Updated Commercial Truck Speed versus Distance–Grade Curves for the Highway Capacity Manual

The current version of the Highway Capacity Manual 2010 (HCM 2010) gives guidance on commercial truck speeds on grade through the use of truck speed versus distance–grade curves, which are located in the HCM 2010, Chapter 11. These curves are designed to allow the analyst to estimate a commercial truck's speed after it travels on a known distance of roadway with a certain grade. Typically, these curves are used to determine an equivalent grade (incline–decline and length) that will result in the same final speed of commercial trucks as would the series of grades making up a composite grade. The HCM 2010 curves were developed as a result of studies that used simulation tools. These simulation tools used a simple lookup table approach for maximum acceleration versus speed and did not account for commercial truck transmission gear–changing capabilities. These shortcomings, along with changes in truck performance over the past 20 years, necessitate an update to the HCM 2010 speed versus distance–grade curves. This study presents the development of updated speed versus distance–grade curves for commercial trucks by using TRUCKSIM. This simulation tool uses a modeling approach of full-vehicle dynamics, which ensures that commercial truck performance is accurately represented for field conditions. The results show that the performance of commercial trucks, as presented in the HCM 2010, is worse than that currently observed. Therefore, the authors recommend that the current speed versus distance–grade curves included in the HCM be replaced with those presented in this study.