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2001
12ème Colloque Européen de Géographie Théorique et Quantitative, St-Valéry-en-Caux, France, 7-11 septembre 2001
210

Conceptual modelling of the interaction between transportation, land use and the environment as a tool for selecting sustainability indicators of urban mobility.

Proposition d'un instrument de sélection d'indicateurs de durabilité en mobilité urbaine: la pertinence conceptuelle de la modélisation systémique dans l'étude des interactions entre les transports, l'utilisation du sol et l'environnement.
Giovanni Fusco

Résumés

Il y a un consensus international sur l'utilisation d'indicateurs pour évaluer la durabilité du développement urbain. Le cadre de référence le plus couramment utilisé pour organiser et sélectionner de tels indicateurs est le schéma Pressions-État-Réponses. Originellement élaboré par l'OCDE pour organiser les indicateurs d'environnement, ses manques sont de plus en plus évidents. Les faiblesses des cadres conceptuels couramment utilisés dans les analyses de durabilité urbaine nous ont amenés à développer un nouveau cadre conceptuel plus cohérent dans la forme d'un schéma systémique. Ce dernier combine l'approche causale du modèle Pressions-État-Réponses avec l'attention aux problématiques de la durabilité urbaine du modèle élargi de métabolisme proposé par Newman et Kenworthy. Dans cet article, le nouveau schéma a été utilisé pour en déduire un cadre systémique pour l'évaluation de la durabilité de la mobilité urbaine. En premier lieu, nous avons adapté le schéma général à l'étude de la mobilité urbaine. Ensuite, à l'aide du logiciel de modélisation conceptuelle Decicion Explorer, nous avons développé ce schéma pour obtenir un modèle conceptuel plus désagrégé. Pour chaque élément du modèle, nous avons donc pu dériver une table de sélection pour guider le choix d'indicateurs pertinents basés sur les liens systémiques de l'élément. Nous avons ainsi pu définir une série d'environ soixante indicateurs de système qui peuvent être utilisés dans l'évaluation de la durabilité de la mobilité urbaine. L'ensemble des indicateurs retenus reflète les liens causaux entre phénomènes et le degré de réalisation des objectifs de durabilité pour la mobilité urbaine. Tous les indicateurs permettent la comparabilité géographique et intègrent le rôle joué par les différents acteurs urbains et les divers modes de transport. Le travail présenté a été effectué dans la première année d'un projet de recherche de doctorat en co-tutelle entre l'Université de Nice-Sophia Antipolis et le Polytechnique de Milan.

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Texte intégral

Introduction

1Assessing the sustainability of urban mobility means for us assessing the interaction between transportation, land use and the environment in light of the key questions of local sustainability. In particular, we can highlight the following aspects:

  • the consumption of resources, also associated with the possibility of future generations to dispose of an appropriate stock of resources;

  • the economic effectiveness, resulting from the economic service produced by the urban mobility system;

  • the environmental effectiveness, resulting from the contribution to local and global environmental problems;

  • the social equity ensured, resulting from the distribution of the costs and the benefits of urban mobility among different social groups;

  • the systemic aspects of sustainability (redundancy, reversibility, auto-regulation) resulting from the internal functioning of the urban mobility system.

2The underlying vision of the sustainable urban mobility is that of a system which is at the same time efficient, effective (both economically and environmentally), socially fair and systemically robust.

3The most common tool used to assess sustainability in cities are indicators of sustainable urban development.

4Indicators are instruments conveying information synthetically through different representations (numbers, graphs, thematic maps, etc.) on a more complex phenomenon having a wider meaning.

5In their simplest form they are parameters or combinations of measured parameters which are particularly significant in the understanding of a phenomenon (for example, population density of a city as an indicator to understand the spatial distribution of its population).

6Indicators can be applied in the assessment of all urban activities (transportation, industry, tourism, etc.). They are a flexible method to study complex phenomena, like the land use – transportation – environment interaction. They allow a descriptive approach to be used even in the absence of a comprehensive theory of the phenomena to be analysed. They can integrate both quantitative and qualitative pieces of information and they answer the practical needs of public decision makers. For all these reasons, their usefulness in assessing urban sustainability is internationally recognised (IIUE 1994, LGMB 1995, ICLEI 1995).

The need for a conceptual framework for urban sustainability

7Nevertheless, the indicator approach presents several limits. First of all, indicators cannot give a complete analysis. They always imply a simplification of the real world, which has to be kept in mind by their users.

8Secondly, choosing the parameters or the combination of parameters to be used as indicators is always difficult. In particular, how to ensure that the selected indicators exhaustively describe the phenomena to be studied ?

9Thirdly, the lack of standardisation often hinders comparability between different geographical contexts. Comparability is in our opinion necessary if scientific knowledge is to be acquired from the use of sustainability indicators.

10The most important point is however, the absence of a shared theoretical model structuring urban sustainability indicators. The use of a theoretical model would also contribute to remove some of the objections previously raised to the indicator approach: it would render explicit the simplifications introduced, guide the selection process, and ensure an exhaustive coverage with respect to the phenomena retained in the model.

11On the common base of system theory, several conceptual frameworks have been proposed to assess urban sustainability. They are systemic schemes, that is, highly aggregated models giving a general framework for system analysis. Their common objective is to model the interaction between urban activities and the environment supporting them (the urban environment on the one side, the global environment on the other). They are often derived from more general considerations on the interactions between human systems (anthroposystems) and ecosystems, the city being a particular case of anthroposystem.

12We can recognise two main groups of conceptual frameworks for urban sustainability: urban metabolism schemes and causal schemes.

Picture 1: Urban metabolism models: the simple model (a) and the extended model (b).

13Urban metabolism schemes conceive the city as an ecosystem. In its simplest form (picture 1a), the city is represented as a set of urban activities receiving an input of resources and producing an output of emissions and wastes.

14This simple systemic model forgets that cities, like other anthroposystems, are not just machines producing emissions and wastes. They also produce numerous goods and services or mere "opportunities" to satisfy the needs of their inhabitants and assure them a certain quality of life.

15Newman and Kenworthy (1998) propose an extended urban metabolism model (picture 1b) better integrating urban sustainability issues: reducing the consumption of resources and the production of emissions and waste, while increasing the living conditions of its inhabitants. Liveability is thus a fundamental output of urban activities in the extended urban metabolism model.

16Urban metabolism schemes describe the flows of inputs and outputs but are not able to highlight the causal links between phenomena.

17Representing causal relationships characterising anthroposystems is, on the contrary, the main aim of causal schemes. The first important causal scheme used to select and organise urban sustainability indicators was the Pressures-State-Response framework, first developed by OECD (1993).

18In the PSR model (picture 2a) urban activities exert pressures on the environment and modify the quality and the quantity of natural resources (the State). Society responses to these changes through measures of environmental, economic or sector policy (the Responses), which feed back on pressures and state.

19The European Environmental Agency developed the PSR framework further in a five element scheme (Smeets et Weterings 1999): Drivers, Pressures, State, Impacts and Responses. Like in the OECD scheme, phenomena are chained by causal links.

20Drivers are the causes producing environmental pressures. This concept is differently interpreted in EEA works: drivers are the demand for goods and services, including underlying phenomena like demography (Peirce 1998), or they are the human activities responding to this demand (EEA-Eurostat 1999).

21Environmental pressures include the consumption of natural resources and the production of pollutants and waste. Pressures, in their turn, change the state of the environment, which can have an impact on human health or on ecosystems. Finally, society responds to these impacts by taking measures affecting the different links of the causal chain.

22This model has been proposed for the analysis of sustainable urban development (IBGE 2000). Its main weakness is to consider urban activities only as sources of environmental pressures, whereas they also produce goods and services resulting (together with the impacts of the urban environment) in living conditions for the urban population.

Picture 2: Causal schemes of the relationships between urban activities and urban environment: PSR model (a) and DPSIR model (b).

23A common weakness of both metabolism and causal schemes is their linearity. The only feedback in the schemes of the PSR type is the action of the responses consciously given by society. The other phenomena are linked as a linear sequence of causes and effects. This can be an acceptable approximation for the pressures-state-impacts chain. The relationships between drivers, pressures and responses are much more complex: positive and negative feedback loops exist between different urban activities, as a result of economic and social mechanisms, policy measures have multiple effects, etc.

24In the assessment of the sustainability of urban activities, these interactions are fundamental to understand the system dynamics. This induced us to develop a more coherent theoretical framework for the assessment of urban sustainability and to use it as a first step to derive a more specific framework for the urban mobility system.

25Our conceptual framework combines the causal approach of the PSR/DPSIR schemes with the main assumption of the extended urban metabolism model: the city is an anthropic system "per excellence" whose main objective is the satisfaction of human needs. The new framework is shown in picture 3. It is composed of five elements:

261. The satisfaction of the needs of the city, that is, the living conditions of its inhabitants.

272. The urban activities developed in order to satisfy these needs.

283. The consumption of resources (both natural and anthropic) by the urban activities.

294. The emission of waste and pollutants in the environment by the urban activities.

305. The state of the urban environment, including both the quality of the natural ecosystem (air, water, forests, etc.) and the quality of the more peculiar aspects of the human ecosystem which is the city (quality of public spaces, of the urban landscape, etc.).

31It is clearly a very aggregated model: every element represents a complex process, which includes several more specific interacting phenomena (like the ones indicated in picture 3). It is a causal framework like the OECD and the EEA models. The links between elements represent cause-effect relationships. Human needs are the cause of urban activities, which cause on the one hand the consumption of resources and the emission of waste and pollutants, but, on the other, the satisfaction of the human needs.

32Emissions in the environment and resource consumption determine changes in the quality of the urban environment (they also contribute to changes in the quality of the regional and the global environments). Finally, the quality of the urban environment, also contributes to the satisfaction of the needs of the inhabitants of the city.

Picture 3: A new systemic framework for urban sustainability: the causal model of urban metabolism.

33The causal links already highlighted by the PSR/DPSIR schemes are to be found in the new model: the environmental quality depends on the pressures on the environment (resources consumed and emissions produced), pressures are generated by urban activities. Every possible societal response will be accounted for as a modification of urban activities, eventually resulting in the reduction of the pressures exerted. The new framework clarifies the causal links behind urban activities: the real drivers are the socio-economic needs of the population; urban activities are developed to answer these needs and interact with them.

34Like the extended urban metabolism model, the new framework highlights that urban activities need an input of resources and produce a double output: waste and pollutants on the one hand, quality of life on the other. The new framework shows that quality of life also depends from environmental quality. A substitution between goods and services produced by urban activities and amenities offered by a good urban environment is not always possible. The acceptable substitution level between natural and human produced elements is a central issue in the different approaches to sustainability (Rechatin et Theys 1997, Camagni 1996).

A systemic framework for the sustainability of urban mobility

35The new causal model of urban metabolism was the starting point to develop a systemic framework specific to the analysis of sustainability of urban mobility.

36In order to better organise elements and links of the graphical model and to perform some preliminary analysis of the resulting causal graph, we used the cognitive mapping software Decision Explorer® by Banxia (Banxia Software 1999).

37Cognitive mapping is a graphic modelling technique enabling the development of systemic representations in the form of causal graphs.

38Every element of the system is represented by a "concept", that is, a box containing a short sentence (eventually a very simple label).

39The modeller will then enter causal links between the concepts (other possible links are temporal, connotative or user-defined).

40In the simple example shown in picture 4, concept B (car use) is a direct cause of concept A (traffic congestion). Concepts C and D are direct consequences of concept A. Finally, concept E is an indirect consequence of concept A: traffic congestion is an indirect cause of air pollution through the phenomenon of pollutant emission.

41The software will later consider this scheme as a causal graph and not just as a pictorial image.

42Working in the user-friendly environment of Decision Explorer, we could easily derive a systemic framework for the sustainability of urban mobility from the more general causal model of urban metabolism. This was done by highlighting the key urban phenomena involved in the transportation-land use-environment interaction.

Picture 4: Example of a conceptual model created with Decision Explorer.

The model is a causal graph whose elements (the concepts) are linked by arrows representing causal relationships.

43Traditionally, sector studies highlight three key-phenomena in the urban mobility system: the formation of the mobility demand, the constitution of transport supply and the very production of mobility.

44But the urban mobility system also includes the localisation of residences and activities within the city. This interacts with other elements of the system: it depends from the distribution of accessibility within the city and it conditions mobility demand and transport supply.

45Finally, the scheme must also include the socio-economic performance of urban transportation (satisfaction of the needs of the city, resource consumption) and the interaction with the natural environment (pollutant emissions, environmental quality).

46As for the needs, we consider that the inhabitants of the city demand two main kinds of services to the urban mobility system:

  • Accessibility (to jobs, services, neighbourhood facilities, etc.).

  • Integrity of their health, especially in the WHO definition: "Health is not only the absence of decease, but a state of whole physical, mental and social well-being". This aspect is linked to the effects of the externalities of the urban mobility system (air pollution, noise, accidents, etc.).

47The resulting scheme is shown in picture 5. In this model the transportation-land use-environment interaction is conceived in the most extended way:

  • Given a certain distribution of urban functions within the city, individuals have to move every day in order to satisfy their needs (formation of a mobility demand, links 1 and 2).

  • The urban mobility system produces this daily mobility (links 3 and 4), the final service given to the inhabitants of the city is the accessibility to the different urban functions (link 5).

  • The distribution of urban functions (residences, activities) depends in its turn from the accessibility of the different urban zones (link 6).

  • The localisation of urban functions conditions accessibility (link 7) and transportation offer (link 8): on the one hand, new land developments require new transportation infrastructures, on the other, some infrastructures and services are only possible for appropriate patterns of land development.

  • The functioning of the transportation system requires resources (link 10) and produces emissions in the environment (link 9). Other resources are needed just to establish a transportation offer (infrastructures, services) regardless of their actual use.

  • Pressures exerted from the transportation system con contribute to global environmental problems (natural resources depletion, global warming, etc., links not shown in the scheme going out of the local urban system) or produce externalities in the city (degradation of the urban environment, links 12 and 13).

  • In its turn, urban environmental quality contributes to the living conditions in the city (satisfaction of the needs of a healthy living environment and of environmental amenities, link 14). Urban environmental quality can be indirectly linked to the localisation process: individuals living in a certain urban area will re-locate within the city if they feel that their need of a healthy environment is not being satisfied.

48This systemic scheme allows us to highlight some peculiarities of urban mobility with respect to other urban activities.

49Daily mobility demand cannot be considered as a need in itself for the inhabitants of the city. It is an element resulting from other needs (Musso et Burlando 1999): individuals express a "demand for the city". What really characterises their quality of life is the participation in social and economic activities of the city. The impossibility of having all urban functions (including residences) concentrated in one only point, compels individuals to travel within the city. The real service offered by the transportation system to the urban population is thus not the quantity of produced mobility, but accessibility to final destinations (accessibility to jobs, retail facilities, services, etc.).

50Also, because of the peculiar structure of the urban transportation market and of the role played in it by public decision makers, there is no automatic match between offer of transportation and daily mobility demand. This means that the offer remains fundamentally a decision variable of public administration and matching offer to demand is a political choice among others.

Picture 5: A new systemic framework for sustainable urban mobility: the causal model of urban metabolism for the activities of the urban mobility system.

A conceptual model for the sustainability of urban mobility

51The systemic scheme for urban mobility is already an interesting tool for selecting and organising indicators of sustainable mobility in the city. For us, it was a first step in the development of a more detailed conceptual model.

52A conceptual model is more than a simple systemic scheme organising indicators. The conceptual model should be sufficiently detailed to enable a deeper comprehension of the broad processes highlighted in the scheme (for example, what aspects of the transportation offer depend from the localisation process ? Which societal needs are satisfied by a good local environmental quality ?).

53The definition of the elements of the model is linked to the level of detail required from the indicator set. Ideally, each element would be represented by one indicator. In reality, each element plays a different role in the functioning of the system and this has important consequences on both the number and the kind of indicators necessary to describe it. It is only at the end of the modelling process that the role played by each element can be fully assessed. In this sense, the development of a detailed conceptual model is a necessary step in order to select system indicators.

54Our conceptual model has been derived from the 8-element scheme using the cognitive mapping software Decision Explorer. Every element of the scheme has been decomposed in 3-4 key-phenomena (represented each by a "concept" on the cognitive map, the concepts retain the colour of the original element of the scheme).

55The resulting model is thus composed of 28 elements, and of a network of causal links among them (picture 6). The links were deduced from a series of empirical works.

56The version of the model shown here is the result of an iterative process. The analysis tools of the cognitive mapping software Decision Explorer allowed us to carry on several coherence tests on the model as it increased in complexity, until we arrived to the final version.

57Through this model we can appreciate the complexity of the urban mobility system: multiple causation, "indirect causal paths", positive and negative feedback loops.

58The conceptual model developed is much more wide-ranging than the classical mathematical models used in transportation planning; it also allows the inclusion of qualitative pieces of information. In particular, it conceives the urban localisation process both as a cause of the observed mobility patterns, and as a consequence of the accessibilities allowed by the transportation system.

59Globally, the developed model is a general model: all possible links between the phenomena are shown and it can be applied to all cities. It is a general conceptual model of the urban mobility system.

60The cognitive map is a very versatile tool allowing us to formalise all the knowledge we have on the system in a graphical way.

61Aside from causal links among the phenomena, we can add connotative links to eventually associate phenomena with sustainability objectives, urban actors and modes of transportation (picture 7).

62With reference to our conception of urban sustainable mobility, we considered four kinds of sustainability goals: efficiency, effectiveness (both economic and environmental), equity and robustness. Clearly, not every element of the system is associated with a sustainability goal, whereas some are associated with several goals (accessibility is for example linked to effectiveness as well as equity goals).

63We also decided the number of urban actors and of transportation modes that had to be integrated in the sustainability assessment.

64The urban actors finally retained are the public administration, urban transportation operators, enterprises and the population.

65As for the modes of transportation, we retained car, transit, motorised two-wheeled vehicles, bicycle and pedestrians. It is important to highlight phenomena in which more than an actor or a transportation mode are concerned. They could require several indicators to take into account the role played by different actors or modes. The offer of transit services concerns both public decision makers and transportation operators. Emissions (of local pollutants, of greenhouse gases, of noise) concern all motorised modes (car, transit, motorised two-wheeled vehicles)

Picture 6: The conceptual model of the urban mobility system. Arrows represent cause-effect links between phenomena.

Picture 6: The conceptual model of the urban mobility system. Arrows represent cause-effect links between phenomena.

Picture 7: Example of association of sustainability objectives, urban actors and modes of transportation to the elements of the causal graph.

Selecting indicators using the conceptual model

66Based on the conceptual model of the urban mobility system, we could later develop a selection table to choose system indicators (picture 8).

67For every concept, the table shows in its columns concepts directly linked to it:

  • direct causes (that is, concepts directly determining it)

  • direct consequences (that is, concepts directly determined by it)

  • associated sustainability objectives

  • urban actors involved

  • transportation modes concerned

68A final column assesses the geographical comparability of the indicators.

69Each row of the table is used to assess a possible indicator for the concept. Indicators were mostly derived from existing literature.

70For each indicator we evaluated its ability to represent the links between concepts, with the objectives, with the urban actors and with the transportation modes. It is a qualitative assessment with three possible values (yes, no, partially) based on the modeller’s knowledge of the indicators.

71If no indicator is able to sufficiently represent the set of links of a given concept, we considered using the combination of two or more indicators to "cover" the phenomenon. Using the information resumed in the table, the indicators thus selected can be assessed as a whole with respect to the coverage of the links.

Picture 8: Example of selection table for system indicators of a given concept.

For direct causes, direct consequences and associated sustainability objectives: "**" = good coverage, "*" = partial coverage, " " = no coverage. For urban actors, transportation modes, geographical comparability: "*" = coverage, " " = no coverage.

72For only 8 concepts out of 28, we could find a comprehensive indicator. That is, an indicator which takes into consideration all of the systemic links of the concept.

73For the remaining 20 concepts we had to use 2 or 3 indicators and exceptionally up to 6 indicators.

74We thus arrived to a final selection of 62 indicators for the 28 phenomena of the urban mobility system (picture 9). The indicators were also selected in order to allow the geographical comparison between different urban systems or different urban areas within the same city.

75A characteristic of our selected set is the importance given to the accessibility indicators (9 out of 62). This is a direct consequence of the many direct links that the accessibility concepts have with the rest of the system, with sustainability objectives, with the different urban actors, etc.

76The majority of indicators can be calculated for the city as a whole or for its sub-units (urban districts, quarters, etc.). Average values for the whole city can be used to assess the effectiveness and efficiency dimensions of the urban mobility system. They allow us to answer questions like: what are the average accessibility levels in the city ? What are the average environmental conditions ? How many resources are consumed per inhabitant ? How many pollutants are emitted per urban hectare ?

77In order to assess the equity dimension of sustainability, we need to calculate the same indicators for urban sub-units. Equity results from the distribution of the costs and the benefits of urban mobility among different social groups. The spatial analysis of the urban territory will reflect this social dimension as long as social differentiation processes will affect the spatial distribution of the inhabitants of the city.

land-use constraints

(contraintes de l'urbanisation)

1

% of plane surface

2

% of different classes in land-use plans

population distribution and composition

(distribution et composition de la population)

3

population distribution among different urban zones (central, peripheral, suburban)

4

population density

5

population structure according to occupational status

6

population structure according to household size

distribution and composition of activities

(distribution et composition des activités)

7

job distribution among different urban zones (central, peripheral, suburban)

8

distribution of jobs in retail and consumer services among different urban zones (central, peripheral, suburban)

building offer

(offre de bâti)

9

housing distribution among different urban zones (central, peripheral, suburban)

10

distribution of business premises among different urban zones (central, peripheral, suburban)

household equipment

(équipement des ménages)

11

household distribution according to number of motor vehicles owned (alternatively, number of motor vehicles per 1000 inhabitants)

12

household distribution according to number of powered two-wheelers owned (alternatively, number of powered two-wheelers per 1000 inhabitants)

13

% of motor vehicles attaining emissions and consumption standards

infrastructure offer

(offre d'infrastructures)

14

parking spaces per 1000 inhabitants and jobs combined

15

public transport infrastructure density

16

road density

17

density of pedestrian routes

18

density of bicycle routes

public transport service offer

(offre de services de transport public)

19

public transport vehicle kilometres of service per capita

20

public transport spatio-temporal coverage ratio (number of vehicle stops per day and per km2)

daily mobility agendas

(agendas quotidiens)

21

trip distribution according to trip motives

22

population distribution according to number of daily trips

spatial structure of mobility demand

(structure spatiale demande de mobilité)

23

spatial distribution of produced trips according to trip destination

24

spatial distribution of attracted trips according to trip origin

25

overall average trip distance

modal choice

(choix du mode de transport)

26

modal split of constrained trips (journey to work and journey to place of study)

27

modal split of non-constrained trips

modal habits

(habitudes modales)

28

% of car-dependent users

29

% of multi-modal users

produced passenger mobility

(mobilité produite par les modes de transport)

30

modal split of produced mobility in passenger kilometres

31

average daily travelled distance per capita

traffic flows on infrastructures

(flux sur les infrastructures)

32

density of traffic flow

33

occupation rate of public parking spaces

speed of transport networks

(vitesse des réseaux de transport)

34

average speed of public transport network

35

average speed of road network

emissions of local pollutants

(émissions de polluants locaux)

36

emission density of total annual local pollutants (CO, NOx, SO2, VOC, particulate matter)

37

total annual acidic emissions per capita

greenhouse gas emissions

(émissions de gaz à effet de serre)

38

total annual emissions of CO2 per capita

noise emissions

(émissions de bruit)

39

density of noisy transport infrastructures

urban land consumption

(consommation de sol urbain)

40

occupation of land for transport infrastructure per capita

time consumption

(consommation de temps)

41

population distribution according to daily time consumption for urban mobility

energy consumption

(consommation énergétique)

42

total annual urban mobility energy consumption per capita

transportation costs for the urban actors

(coût des transports pour les acteurs urbains)

43

urban mobility cost as percentage of metropolitan GDP

44

% of household revenues spent on urban mobility

45

modal split of public spending for urban mobility

air quality

(qualité de l'air)

46

annual air quality standard exceedences

quality of urban spaces

(qualité des espaces urbains)

47

open public space area of good environmental quality per capita

quality of natural spaces

(qualité des espaces naturels)

48

natural spaces as percentage of total urban surface

49

% of trees in good state in natural spaces

accessibility to the city functions

(accessibilité aux fonctions de la ville)

50

accessibility to jobs: number of jobs at less than 20 min by car

51

accessibility to jobs: number of jobs at less than 20 min by public transport

52

accessibility to retail and services: number of jobs in retail and consumer services at less than 20 min by car

53

accessibility to retail and services: number of jobs in retail and consumer services at less than 20 min by public transport

54

accessibility to population: number of inhabitants at less than 20 min by car

55

accessibility to population: number of inhabitants at less than 20 min by public transport

accessibility to neighbourhood functions

(accessibilité de proximité)

56

local accessibility to jobs: number of jobs at less than 20 min on foot / 10 min by bicycle

57

local accessibility to retail and services: number of jobs in retail and consumer services at less than 20 min on foot / 10 min by bicycle

58

local accessibility to population: number of inhabitants at less than 20 min on foot / 10 min by bicycle

health impacts of pollution

(impacts sur la population)

59

% of population exposed to more than 60 dB LAeq during the day or 55 dB LAeq at night

60

incidence of respiratory decease

population safety

(sécurité de la population)

61

modal split of road accident casualties

62

road accident fatalities per vehicle-km

Picture 9: The selected set of sysem indicators according to the phenomena to which are referred.

78Finally, the systemic aspects of a sustainable urban mobility can hardly be apprehended through specific indicators (although some indicators of modal split could be used to assess the modal diversity of the system). It is only through the analysis of the system dynamics (based both on indicators directly linked to sustainability objectives and on indicators describing the functioning or the urban mobility system) that the most important systemic aspects can be assessed. In particular, historic series of the indicators can be used to assess, for a given urban system, the relative strength of the different systemic links shown in the conceptual model. It will then be possible, for example, to determine which loops (negative or positive) play the most important role in the system, which elements introduced irreversibilities in the functioning of the system, etc.

79The central point of our selection process is that descriptors were not selected to be individually good sustainability indicators: it is only the set of indicators in its wholeness, which becomes a pertinent tool for assessing sustainability

80The proposed selection process presents other interesting features.

81Firstly, it is open: the conceptual model can at any time be changed, allowing an evolution of the comprehension of the system, of the sustainability objectives, etc. The links with the urban actors could be assessed by the urban actors themselves in a participatory process.

82Secondly, it is flexible: if an indicator is not measurable, the selecting process will indicate the second best indicators to be used. Finally, it is based on a systemic view: indicators of a given phenomenon are selected to cover the systemic links that the phenomenon has with the rest of the system.

83We carried out a first critical assessment of the method.

84Our selection process is fundamentally based on one-step links around each concept. The question arose whether, by doing so, we had been correctly appreciating the relative importance of the different concepts.

85We then carried out a series of exploratory analyses of the model using the analysis tools of Decision Explorer. Three kind of analysis have been retained (a more detailed presentation of these analysis tools is to be found in Banxia Software 1999):

  • Domain analyses, taking into consideration, like our selection tables, direct links around each concept.

  • Centrality analysis, taking into consideration direct and indirect links around each concept through a particular weighing procedure.

  • Potency analysis, considering the ability of each concept to influence the whole set of sustainability objectives through direct as well as indirect causal paths.

86The three analyses resulted in highlighting 13 concepts out of a total of 28 as the most "crucial" in the model.

87Retrospectively, we could observe that our selection process reflects this result: by dedicating 39 indicators to the 13 "crucial" concepts, it explicitly gave more importance to these concepts within the selected set of indicators.

88We want to stress that a thorough critical assessment of the method can only follow a field application, which we will carry out in the course of our research project for the cities of Nice and Genoa.

Conclusions

89Appropriate conceptual frameworks are needed to select and organise indicators of urban sustainability. The weaknesses of the conceptual frameworks currently used in urban sustainability analysis induced us to to develop a more coherent theoretical framework in the form of a systemic scheme. The new scheme combines the causal approach of the Pressure-State-Responses model and the attention to urban sustainability issues of the extended urban metabolism model. It is a general scheme, which can be applied to every urban activity.

90In this paper, the new scheme was used to derive a systemic framework for the assessment of the sustainability of urban mobility. First, we elaborated a systemic scheme for the urban mobility system composed of the eight main phenomena of the land use-transportation-environment interaction. Successively, using the cognitive mapping software Decision Explorer, we further developed this scheme to obtain a more disaggregated conceptual model. A selection table could be derived for every element of the model to guide the selection of pertinent indicators based on the systemic links of the element. We could thus define a set of some sixty system indicators to be used in the assessment of the sustainability of urban mobility.

91The selected set of indicators reflects the causal links between phenomena and can monitor the attainment of sustainability objectives for the urban mobility system.

92All indicators allow geographical comparison and integrate the role played by different urban actors and modes of transportation.

93We want to stress that the proposed conceptual model is a general model valid for all cities.

94Different causal links can play a smaller or bigger role in particular contexts. After evaluating the model for a particular city it would be possible to discriminate hierarchical groups of indicators within the set. It will also be possible to assess the relative importance of the different systemic links and to judge the system dynamics in the light of urban sustainability. In the course of our future research work, we will carry on these operations for the cities of Nice and Genoa.

© CYBERGEO

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Référence électronique

Giovanni Fusco, « Conceptual modelling of the interaction between transportation, land use and the environment as a tool for selecting sustainability indicators of urban mobility. », Cybergeo : European Journal of Geography [En ligne], Dossiers, document 210, mis en ligne le 21 mars 2002, consulté le 12 juillet 2020. URL : http://journals.openedition.org/cybergeo/1590 ; DOI : https://doi.org/10.4000/cybergeo.1590

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Auteur

Giovanni Fusco

giovanni.fusco@unice.frPolitecnico di Milano, DiAP, Piazza Leonardo da Vinci 20, 20100 Milano, Italy
Université de Nice-Sophia Antipolis
UMR 6012 ESPACE, 98 Bd Herriot – BP 3209, 06204 Nice, France

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