The selection of general alignment and gradient for a road is posed as a unique design decision driven by the objective of minimizing total life-cycle costs of construction and vehicle operation. Inputs to the design decision process include the terrain between the origin and destination; the roadway geometry and surface type; the vehicle volume, mix, and growth rate; unit construction and vehicle operating costs; design life; and interest rate. Outputs include combinations of horizontal alignments and piecewise gradients, representing various optima based on combinations of the life-cycle cost components. These outputs provide the basis for the subsequent design decisions. The analytical procedure includes a basic cost model that reduces the terrain to a number of grade-constrained construction surfaces by using linear programming and a route selection model that computes the life-cycle costs of various alternative alignments over the surfaces and selects the ones with the least cost by using shortest path and next-best path techniques. The implications of a total-cost approach for horizontal and vertical design standards are discussed. The overall implication, however, is the potential obsolescence of predetermined geometric design standards for other than urban roads and intersections, because these standards can be uniquely determined as outputs of an analytical process. This paper appears in transportation research record no. 1291, Fifth international conference on low-volume roads, may 19-23, 1991, raleigh, north carolina, volume 2.
Abstract