Achieving consistent geometric design is an important goal in highway desig
n to ensure obtaining safe, economical, and smooth traffic operation; Exist
ing operating speed models for design consistency in North America and Euro
pe are mainly based on two-dimensional (2D) analysis of highway horizontal
alignments. This gaper develops operating speed models for two-lane rural h
ighways 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 an Highway 61 and Highway 102 in Ontario. The results sho
w that there is a significant difference between the predicted operating sp
eed using the 2D and 3D models. Therefore, it is recommended that the devel
oped 3D models be used in highway consistency analysis and evaluation.