In order to build a model capable of simulating urban dynamics, two interrelated spatial systems have to be modelled together: the residential system and the daily mobility system. For this, MobiSim contains several modules that are interconnected and calibrated with appropriate data. The modules can be used in combination or separately.
- Starting with appropriate socio-demographic and geographic databases, MobiSim Synthetizer automatically generates a synthetic population of individuals grouped into households and located in dwellings.
- Individuals have a series of characteristics (sex, age, activity, education, marital status, income).
- Households are made up of individuals. Household composition is defined according to the socio-demographic structure of the population at an aggregated level.
- Dwellings are characterized by several attributes (type, number of rooms, occupancy status, parking spaces) and are located on the floors of residential buildings.
Urban shapes and functions
- Buildings are represented on a metric scale; they are the basic spatial entities in which dwellings, facilities and other economic activities are located.
- Facilities and other economic activities (e.g. shops, public services, schools, industrial outlets) are classified in accordance with NACE European nomenclature; they are characterized by surface area and number of employees. Facilities are also characterized by frequency of use.
- Facilities and other economic activities can be reached by individuals using different modes of transport (basically driving, public transport, cycling and walking, but any other transport mode can be added). Modelling of travel by individual transport modes takes into account the characteristics of traffic lanes (speed, capacity, traffic direction). Travel modelling by public transport modes takes into account the location of transit stops and their timetables.
In order to analyse population trajectories in space and time, demographic evolutions are modelled on the scale of individuals and households:
- Life-cycle events (e.g. birth, ageing, death, union, separation, child emancipation) are endogenously simulated by MobiSim to represent the evolution of individual and household characteristics over time.
- Residential migrations into and out of the study area are taken into account: at each simulation step, a given proportion of households of each type enters or leaves the study area.
Residential development is worked into the simulation process on three spatial scales:
- On the scale of the entire study area: the user sets the number of dwellings built each year, the proportion of individual houses and flats, and the dwelling sizes (number of rooms) for each type of dwelling.
- On a finer scale: newly created dwellings are spatially distributed. The spatial distribution may be made dependent on the size of built clusters (e.g. most newly created dwellings are located within the largest built cluster) or ad hoc zoning (e.g. a typology of local communities).
- On a very local scale: new buildings are constructed to accommodate newly created dwellings. Buildings are located in areas defined as worth urbanizing and so outside of non-developable areas (restricted zones, protected natural areas). All buildings are cubes with the same 20 × 20 m footprint. The shape of the simulated built pattern (local density, fractal dimension) is defined exogenously.
Future infrastructures are taken into account as a set of parameters spread across the various MobiSim modules rather than within any specific module. These parameters allow realistic planning projects to be introduced into MobiSim in the form of scenarios involving:
- Future change of facility locations including planning investments (e.g. creation of amenities, new districts) or new shops and services associated with urban growth, demographic evolution and change of catchment areas.
- Future change of transport systems through change in the spatial or temporal structure of public transport systems (e.g. new lines or stations, new timetables) and/or by changing the characteristics of routes and roads (e.g. speed, capacity, modal use).
Changes to facility locations and transport systems can be introduced at any simulation step to mimic the continuation of planning and investment schedules over the medium and long term.
Daily and residential mobility
MobiSim includes a transport module based on the classical four-step economic model. In the modelling process, the four steps are grouped two by two enabling interaction between the model steps:
- Schedules and trip distribution are two connected processes determining daily individual mobility (in straight lines from a point to another based on a gravitation model) according to individuals and households’ activity planning (for any period of a 24-hour day), including trip chains and peregrinations. The number and type of activities are defined by the user.
- Modal choice and route calculation allow traffic to be mapped for all modes of transport meaning that hot spots of frequentation and congestion in space and time can be identified. Modal split is based on a generalized costs logit model while route calculation uses Dijkstra’s classical shortest path algorithm.
The model determines which households move from where to where in each simulation year. It is based on households’ appraisal of the attributes of their current dwelling and their current residential environment as well as the attributes of vacant dwelling units and their residential environments.
- Each dwelling is characterized by its intrinsic attributes (type, size, occupancy status, cost) and the attributes of its residential environment. The local residential environment (neighbourhood) differs from the global residential environment (defined with respect to other places: city centre, employment places).
- A household does not consider all possible residential destinations: it has limited knowledge of existing opportunities.
- Economic variables (housing costs, transport costs and income) are taken into account in the same way as variables characterizing household life cycles and life styles.
- Households’ propensity to move results from the combination of three variables: residential satisfaction, intrinsic individual mobility (depending on life-cycle stage) and global value of moving within the study area. Calculating the global value of moving involves global variables associated with the local housing market (quantity and quality of vacant housing).
- Migration decisions are determined by individual factors and collective references. The role of collective references is modelled through variables characterizing the study area globally: statistical distribution of access times to the city centre and generalized access costs to places of employment; global value from moving within the study area.