Rural two-lane highway shoulder and lane width policy evaluation using multiobjective optimization

Lane and shoulder widths are important highway safety factors because wider lanes and shoulders generally help to reduce crashes. In practice however, due to physical limitations of the right-of-way or statutory restrictions, it may be the case that the existing overall width of the roadway (lanes plus shoulders) cannot exceed a certain value. For a given overall roadway width constraint, wider lanes mean there will be smaller space for shoulders, and vice versa. In such zero-sum situations, it is not certain if a cross-sectional configuration with wide lanes and narrow shoulders is safer compared with one with narrow lanes and wide shoulders. Clearly, for a given total roadway width (TRW), the trade-off between shoulder and lane widths should be evaluated in order to maximize the user benefits (safety) without unduly incurring excessive agency life-cycle costs of construction and preservation. The TRW allocations to the lane and shoulder, expressed as the ratio of lane width to shoulder width, will generally depend on the TRW; lane and shoulder pavement material types and costs; and traffic volume, among other factors. In order to optimize the lane and shoulder width allocations for a given TRW, this paper proposes a framework that formulates the problem as an optimization problem with the objective of minimizing the total (agency and user) life-cycle cost of the roadway. First, the relationships are established between lane/shoulder widths and the resulting user costs (crashes), and also between lane/shoulder widths and the associated agency costs of lane/shoulder construction and preservation. The sensitivity of the optimal solution to different evaluation inputs (such as the relative weights between the agency and user costs) is also analyzed. Using the developed framework, the paper presents a number of decision support charts that can be used by highway agencies to determine the optimal lane and shoulder widths under a given set of conditions including the highway functional class and the total available roadway width. The flexibility of the proposed optimization framework to accommodate risk compensation effects was also demonstrated.