Modeling 85th Percentile Speed Using Spatially Evaluated Free-Flow Vehicles for Consistency-Based Geometric Design

This paper investigates the free-flow condition suitable for four-lane divided highways and proposes spatial free-flow speed–based operating speed prediction models. The 85th percentile speed ($V_{85}$) of spatially evaluated free-flow vehicles (passenger car, light commercial vehicle, and heavy commercial vehicle) at the curve center were analyzed with respect to various geometric parameters to develop these models. Among various multiple linear regression techniques explored, the backward elimination method was found to be the best for the adopted data. The curve length ($CL$), deflection angle ($\mathrm{\Delta }$), and preceding tangent length ($PTL$) were identified as the explanatory variables of the developed models. These models were validated for three different sites located in the western and northeastern parts of India. The explanatory variables $\mathrm{\Delta }$ and $CL$ can indicate curve appearance; this is in conformation with the design guideline’s recommendation of adjusting $\mathrm{\Delta }$ and $CL$ for a directional and smooth appearance. Therefore, the proposed models can be used to integrate the quantitative and qualitative guidelines for horizontal curve design. Further, they support the design guidelines’ recommendation to avoid a longer tangent prior to a sharp curve. Sensitivity analysis indicated the passenger car (PC) as the most critical vehicle category. Hence, the speed prediction model of PC was used for developing the nomograms for consistency-based geometric design of horizontal curves along four-lane divided highways.