Environmentally Conscious Highway Design for Vertical Grades

This paper provides guidelines and tools for quantitative evaluations by means of fuel consumption and emissions for various vertical grade design conditions. Detailed vehicle operation conditions were represented by second-by-second speed profiles generated with the truck dynamic model and nonuniform acceleration and deceleration models. The rates of fuel consumption and emissions based on vehicle-specific power and speeds were extracted by using the recently developed Motor Vehicle Emission Simulator. The generated speed profiles were matched with the extracted rates and aggregated during a trip on the grades. As a result, the design vehicle consumed more than five times the amount of fuel on a 3,000-m graded segment when the vehicle had a speed reduction of greater than 20 km/h, as opposed to the vehicle with a speed reduction of less than 10 km/h. For emissions—carbon dioxide, carbon monoxide, oxides of nitrogen, hydrocarbons, and particulate matter of 2.5 microns or less—there were up to fivefold increases on the segment. Thus, controlling a speed reduction of less than 20 km/h can minimize adverse environmental effects from vehicle movements. In addition, this study shows that environmentally conscious vertical grade design is feasible and economically beneficial throughout the life of the highway. The provided guidelines and tools can reduce uncertainty for designing environmentally conscious highways and can be used as part of the highway design process.