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 ▲]

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 ▲]

Housing land transaction data and structural econometric estimation of preference parameters for urban economic simulation models

in Data in Brief (2015), 5

This paper describes a dataset of 6284 land transactions prices and plot surfaces in 3 medium-sized cities in France (Besançon, Dijon and Brest). The dataset includes road accessibility as obtained from a ... [more ▼]

This paper describes a dataset of 6284 land transactions prices and plot surfaces in 3 medium-sized cities in France (Besançon, Dijon and Brest). The dataset includes road accessibility as obtained from a minimization algorithm, and the amount of green space available to households in the neighborhood of the transactions, as evaluated from a land cover dataset. Further to the data presentation, the paper describes how these variables can be used to estimate the non-observable parameters of a residential choice function explicitly derived from a microeconomic model. The estimates are used by Caruso et al. (2015) to run a calibrated microeconomic urban growth simulation model where households are assumed to trade-off accessibility and local green space amenities. [less ▲]

How green neighbourhoods make cities more compact? A 2D microeconomic perspective

Scientific Conference (2014)

We analyse urban growth forms by means of a 2D microeconomic model with no symmetry assumption where households value green space at neighbourhood scale. We analytically demonstrate that cities can grow ... [more ▼]

We analyse urban growth forms by means of a 2D microeconomic model with no symmetry assumption 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 emphasizing the importance of neighbourhood planning in particular for facilitating short trips and view to 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 agricultural land or not, hence impact on the emergence of sprawl patterns. We conclude that carefully addressing the spatial arrangement of green space and activities within neighbourhoods may constitute an effective policy lever in some urban contexts and a positive incentive to a more compact development. [less ▲]

Emergence of scattered and leapfrog urban development from analytical results to complex simulation outputs with realistic calibration

Scientific Conference (2013)

How neighbourhood interactions influence urban sprawl

Scientific Conference (2011)

S-GHOST: Un modèle d'auto-organisation de l'étalement urbain et du réseau de transport

in Antoni, Jean-Philippe (Ed.) Modéliser la ville: formes urbaines et politiques de transport (2011)

Transition phases and leapfrogging in urban sprawl patterns: a simulation and analytical approach with SGHOST

Scientific Conference (2009)

Morphological similarities between DBM and an economic geography model of city growth

in Zhou, Jie (Ed.) Complex Sciences (2009)

An urban microeconomic model of households evolving in a 2D cellular automata allows to simulate the growth of a metropolitan area where land is devoted to housing, road network and agricultural/green ... [more ▼]

An urban microeconomic model of households evolving in a 2D cellular automata allows to simulate the growth of a metropolitan area where land is devoted to housing, road network and agricultural/green areas. This system is self-organised: based on individualistic decisions of economic agents who compete on the land market, the model generates a metropolitan area with houses, roads, and agriculture. Several simulation are performed. The results show strong similarities with physical Dieletric breakdown models (DBM). In particular, phase transitions in the urban morphology occur when a control parameter reaches critical values. Population density in our model and the electric potential in DBM play similar roles, which can explain these resemblances. [less ▲]

Surprising similarities between DBM models and an economic geography model of city growth

Scientific Conference (2008)