Prediction of Operating Speed on Three-Dimensional Highway Alignments

Achieving consistent geometric design is an important goal in highway design to ensure obtaining safe, economical, and smooth traffic operation. Existing operating speed models for design consistency in North America and Europe are mainly based on two-dimensional (2D) analysis of highway horizontal alignments. This paper develops operating speed models for two-lane rural highways that account for the three-dimensional (3D) nature of highways. The models will help highway designers to predict operating speed and evaluate design consistency more accurately, and thus aid highway safety. Two types of 3D combinations were considered: a horizontal curve combined with a sag vertical curve and a horizontal curve combined with a crest vertical curve. Regression analysis was used to develop the operating speed models based on data collected on Highway 61 and Highway 102 in Ontario. The results show that there is a significant difference between the predicted operating speed using the 2D and 3D models. Therefore, it is recommended that the developed 3D models be used in highway consistency analysis and evaluation.