UC Irvine Institute of Transportation Studies

This paper addresses deficiencies found in the current practice of highway geometric design and presents an alternative approach which achieves more meaningful and cost effective design. Current design practice suffers from vague definition of the design speed concept, is very inflexible, is insensitive to traffic volume and composition, does not explicitly consider cost factors, and is very costly. These deficiencies are primarily attributed to the deterministic approach utilized by current design practice. While all factors involved in the geometric design process (i.e., speed, friction, reaction time, etc.) are stochastic in nature and are fully distributed among the road users, the current approach relies on a single arbitrarily chosen value to represent each factor. 

This paper presents an alternative approach to geometric design of highways. This approach is fully sensitive to the real conditions of the design problem at hand (i.e., the traffic volume and composition), because it incorporates the stochastic nature of the various factors involved into the design process. The proposed approach also achieves an optimal, or a cost-effective, design which takes into account all the cost elements associated with the highway. An empirical example of horizontal curve design is presented to demonstrate the advantages of the proposed approach.