Urban Transportation System Optimum: the Effect of Accounting for the Interaction of Buses and Cars

Urban Transportation System Optimum: the Effect of Accounting for the Interaction of Buses and Cars


C A Guevara, Massachusetts Institute of Technology and Universidad de Los Andes, CL; E Valdés, J Dávila, Universidad de Los Andes, CL


Novel externality components, related with the interaction of buses and cars, are derived. Small simulating model and city wide model, based in real data, show that this framework implies relevant changes in optimal tolls


This research conveyed the analysis of the effect of the interaction of buses and cars, in the determination of optimal congestion tolls. First, based on the microeconomic analysis of equilibrium conditions, general expressions for the externality components were derived.

Afterwards, equilibrium under those conditions was simulated by means of two experimental mode choice and traffic assignment models. These exercises showed three main results: first, that for mixed use roads, optimal trip assignment implies car tolls which are significantly higher than usually estimated and, consequently, optimal car flow decrease for such type of roads as total demand grows; second, that devoting some lanes of mixed use roads for the exclusive use of buses, would yield equilibrium conditions which just partially resemble optimal road pricing; and third, that when bus system paths are not optimized, considering the external cost over bus riders in the calculation of tolls, would result in a situation which is socially worst compared to the case when this external cost is not considered.

The final stage in this research corresponded to the evaluation of these theoretical findings in an empirical model. This was attained by testing different schemes of road usage and tolls with the four-stages simultaneous equilibrium software ESTRAUS calibrated with real data from the year 2001 for the city of Santiago de Chile. This empirical evidence reinforced the conclusions attained in terms that optimal tolls become higher than usually calculated. Among other results, evidence also showed that reserved bus-lanes or streets were better than both practical cordon pricing and link specific Pigouvian, both of which resulted in a negative variation in social welfare. This unexpected last result was attributed to the facts that the differences in users? value of time was not considered in the calculation of link tolls, and that bus paths were also not optimized.


Association for European Transport