to generate urbanisation patterns with the aim to replicate at best observed expansion footprints, including matching observed and simulated fractal dimensions. In applied cases, with the addition of constraints at multiple scales (land constraints, threshold per zones, etc.) simulation models seem to perform rather well and obtain sound urban fractal dimensions. Models that are more parsimonious in parameters however do not seem to perform as well. Exceptions are those models directly inspired from physics such as DLA (Diffusion Limited Aggregation) or DBM (Dielectric Breakdown Models) but these are frustrating when it comes to behavioural or economic interpretation. Models with explicit micro-economic component in a monocentric setting also seem to lag behind in terms of fractal performance: unless exogenous spatial heterogeneity is provided, the spatial outcome of these models is too homogenous to resemble real cities, despite agglomeration and dispersion processes at neighbourhood scale and despite the self-emergence of road networks and subsequent open land lock-ins. Rather than resolving to exogenous polycentric setting or exogenous stochasticity that would provide better looking outcomes, we investigate this insatisfaction by exploring the results of an augmented micro-economic simulation model on a theoretical monocentric space. The innovations are brought along three rationales: Firstly, an assumption is made that the length of the infrastructure network should feed back into households budget. Cities cannot expand too quickly not only because of unitary commuting costs but also infrastructure costs. We therefore implement an infrastructure tax that should lead to agglomeration or a more efficient generation of roads from the city perspective. Secondly, we assume that the infill of undeveloped spaces by new residents is limited by residents who settled earlier in the city and refuse important utility losses in terms of open green space. This leads to relaxing the assumption of a dynamic adaptation of rents and building stock trough time. Free entry and placement is somehow limited by a public authority that keeps utility at its higher possible state at each time step. Thirdly, we abandon the assumption that neighbourhood quality is related to the density of available activities or the density of green space within a given neighbourhood, but replace this with the access to a diversity of urban and green opportunities depending on their use frequency (daily walk, playground, hiking in forest,…). This is a very important change in geocomputational terms since simple focal functions can no longer be used to represent externalities in simulation models. The gradual construction of roads and houses change gradually the nature of the landscape and the value taken out of it, typically by dividing green patches into parts, creating detours to access bigger parks, shadowing effects, etc. In addition, this requires that landscape objects are represented as vectors, not cells, which is a second important change in geocomputational terms. To some extent this brings urban simulation models closer to landscape ecology and graph-based approaches. In this paper we explore and contrast the effects of the three mechanisms mentioned above on the resulting urban morphology. [less ▲]

A perspective on urban economics and geographical agent-based models. (Keynote Speech)

Scientific Conference (2015, September)

In this presentation, we will argue that there are many benefits to further liaise urban economic models and geographically -aware agent-based simulation models for further understanding the structure and ... [more ▼]

In this presentation, we will argue that there are many benefits to further liaise urban economic models and geographically -aware agent-based simulation models for further understanding the structure and dynamics of cities, especially at the scale of city regions but also, eventually, for contributing micro understanding of changes in a system of cities. At first sight it appears that these two fields of urban research are very separated: urban economics relies mostly on mathematical proofs, a parsimonious set of interactions, static equilibria and a rather homogeneous space and set of agents; while geographical agent-based models emphasize the dynamics and complexity of cities, the heterogeneity of agents and space, and rely solely on computational methods. The presentation will showcase a series of theoretical and applied research to stress the idea that the two can fruitfully be combined and that results are then palatable to both urban economists and geographers and can therefore participate to consolidating a wider and formalised urban theory. The geographical turn towards agent-based approaches, compared to more aggregate models and behaviourally-blind geocomputational methods, is actually a chance for geographers to formalise market as well as non-market interactions in a manner that is consistent with urban economic theory and enhance the treatment of space in this theory. Relying on a formal description of agents utility permits to discuss normative instruments where environmental and welfare impacts can be traded-off by policy makers. Furthermore,, land or housing market outcomes of the models and the analytical efforts actually help to reduce parametric space and calibrate or validate models against independent data. [less ▲]

Dielectric breakdown and urban growth: Morphological similarities or conceptual links?

Scientific Conference (2015, September)

Urban dynamics generate sprawl patterns that reveals in many cases fractal-like properties. This is the reason why a couple of research projects focused on how to simulate the growth of such patterns ... [more ▼]

Urban dynamics generate sprawl patterns that reveals in many cases fractal-like properties. This is the reason why a couple of research projects focused on how to simulate the growth of such patterns while respecting their fractal features. It is well-known that cellular automata can be used for generating random fractal structures. This holds e.g. for certain models developed in physics like DLA (Diffusion Limited Aggregation) or DBM (Dielectric Breakdown Models) and thus such models inspired urban growth simulation models. However in physics these models are based on a transcription of the underlying physical laws, combining electrodynamics and thermodynamics. Hence DLA and DBM are not just morpho-descriptive, but refer to explanatory approaches. However it seems difficult to establish a direct causal link between these approaches and urban growth. The goal of several recent models was to introduce a more explanatory approach for simulating the emergence of urban patterns by means of cellular automata. Following this objective, micro-economic reasoning has been used for describing households’ residential choice behaviour and constituting the driving force of cellular automata. These models assume that households settle down subsequently in the vicinity of a preexisting CBD where jobs are localized. Households have preferences for social and green amenities in their neighbourhood. Even if these approaches can explain some properties of urban growth processes, like leapfrogging, the patterns generated do not really show fractal properties, even if some morphological analogies with DBM have been identified. Moreover in the enunciated models, the dynamics is driven only by the evaluation of households who to want to settle down in the already existing city region. The City Administration is supposed to construct new road segments without consequences for the budget of the households. This is questionable. Here we introduce a model that starts from a pre-existing cross-like street network with a CBD at the crossing point where all kinds of services, shopping amenities and jobs are concentrated. However we introduce a series of new mechanisms. First, new arriving households use the utility of already located households as a reference. Second a tax per household is introduced for maintaining the existing street network and the infrastructures of the pre-existing centre. New households can improve the utility of urban population by contributing to these fixed costs. However if a well evaluated site needs to be connected by a new road segment to the existing street network, this will increase the costs for total maintenance of the street network and thus the tax. Third, each household wants to benefit from green amenities for different use (daily walks, playground, hiking…) but the construction of side branches of the road network can impede direct accesses to open landscapes and can generate supplementary travel costs thus impacting the budget of households. This “shadowing effect” reminds some mechanisms of the DBM-models. The model thus combines different aspects acting positively or negatively on the households’ utility and budget. According to the expected changes to their indirect utility function, the households who have settled down earlier in the city will not be in favour of accepting new households. This paper focuses on the conceptualization of the model and on the morphological properties of the emerging patterns. It is illustrated by several simulation results. [less ▲]

Land Use and Transport Interaction Models - Where is the limit?

Scientific Conference (2015, July 08)

Land Use and Transport Interaction (LUTI) models are precious tools to integrating the many impacts and feedbacks of the location of activities on transport infrastructures and vice versa. In that sense ... [more ▼]

Land Use and Transport Interaction (LUTI) models are precious tools to integrating the many impacts and feedbacks of the location of activities on transport infrastructures and vice versa. In that sense, applying LUTI models is key to delivering regulation and planning options for urban and transport sustainability. For they have been used in practice to guide urban planners and help transport policy since the 1960’s and the pioneering work of Lowry (1964). The effectiveness of LUTI models as decision support tools is generally well accepted by transport and planning researchers and by practitioners, despite sailing through troubled waters between consultancy secrets, politically led options, data problems, rule of thumb calibration, and model openness and transparency. At the turn of the millennium, LUTI models have developed from aggregate zone-based models to micro representation of space and disaggregated representation of agents, following increased computing capacity and availability of better GIS and individual data. As demonstrated by Wegener (2011) this modeling shift however goes with costs that impede empirical validation and further adoption in planning. LUTI modelers have long recognized that the different processes within LUTI models act and interface at different speeds (Wegener, 1986). The further granularity accompanying disaggregation then leads to dynamics that are trickier to handle. As argued by Anas (2013), LUTI models need clearer definitions and a stricter use of urban economics concepts. Likewise, we argue in this paper that LUTI models also need to take better care of geographical knowledge and spatial biases. They require the analysis of the robustness of model outcomes to the choice of spatial units and MAUP, which is analyzed by Jones et al. (2013) but also the effect of changing urban system boundaries, which is under focus here. Defining the limits of a coherent study area for modeling is actually questioning the delimitation of a city or urban region. This is obviously not a new question to geographers and economists but it impacts deeply on how inner stocks (population, firms, …) and external flows (traffic, labor,…) are modeled in LUTI models and therefore on their outcome. From intuition and practice, we hypothesize that there is strong inertia in LUTI models outcomes because the many parameters and variables that are present in these models are eventually strongly constrained by the geographical structure considered (monocentric, polycentric, including exurbs or not, etc.). Where the outer limit of a model is traced not only impacts the internal components of the city but also questions the problem of the limits between two cities, which is not trivial. First, we perform a meta-analysis of recent LUTI applications in European contexts based on 19 peer-reviewed articles. The lack of definition of the study area is striking. Interestingly in the first LUTI implementation, Lowry (1964) explicitly mentioned the use of an estimate of the commutershed of Pittsburgh for the next 20 years. The lack of explicit choice in later literature with models of increased complexity, stresses the need for guidelines that could improve practice in order to improve the comparability of applications and the generalization of results where possible. Second, we perform simulations on a synthetic city system using UrbanSim (Waddell et al., 2003). We gradually vary the spatial limits of the system from an inner center monocentric system to a polycentric city region. We also vary population and employment endowments, hence commuting patterns. Our simulations show that LUTI results are highly impacted by the change of limits and therefore suggest a reason why LUTI models are sensitive to large parametric shocks only. Our paper confirms that the absence of a strict theoretical rationale for city delineation weakens the effectiveness of LUTI models. References Anas, A. 2013. A response to the guest editorial: economics as the science for urban modeling. Environment and Planning B, 40 (6), 955 – 958 Jones, J., Peeters, D and Thomas, I. 2013. On the Influence of Scale on Urban Planning Evaluations by LUTI models. ASRDLF Congress 2013. Lowry, I.S. 1964. A model of metropolis. Memorandum RM 4035 Rand Corporation, Santa-Monica. 136p Waddell, P., Borning, A., Noth, M., Freier, N., Becke, M. and Ulfarsson, G. 2003. Microsimulation of Urban Development and Location Choices: Design and Implementation of UrbanSim. Networks and Spatial Economics, 3 (1), 43-67 Wegener, M., Gnad, F., Vannahme, M. 1986. The time scale of urban change. In Hutchinson, B and Batty, M. (Eds), Advances in Urban Systems Modelling. North-Holland, Amsterdam, 145–197. Wegener, M. 2011. From macro to micro - how much micro is too much? Transport Reviews, 31, 161–177 [less ▲]

Emergence of leapfrogging from residential choice with endogenous green space: analytical results

in Journal of Regional Science (2015), 55(3), 491-512

Leapfrog development is a typical form of sprawl. This paper aims at analyzing the existence, size, and persistence of leapfrogging in a dynamic urban economic model with endogenous green amenities. We ... [more ▼]

Leapfrog development is a typical form of sprawl. This paper aims at analyzing the existence, size, and persistence of leapfrogging in a dynamic urban economic model with endogenous green amenities. We analyze whether incoming households choose to settle at the fringe of the city or to jump further away depending on their preferences and the structure of the city. We first provide an analytical treatment of the conditions and characteristics under which a first leapfrog occurs and show how the optimal choice is affected by the size of the city, income, commuting costs, as well as the size of the area where green amenities are considered. We then study how further leapfrogging and multiple urban rings may appear and be maintained in the long-run equilibrium, and how infill processes take place through time. [less ▲]

Développements et structures de la coopération trans-frontalière dans la Grande Région SaarLorLux

E-print/Working paper (2015)

In diesem Artikel werden die internationalen, interregionalen und grenznahen Strukturen der Zusammenarbeit in der Großregion SaarLorLux vorgestellt, die in den letzten Jahrzehnten entstanden sind. Die ... [more ▼]

In diesem Artikel werden die internationalen, interregionalen und grenznahen Strukturen der Zusammenarbeit in der Großregion SaarLorLux vorgestellt, die in den letzten Jahrzehnten entstanden sind. Die Untersuchung beginnt mit den späten 1960er Jahren, konzentriert sich dann auf die 1990er Jahre, in denen sich die grenzüberschreitende Zusammenarbeit innerhalb der EU stark entwickelte, und endet in den 2000er Jahren, in denen eine deutliche Konsolidierung der grenzüberschreitenden Zusammenarbeit zu verzeichnen ist. Alle Entwicklungen unterliegen den entsprechenden Rahmenbedingungen, die u.a. von Brüssel vorgegeben werden, und sind je nach Akteur unterschiedlich ausgeprägt. [less ▲]

L'industrie automobile dans la Grande Région SaarLorLux en 2011 - évolutions actuelles, défis et solutions possibles

E-print/Working paper (2015)

With almost 150,000 employees and more than 600 companies, the automotive industry is one of the most important sectors in the Greater Region. However, in the more recent past, globalisation ... [more ▼]

With almost 150,000 employees and more than 600 companies, the automotive industry is one of the most important sectors in the Greater Region. However, in the more recent past, globalisation, technological development and, above all, the global economic and financial crisis have had a strong influence on the sector and have posed challenges. This text attempts to review these developments and discuss possible strategies and measures to meet the challenges. [less ▲]

Greener and larger neighbourhoods make cities more sustainable! A 2D urban economics perspective

in Computers, Environment and Urban Systems (2015)

We analyse urban growth forms by means of a 2D microeconomic model where households value green space at neighbourhood scale. We analytically demonstrate that cities can grow more densely when households ... [more ▼]

We analyse urban growth forms by means of a 2D microeconomic model where households value green space at neighbourhood scale. We analytically demonstrate that cities can grow more densely when households have the possibility to enlarge the neighbourhood in which they value green space, thus emphasising the importance of neighbourhood planning in particular for facilitating short trips and views of green amenities. We also show by simulation that the size and form of the city, relative to the size and form of neighbourhoods, impact on the decision of households to leapfrog land or not, thus impacting on the emergence of scattered urbanisation patterns. We conclude that carefully addressing the spatial arrangement of green space and buildings and facilitating trips within neighbourhood units constitute an effective policy lever and an attractive way to deliver more sustainable cities. We further argue that our theoretical experiment with complementary analytical and computer-based simulation provides micro-economic reasoning to the main elements of the Garden City and neighbourhood unit planning concepts. [less ▲]

Workplace Relocation and Mobility Changes in a Transnational Metropolitan Area: The Case of the University of Luxembourg

in Transportation Research Procedia (2014, December), 4

The aim of this paper is to study the utility variation related to the commuting mobility of University staff members due to their future workplace relocation. During the year 2012, a travel survey was ... [more ▼]

The aim of this paper is to study the utility variation related to the commuting mobility of University staff members due to their future workplace relocation. During the year 2012, a travel survey was completed by a total of 397 staff members, representing 36.4% of the university employees, who filled in a questionnaire which revealed complex decision making patterns due to the special traveling scenario involving four countries at once. A Multinomial Logit model has been used to anticipate the impact of university relocation from the capital city to a developing area in the south of the country which will happen between 2015 and 2018 and that will affect most of the employees. The effects of several Travel Demand Management measures are discussed based on the analysis of alternative scenarios [less ▲]