Reconciling Speed Limits with Design Speeds

In recent years, context-sensitive highway design has been promoted to ensure that designers consider the environmental, scenic, aesthetic, historic, community, and preservation aspects of the road. Context-sensitive design may lead to situations where the current design standards cannot be met because of restricting local conditions. Indiana has road sections designed and built some time ago. In a considerable number of roads with the statutory limit of 55 mph (90 km/h), the road geometry does not meet the current standards. At individual intersections and on curves, advisory speeds are posted together with warning signs. Although this solution increases the safety of road users and allows for traveling at reasonably high speeds outside of these segments, the final solution is to upgrade their geometry to the desirable level. This report presents models that predict user-selected percentile free-flow speeds on two-lane rural and four-lane rural and suburban highways. The percentile speeds are computed as a linear combination of the mean speed and the standard deviation in panel data models with random effects. The developed percentile speed models involve more design variables than typical speed models, and separate the mean speed factors from the speed dispersion factors. These benefits ease the model interpretation and its use in highway design. The study results should help designers bring the predicted speed to the desired speed as close as possible given the budget constraints. Engineering judgment can then be applied to balance safety and construction cost in highway improvement projects.