Impact of Road Geometry on Vehicle Energy Consumption and CO2 Emissions: An Energy-Efficiency Rating Methodology

Abstract

This study presents a methodology for classifying road traffic energy efficiency. The indicators defined discriminate the impact of the road vertical and horizontal alignments upon energy consumption, disclosing the improvement potential of the road as a function of the traffic origin–destination matrix. The methodologic approach is based on basic physical principals, thus guarantying its generality, as opposed to the usual empirical mesoscale approaches. A simplified algebraic procedure is also proposed, resorting to simplified driving cycles and a constant speed assumption (CSA), thus avoiding the intricacy of microscale/microsimulation models. The simplified methodology was validated against field data acquired on the Portuguese highway A25. A microscale vehicle specific power analysis combined with detailed fuel models is compared against CSA results. The findings demonstrate its adequacy for free-flow traffic conditions and the importance of classifying road traffic energy-efficiency. For the case studied, it was found that 49.5% of the round trip propulsive energy expended by a 37-ton truck on the A25, a modern road, was degraded as heat through braking. The difference found between the microscale analysis and CSA approach is 0.8%, despite the speed unevenness, varying between 32 and 96 km/h, with a standard deviation of 24% of the average speed.