It is well known that the operating speed varies continuously from the approach tangent to curve on the highways. In other words, it indicates the lack of consistency in geometric design element or combination of the design elements which fail to meet the driver's expectations to maintain the consistent operating speed. It is necessary to evaluate the inconsistency in highway geometric design elements by continuous measurement of the operating speed along the tangent to curve. The current study analyses and models the speed differential along the approach tangent-to-curve of the highway alignment. The data was collected for the heavy vehicles plying on the rural state highway and the major district road having geometrically constrained elements. High-end GPS data loggers were fixed in the vehicles to collect the required data. Variables such as speed, lateral-longitudinal acceleration/deceleration and temporal and spatial coordinates, of the vehicles were analysed, and models were developed to predict the 85th percentile speed differential (Δ85V) using continuous speed profile data. The analysis results revealed that, most of the times vehicle starts to decelerate near the approach tangent which continuous to the curve portion where it reaches minimum speed and again accelerates in the curve portion to the departure tangent till it attains the free-flow speed. The insights from the study can be used to evaluate the consistency in the geometric design elements of the rural highways for the heavy vehicles observed in developing world traffic. © 2020 The Authors. Published by Elsevier B.V.