Previous studies documented that a uniform design speed does not necessarily guarantee design consistency on rural two-lane facilities. Since a similar process is also followed for four-lane rural highways, it is reasonable to assume that similar inconsistencies could be found on such roadways. The operating speed-based method has been extensively used in other countries as the primary methods to examine design consistency. Numerous studies have been completed on rural two-lane highways for predicting operating speeds and evaluating design consistency. However, few studies have considered these issues for rural four-lane highways. Therefore, prediction models for rural four-lane highways are needed. This study aims to develop models to predict operating speeds on horizontal curves of rural four-lane highways. A parallel study documented that speeds on inside and outside lanes are different and therefore, two multiple linear regression models are developed. For the inside lane, the significant factors are: shoulder type, median type, pavement type, approaching section grade, and horizontal curve length. For the outside lane, the above factors were included as well as: left shoulder width, curve radius, and presence of approaching curve. The factors in the two models indicate that the curve itself mainly influences a drivers speed choice. The models were validated using the data splitting approach and the validation shows that there are no statistical differences between the predicted and field-observed operating speeds.
Abstract