Improving Consideration of Redundancy of Itineraries in Introducing a Logsum of Utilities –application in Global V9



Improving Consideration of Redundancy of Itineraries in Introducing a Logsum of Utilities –application in Global V9

Authors

Derome Charlotte, Ratp

Description

Forecasting traffic for a public transportation project which is redundant with an existing public transport infrastructure is particularly delicate. This paper gives a technical comparison of robust estimation techniques which exists.

Abstract

In the 1970’s, when RATP began the internal development of GLOBAL, its transport model, traffic forecasts were mainly about the overall use of the new lines. Since then, the model has been improved and is used in many projects of urban railways, subway extensions and recently for trams. Today, GLOBAL is thus used to simulate a very complex and interconnected network, consisting of different modes (subway, urban railway and trams), and located over a large area (Ile-de-France is more than 12,000 km² wide and represents 12,000,000 people and 6,000,000 jobs).
The constraints of the context add to this complexity. Indeed, in a dense public transportation network such as Ile-de-France, in some cases, several paths can be used to go from one place to another. For example, the Parisian project “EOLE” offers a new transport public alternative to go from “La Defense” to “Saint Lazare”. In such situations, it becomes complicated to include in the model the redundancy of itineraries in public transportation and to evaluate the modal share. Handling these cases allows us to further improve the quality of the model.
Modal choice is generally shaped as a logit of utilities. Utilities are functions of explanatory variables, which describe the characteristics of transport supply: price, travel time, number of connections, etc. In the case of several possible itineraries, one may wonder which itinerary to take into account to calculate the public transport utility. Three different calculation methods exist: the shortest path (later named “SPath”), based on the shortest path’s characteristics, the weighted average method is the average of itineraries weighted by their respective share (later named “WA”) and the Logsum method, in which the exponential of each itinerary’s utility is added and calculated as a logarithm (later named “LOGSUM”).
There are extended researches documenting LOGSUM as the best way to evaluate the redundancy of itineraries (Mc Fadden / Nobel Prize in Economics, Ben-Akiva and Lerman; Gaudry and Quinet). Mathematically, LOGSUM produces utilities greater than SPath, which produces greater utilities than WA. One may conclude that the same order is to be expected in terms of public transit modal share. In reality, the calibration of these parameters allows us to reset them and reproduce modal shares. In the end, the exercise showed very little differences in level between utilities and did not help discriminating a particular method. At the time we began this analysis, our model was based on the SPath. We then decided to focus on the differences between SPath and LOGSUM. Specific tests on the actual public transport network were undertaken. These tests consisted in calculating the public transport modal share for an itinerary with two independent paths, another one with three interconnected paths, etc. Small modal split differentials between SPath and LOGSUM were observed. Finally, we tested the behavior of LOGSUM in specific situations on the future public transport network. These tests highlighted the fact that public transport modal share calculated with LOGSUM is higher than with SPath in the case of redundancy of itineraries, and smaller in the case of a single one.
To conclude, LOGSUM produces increases and decreases of transport public modal share which are not significant (in the way it has no effect on the calculation of the capacity design), but more realistic. Thus, LOGSUM was integrated to our model as it is technically more rigorous, scientifically more precise and in line with our previous results.

Publisher

Association for European Transport