Dany Nguyen-Luong
The Ile-de-France Region (12,000 sq. km) has developed a transportation model to forecast traffic of public and private transport.
Two packages have been used : ArcInfo and the transportation planning software MinUTP. The first, which has been used for six years to deal with land occupancy, is here used to manage the data as input to the second : networks and socio-economic data. The traffic generation happens in ArcInfo. Then, output is sent to MinUTP, which calculates the distribution, the mode choice and the assignment to the networks. ArcInfo is then used for post-processing : it allows analysis of the environmental impact and the economic evaluation, and also map display.
An interface has been developed in ARC Macro Language to allow the two products to exchange input and output.
Keywords : transportation , model , interface
The Ile-de-France is the French administrative region around Paris and its suburbs. It covers 12,000 sq. km, with 10.7 millions inhabitants and 5.1 million workers. The Regional Council finances investments in the fields of environment, housing, schools and especially transportation, which require a lot of preliminary planning studies.
The IAURIF (200 experts) has developed for the Regional Council a global transportation model to forecast traffic of public and private transport. The adopted methodology was to integrate GIS and transportation modelling.
This paper will give the point of view of the final user, the one who is daily in front of the screen, trying to run the model, to analyse the results and to produce traffic maps suitable for reports or presentations.
The IAURIF has been organising and installing the Regional Geographical Information System for six years with ArcInfo : about fifty information layers on subjects like land use (the reference dataset), population, environment, transportation, facilities ... Today, our GIS is well known among those involved in urban planning in the Ile-de-France and is much sought after.
Two years ago, we purchased one of the most widely used transportation planning software products : MinUTP.
The goal was to get a multimodal model, which allows simulations of both public and private transport. We have chosen to develop a four step classical model, refined at the level of trip purposes and enriched with a lot of original data coming from the GIS database.
After the study area is partitioned into zones and the networks built in mathematical graphs, the first step is the generation of trips : production and attraction of each zone. Then the second step consists of the distribution of the trips to all zones using a standard gravity model equation. It results in an origin-destination matrix of trips. This is split in the third step into two matrices (private transport and public transport) by a mode choice model. Finally, each matrix is assigned to the routes of a network according to minimum paths between all zone pairs. The results are traffic volumes on each link of the network and ratios such as volume/capacity.
At first, we studied the potential of ArcInfo and MinUTP for each step of the modelling in order to use the advantages of each in an integrated environment. So, we found out both the advantages and disadvantages in each one :
| - the powerful module ARCEDIT allows update of the networks at a high level of detail.
- you can easily handle zonal data thanks to the polygon topology. - with its complete plotting capabilities, the drawing module enables production of high quality maps. - you may use also ArcView. | - the main functions of distribution, mode choice and assignment contain a lot of parameters and give the possibility of fine tuning.
- the turning movements (penalties and prohibitors) are easily described in an ASCII file. - fast computation | |
| - the module Network in 1994 wasn't easy to use, especially the definition of the turning movements by turntables.
- no assignment function. - no user-friendly graphical interface for ARCPLOT.
- the public transport services can only be described using dynamic segmentation. | - historically, a software developed in FORTRAN language and installed on proprietary systems. The PC version remains heavy to use, still running with batch processing.
- the input files formats are very specific, so constraining. - output maps are very plain (no vertex entity, no data background) and the module for plotting is difficult to use. |
So, the form of the integrated environment of GIS
and transportation model depends on the easy-to-operate level
you expect. We have chosen the following chain of modelling :
| MINUTP | ||
| DATA PREPARATION | - Networks (topology, geometry)
- Zoning | - Networks in specific format
- Centroids |
| - Socio-economic data | ||
| GENERATION | Productions and attractions by trip purposes | |
| DISTRIBUTION |
| an origin-destination matrix |
| MODE CHOICE | Splitting process into two matrices | |
| ASSIGNMENT | Assignment procedure on networks with iterative capacity restraint | |
| POST-PROCESSING | - Map display (traffic volumes, isochrones )
- Indicators |
So, ArcInfo is used up and down the chain with MinUTP as the motor.
Practically, how is the system organised ?
ArcInfo runs on a workstation and MinUTP on a PC, both are linked
by the Ethernet network and the PC/NFS communication software.
ArcInfo is a powerful tool to manipulate polygons. You can adjust simply zone boundaries and recompute the proper zone centroid coordinates by overlaying the polygons with the coverage of population densities. The socio-economic information have been collected at the smallest administrative level (the commune) and stored in the GIS database. A correspondence table between the zones and the communes is established.
An ARC Macro Language (AML) program chains up the following tasks :
Base networks
The networks are extracted from the lines in the land use and then built into graphs. Some functions of ArcInfo are very helpful for validating the graphs structure and connectivity. The advantages of working under ARCEDIT rather than in the MinUTP editor are the precision of the length data and the geographic coherence with the other layers of the GIS.
For the roads, any characteristics needed for processing can be exported : name, kind of road, one-way road, number of lanes, speed class, capacity class, toll.
For the public network, you just have to export the base network (since the services are described in MinUTP) and the names of the stations.
The coordinates files are extracted from the node topology.
You have to be careful to create a complete link between the networks in both ArcInfo and MinUTP . Hence the constraint to assign and manage a unique record identifier for each arc. But because of the one-way roads, you have to maintain the "From-node" and the "To-node" values, so there are three identifiers per arc.
Generation
Each zone is described by socio-economic characteristics and some parameters such as mobility. An AML program solves the production and attraction equations to produce the trip ends for each zone. A factoring process is implemented so that total productions match total attractions.
The difficulty is to transfer all these data in the appropriate MinUTP fixed-field format. So, an AML program has been written to export arc and node attribute table to MinUTP, as well as the generation file and the coordinates files.
The three following steps of modelling happen in MinUTP. The files passed by ArcInfo are read as input to the network editor and the distribution module. Then, the matrix manipulation module executes a mode choice model. And lastly, the matrices are assigned to the networks.
One drawback of MinUTP is that the results of assignment are awfully
presented, so you have to sort them before analysis. A MinUTP
subroutine is written to arrange the results and output a text
file containing the identifier and any processed variable for
each oriented link: traffic volumes, congested speed, congested
time, node-to-node movements.
This last output file is passed to ArcInfo and joined to the arc attribute table using the identifier as the common key.
Thanks to the buffering function and a look-up-table, you can plot attractive maps by adding any background coverage to the traffic volumes : land use, names of town, zone boundaries, observed traffic and so on.
Other kinds of maps are very often required in a transportation study: traffic flow assigned to a specific link, isochrones.
All these maps can be incorporated directly in reports or presentations.
Now, you can perform geographical analysis with ArcInfo :
- Once the processed variables have been integrated in the arc attribute table, any interesting indicators related to land use information and socio-economic characteristics can be calculated : number of vehicles/urban area square meter, number of vehicles/kind of road, roads classified by degree of saturation and so on. These indicators make the analysis and comparisons between scenarios at the decision process level easier.
- Environmental impacts : some buffer spatial analysis around the arcs and statistics can be done. For example, noise and pollution emissions can be calculated from the traffic volumes. Then ratios using the number of submitted population in the corridor provide some interesting criteria.
- Economic and strategic evaluation : towns, sites, facilities (data located in the GIS) can be classified in function of the traffic volume or the congested time accessibility.
The non-technical GIS and MinUTP user will work with the software ArcView to access, query, display, focus on a particular area and plot the assignments results.
The interest of integrating GIS and transportation modelling is undeniable. A set of AML programs has been implemented to convert data from ArcInfo and MinUTP, and vice versa. Such data sharing is intended to take advantage of the best functionality of each product : classical functions of simulation in MinUTP and a better set of display and analysis capabilities in ArcInfo. Thus, all data (traffic, land use, socio-economic data) are assembled in a unique platform and can be overlaid to ease interpretation . This integration also allows to make fast assignment and so we can devote more time to analysis, which is the most difficult part in a transportation study.
Mr Dany NGUYEN-LUONG
Institut d'Aménagement et d'Urbanisme de la Région
d'Ile-de-France - IAURIF (Institute for Urban Planning and Development
in the Ile-de-France Region)
Address : 251, rue de Vaugirard
75740 PARIS cedex 15 - FRANCE
Telephone: (33) 1-40-43-77-44
Fax : (33) 1-40-43-76-02