The marginal access cost: a new indicator for sustainable urban development

Accessibility is a key concept to achieve sustainable development in urban areas, by means of integrated transport and land-use policies. It can be a great support to those decision-making processes related to urban development problems (e.g. location of new dwellings or activities) as well as to evaluate transport plans (e.g. new transport infrastructures). In fact, accessibility measures allow either to assess the ease of the activities located in a zone to be reached by potential users (clients, workers, carriers, service providers, etc.), i.e. “passive” or “place” accessibility (Hansen, 1959; Ben-Akiva and Lerman, 1985), or to assess how people living in a zone, could reach activities located in other zones of the urban areas, i.e. the “active” accessibility (Cascetta, 2012). These indicators are typically defined from the users (i.e. residents or activities) perspective, in that they include individual travel times and costs but do not cope with environmental issues such as the impacts of mobility on environment, e.g. in terms of pollutant emissions. In this paper we present a new synthetic area-type indicator to support city planners in identifying new development areas for different kind of activities (e.g. commerce). In particular the indicators computes the “marginal access cost”, that is the incremental (internal and external) transport-related cost (expressed in monetary terms), induced by the location of one new activity in a specific zone “o” of the study area. This is based on the fact that new activities induce incremental mobility costs related to the individual trips to reach those activities (Internal costs), which in turn produce additional traffic and environmental transport-related costs (External cost). “Activities” are expressed in terms of people employed in the activity itself. By “one activity in the specific zone o”, we mean an increment of “n” employees in zone “o”, n being the average number of employees per activity and economic sector (e.g. commerce) in the study area. Internal costs includes the generalised travel cost (times and monetary cost on auto and public transport) both of people employed in the new activity (i.e. commuting cost) and of people “attracted” by that activity for other purposes (e.g. shopping). External costs are computed as the pollutant emissions related to the additional trips by car (converted in monetary units, using the conversion factors proposed in the literature). A residential choice location model (Nuzzolo and Coppola, 2007) has been adopted to estimate the spatial distribution of new employees in the specific zone “o”. Given the residential zones, it is straightforward to compute the incremental OD commuting trips, provided that the destination (working) zone of such employees is known, and thus, it is possible to estimate the incremental generalized travel cost for commuting trips. The incremental number of trips for “other purposes” is estimated using a trip generation-distribution model, which include the probability of the zone as a function of the number of activities located in the zone. Finally, a model choice model split these new trips (commuting + other purposes) between auto and public transport modes. This allows to estimating the vehicle-km on auto which are finally converted into pollutant emission and then into external (monetary) cost. Since such impacts vary from zone to zone, the proposed indicator gives an estimation of the differentiated impacts of developing activities in different zones of the urban area in compliance with the goal of reducing the overall transport costs. Using the urban area of Rome (2,7 million of residents; 1 million of employment) as test bed, different “zone accessibility profiles” have been identified according to accessibility to public transport and proximity to densely urbanised areas. Preliminary results show that zones with high residential density and high accessibility to public transport have lower marginal access costs, both in terms of internal components (i.e. individual trips), due to higher proximity to residential areas than peripheral ones, and in terms of external components (i.e. transport-related environmental impacts), due to a lower number of induced trips by car. The marginal access costs has been finally applied to validate the urban development plans of the municipality aiming at reducing the overall transport costs by integrated land use and transport policies. Our methodology allows to clearly identifying those zones where to improve public transport accessibility and those in which foster more dense land use patterns