Evaluation of highway consistency and safety: Practical application

Current design practices in North America are based mainly on 2D analysis of the highway alignment and do not account for the operating speed adopted by drivers. A more accurate approach should consider the interaction among the horizontal alignment, vertical alignment, and cross section based on the predicted operating speed. By combining the operating speed models along with 3D sight distance and vehicle dynamics models, a practical application of this approach is presented using an 8-km segment of Highway 61 (Ontario, Canada). By comparing the profile of predicted operating speed to the profile of allowable speed, based on 3D sight distance and vehicle dynamics, the consistency or deficiency in the highway segment is identified. The paper demonstrates the applicability of the 3D models to real highway segments and their usefulness in evaluating the adequacy of new designs or roads already in service. As the difference between the 2D and 3D results is significant, the 2D analysis is proven to be inadequate for such an evaluation, and the need for a 3D analysis is evident. Also, field measurements show that the accuracy of operating speed models is considerably compromised on combined horizontal and vertical curves. Thus, the results indicate the need to develop operating speed models that account for the effect of combined alignments and nighttime driving.