6 Disequilibrium Network Design: A New Paradigm for Transportation Planning and Control Terry L. Friesz Departments of Systems Engineering and Operations Research & Engineering George Mason University Fairfax, VA 22030, USA 6.1 Introduction Samir Shah PB Farradyne Inc Rockville, MD 20852 USA David Bernstein Dept. of Civil Engineering and Operations Research Princeton University Princeton, NJ 08544, USA In a very broad sense, the network design problem (NDP) is a topic that has captured the attention of many researchers. This is mainly due to the immense importance of strategic capital investment decisions involving transportation infrastructure. For the purposes of this paper we limit the scope of the network design problem to highway systems. Design related decisions relevant to highways include a rich and wide variety of strategic (e.g., new right of way), tactical (one way street assignment, HOV assignment) and operational (traffic signalization, ramp metering) decisions that typically arise in transportation planning. The equilibrium network design problem is to find an optimal network design in terms of additional facilities or capacity enhancements, when the network flow pattern is constrained to be an equilibrium. The much reported occurrence of Braess' paradox (Murchland, 1970) requires that design models have Wardropian user equilibrium (Wardrop, 1952) constraints. The transportation research literature includes many such equilibrium design models wherein equilibrium constraints are generally articulated as an equivalent optimization problem (Abdulaal and LeBlanc, 1979; LeBlanc,1975) or as an equivalent variational inequality problem (Friesz et aI., 1990; 1992; 1993b; Marcotte, 1986). Unfortunately such equilibrium network design models presuppose a static environment and completely ignore the impacts of potential disequilibria which can arise due to perturbations in the capacity of the network infrastructure. Such a static perspective may lead to the occurrence of a temporal version of Braess' Paradox. The "temporal Braess' Paradox" does not yet enjoy a standard definition, but in the context of the present discussion can be viewed as occurring when a capacity altering action lowers (or leaves unchanged) overall delay in the present and near future but increases overall delay at some more distant time. Such outcomes are possible when the present value of disequilibrium impacts is substantially negative, as can occur when the immediate disequilibrium response is a sharp congestion increase or when the disequilibrium response is a mild congestion increase of relatively long duration. An example is provided by a highway construction project intended to enhance capacity but which produces traffic congestion for weeks or months prior to its completion; in this case, the present value of construction impacts may be sufficiently negative to eradicate all positive benefits in the post-construction period. This is particularly true in standard cost benefit kind of analyses, wherein near term benefits are more heavily discounted than long term L. Lundqvist et al. (eds.), Network Infrastructure and the Urban Environment © Springer-Verlag Berlin · Heidelberg 1998