Review and Evaluation of Interchange Ramp Design Considerations for Facilities Without Frontage Roads

Based on a recent change in TxDOT policy, frontage roads are not to be included along controlled-access highways unless a study indicates that the frontage road improves safety, improves operations, lowers overall facility costs, or provides essential access. Interchange design options that do not include frontage roads are to be considered for all new freeway construction. Ramps in non-frontage-road settings are more complicated to design than those in frontage-road settings for several reasons. Adequate ramp length, appropriate horizontal and vertical curvature, and flaring to increase storage area at the crossroad intersection should all be used to design safe and efficient ramps for non-frontage-road settings. However, current design procedures available in standard TxDOT reference documents are focused on ramp design for frontage-road settings. The objective of this research project is to develop recommended design procedures for interchange ramps on facilities without frontage roads. This report describes the findings from the first year of a two-year project. Current ramp design practices were reviewed, and the operational features of interchange ramps in non-frontage-road settings were evaluated. Both the diamond and partial cloverleaf interchange forms were considered. The findings from the evaluation of the field and simulation data indicate that a rational approach to interchange type selection and operational evaluation is feasible using the critical movement analysis approach. A procedure for using this approach is documented in the Appendix.