Interactive Localisation System for Urban Planning Issues - Efficient Decision Support Systems: Practice and Challenges from Current to Future - page 465

difference between the potential of the source node, i.e., the starting point, and the
destination node, namely the node where the facility is located;
- the current in each link of the network which represents the traffic variation induced by
the facility;
- the current in the node where the facility is located which describes the degree of
utilization of the facility.
- It is worth noting that the network solution, relying on the analogy between voltage at
the nodes and travel times, and on the analogy between currents in the links and
induced traffic, leads to the determination of the minimum of the cost function C (Di
Barba and Savini, 1996). Indeed, according to (Maxwell 1892), in a circuit made up by
resistors, independent voltage and current sources, the currents tend to reach a
distribution such that the dissipated power is minimum.
It is interesting to note that the total cost gives the number of vehicles the chosen location
induces in the road network, so its minimization means the minimization of induced traffic
pollution.
The advantage of the modeling analogy pursued is mainly tied to the possibility of
exploiting the huge capabilities of commercial electrical networks analyzers to determine
the relevant parameters characterizing the behavior of the clients' population with respect to
a certain choice of the facilities location.
To this end, one of the most common tools is SPICE, which can be regarded as a standard all
over the world to analyze and simulate electric networks (Nagle 1975).
The problem of solving the electric networks which accounts for the effects on the
underlying transportation network of the facility location choice belongs to the class of
problems that can be easily dealt with by SPICE. Moreover, the computing time is such that
even a general-purpose PC can be used. Once that the features of the considered network
have been acquired by SPICE, it computes the voltage at each node and the currents in the
links.

4. The system ULISSE

The main purpose of ULISSE consists of creating an unmediated interaction with context
data and of supporting the urban planner in the investigation of planning choice effects.
The first difficulty is that no circuit simulation program has a user interface that can be
adapted to the urban context; the only option would be to translate the viability graph into a
net-list describing the whole graph element by element, but this operation would lose any
geographical information defining the location of the graph nodes.
On the other hand, Geographical Information Systems (GIS) cannot be of any help either,
since the problem lies in the way circuit parameters are managed and related to
geographical information. So the choice author made has been of developing an ad-hoc tool
that could interact with SPICE to process the traffic network data, yet let urban planners
investigate their alternatives in a totally graphical way.
A typical working session would begin with the designer setting up the planning scenario as
follows:
- read a map of the town or the area to be studied;

- mark on the map, and enter the respective parameters of, facility locations, client cells

(nodes that represent the population of a neighborhood), and transportation network
additional nodes;