Probabilistic and Economic Factors in Highway Geometric Design

This paper presents an integrated approach to geometric highway design which incorporates probabilistic and economic factors. It assumes that the characteristics of the traffic flow on the road are stochastic in nature. These traffic flow variables are inputs into physical and behavioral relationships that determine the properties of alternative designs. The choice of a specific design among the many possibilities is based on the minimization of an expected cost function that includes an explicit tradeoff between road users cost on the one hand and construction and maintenance costs on the other. The approach is demonstrated by an application to the design of a climbing lane. We postulate distributions of car and truck speeds on upgrades, formulate an expected cost function, and obtain a closed form solution for the optimal location of the climbing lane. It is shown that the current AASHO procedure, as an example, is a special case of the optimal solution based on simplified truck performance assumptions and a choice of a critical speed difference. A numerical example demonstrates the use of the proposed approach and the potential savings from an optimal location of a climbing lane.