Network reliability-based optimal toll design

Journal of Advanced Transportation, Fall, 2008 by Hao Li, Michiel C.J. Bliemer, Piet H.L. Bovy

Optimal toll design from a network reliability point of view is addressed in this paper. Improving network reliability is proposed as a policy objective of road pricing. A reliability-based optimal toll design model, where on the upper level network performance including travel time reliability is optimized, while on the lower level a dynamic user-equilibrium is achieved, is presented. Road authorities aim to optimize network travel time reliability by setting tolls in a network design problem. Travelers are influenced by these tolls and make route and trip decisions by considering travel times and tolls. Network performance reliability is analyzed for a degradable network with elastic and fluctuated travel demand, which integrates reliability and uncertainty, dynamic network equilibrium models, and Monte Carlo methods. The proposed model is applied to a small hypothesized network for which optimal tolls are derived. The network travel time reliability is indeed improved after implementing optimal tolling system. Trips may have a somewhat higher, but more reliable, travel time.

Introduction

Road pricing has been advocated as a potentially powerful travel demand management (TDM) strategy capable of significantly influencing travel demand characteristics (Cain et al., 2001; Small et al., 2005). Generally, the policy objectives of road pricing can be summarized as: managing demand, optimizing congestion level, reducing environmental impacts, maximizing social welfare gains, raising revenues to recoup maintenance cost and construction cost, etc. The optimal charges obtained with different policy objectives are different.

Nowadays, reliability is becoming an increasingly important performance characteristic of road networks and transport services. Reliability is generally defined as the ability of a system or process to perform a required function under given environmental and operational conditions and for a stated period of time (see Hoyland and Rausand, 1994). With regard to road networks, the operational conditions could be normal and abnormal. The required function can be described in terms of output variables like travel time, throughput, accessibility, equity, and so forth (Yin and Ieda, 2001). Reliability, however defined, provides a measure of the stability of the quality of service, which the transport system can offer to its users. Whereas road pricing has been mainly considered to relieve congestion, it will be argued in this paper that it can also help provide more reliable transport services to the users.

To our best knowledge, no studies have adopted network travel time reliability as an objective of road pricing to design toll systems. Some researchers (e.g., Brownstone and Small, 2005) suggested to evaluate road pricing from a network reliability perspective. Some other researchers (Chan and Lam, 2005; Li et al., 2006) have investigated the impacts of road pricing on network reliability. This paper proposes an optimal toll design model with network travel time reliability as one of the policy objectives.

Definition of Network Reliability-Based Toll Design Problem

In an optimal toll design problem, different optimal toll schemes will result from different policy objectives. Nowadays reliability of a road network, the stability of the service the road authority can provide to its users, is becoming increasingly important. Designing toll levels from such a reliability perspective is a new proposition of the road pricing objective. Conceptually, in the short term, road pricing influences travelers' trip choices, departure time choices, mode choices, and route choices. Trip choices and mode shifts together may result in a drop of travel demand during the tolling period. Departure time choices, in which travelers depart earlier or later to avoid traveling in the tolled period, lead to temporal shifts of travel demand, thus a decrease of travel demand as well within the tolling period. Consequently, decreased travel demand and route flow shifts in the charged period are the prevailing determinants of network reliability.

In this paper, it is assumed that the road authority aims to improve road network reliability and to optimize system performance (i.e., improve reliability and minimize travel time on a network level). The tolls set by the road authority are assumed to influence travelers in their route and trip decisions where the decision to make a trip or not during the tolled period represents temporal and modal shifts as well. In addition, the travelers are assumed to take only travel time and tolls into account when making these decisions, however not trip reliability. Therefore, in this paper reliability is predominantly viewed as a network objective from a planning point of view, however it could be extended towards a user's objective as well (for future work). Network performance reliability will be analyzed considering stochastic network characteristics (i.e., stochastic link capacities), elastic demand and fluctuated travel demand from day-to-day. A network reliability-based optimal toll design model, by choosing the optimal toll levels for a subset of links, subject to dynamic equilibrium traffic assignment is proposed in this paper. A dynamic travel analysis is followed because it will give more accurate estimates of congestion patterns and derived trip characteristics such as travel times. Due to the integrated stochastic properties, dynamic assignment, elastic demands, and various toll schemes, this reliability-based toll design problem becomes very complex, that's why the application is only done to simplistic networks so far.