Vehicle Fuel Consumption And Pavement Characteristics
Vehicle fuel consumption and associated emissions from combustion are influenced by a large number of factors including vehicle and cargo mass, engine size and type, fuel type, tire type and inflation, driving behavior, vehicle maintenance, grades and curves, traffic congestion, traffic control, wind, and several other factors, as well as the number of miles traveled. In fact, many of these have a greater influence on fuel economy than pavement characteristics. However, pavements can influence the fuel efficiency of vehicles-and therefore the associated GHG and air pollution emissions as well-through three mechanisms that together are called pavement-related rolling resistance. A discussion of the basic concepts of rolling resistance considering the total system of the vehicle components, pavement and road geometry, and measurement techniques is included in a report edited by Sandberg (2011). Another report (Jackson et al. 2011) also includes a summary of the principles of rolling resistance and its measurement. The pavement influences on these rolling resistance mechanisms are summarized as follows:
- Roughness-consumption of vehicle energy through the working of the suspension system and drive train components, and deformation of tire sidewalls as the wheels pass over deviations from a flat surface in the wheelpath with wavelengths greater than 1.6 ft (0.5 m) and less than 164 ft (50 m). The working of these vehicle components converts mechanical energy into heat that is then dissipated into the air, requiring greater work by the engine than would be necessary to propel it along a flat surface. Roughness is both built into the pavement during construction and materializes over time as the pavement ages and distresses develop, and is further influenced by subsequent maintenance and rehabilitation treatment applications and timing. Roughness on some pavement types can undergo relatively small changes with daily temperature fluctuations. For a given roughness condition, this rolling resistance mechanism affects all vehicles all the time.
- Macrotexture-consumption of vehicle energy through the viscoelastic working of the deformable tire tread rubber in the tire-pavement contact patch as it passes over positive surface macrotexture, converting it into heat dissipated into the rest of the tire and into the air. Positive macrotexture is produced by stones or other texture protruding above the average plane of the pavement surface with wavelengths of 0.2 to 2 inches (5 to 51 mm). It is the primary pavement characteristic controlling surface friction at high speeds under wet conditions and the associated potential for hydroplaning (Anderson et al. 1998; Panagouli and Kokkalis 1998; Flintsch et al. 2002). Pavements serving high-speed vehicles must have a minimum amount of surface macrotexture and/or sufficient permeability to remove water films from the pavement surface so that frictional resistance is maintained for steering and braking. Macrotexture is provided by the characteristics of the surfacing materials (primarily relevant to asphalt surfaces) and texturing (primarily relevant to concrete surfaces), as well as subsequent maintenance and rehabilitation timing and treatment type. Macrotexture does not change due to daily or seasonal temperature and moisture conditions, although it can increase or decrease with age depending on the pavement surface materials, texture type, traffic, climate and use of chains or studded tires. For a given macrotexture, this rolling resistance mechanism affects all vehicles all the time.
- Structural Responsiveness-consumption of vehicle energy in the pavement itself through deformation of pavement materials under passing vehicles, including delayed deformation of viscoelastic materials and other damping effects that consume energy in the pavement and subgrade. This mechanism has also been characterized in terms of the delayed deformation of the pavement under the wheel such that the moving wheel is continually on a slope (Flugge 1975; Chupin, Piau, and Chabot 2013). Pavement structural responsiveness to loading is determined by layer thicknesses, stiffnesses and material types that determine viscoelastic and elastic pavement response under different conditions of wheel loading and vehicle speed, and temperature and moisture conditions. For a given pavement structure, the effect of this mechanism on viscoelastic materials such as asphalt can be highly dependent on daily and seasonal changes in pavement temperatures (particularly near the surface), and is more sensitive to vehicle speeds and loading than are roughness and macrotexture. Structural responsiveness can change with time.
Practices that are available to pavement managers, designers, and specification developers that might be optimized to help meet GHG emission, energy use, and other environmental objectives associated with the influence of pavement characteristics on vehicle fuel economy are summarized in table 1.
| Vehicle Fuel Consumption and Pavement Objective | Vehicle Fuel Consumption Sustainability Improving Strategy | Economic Impact | Environmental Impact | Societal Impact |
|---|---|---|---|---|
| Reduce Fuel Use Due to Roughness | Implement pavement design process that considers smoothness over the pavement life as a key design parameter, especially for high traffic volume routes. | Potential for small to moderate increases in initial costs but reduced life-cycle costs due to longer pavement lives. Reduced vehicle operating costs for road users. | Reduced environmental impact due to less fuel use, particularly on high traffic volume routes. | Improved economic efficiency. |
| Implement construction specifications to incentivize maximum possible smoothness, especially for high traffic volume routes. | Potential for small increases in construction costs, reduced life-cycle costs due to longer treatment lives from reduction in dynamic loading. Reduced vehicle operating costs for road users. | Reduced environmental impact due to less fuel use, particularly on high traffic volume routes. | Improved economic efficiency. | |
| Optimize timing of maintenance and rehabilitation based on IRI trigger value and traffic volume. | Potentially increased agency initial costs if results in earlier treatment than current practice. Potentially reduced agency life-cycle cost from pavement preservation. Reduced vehicle operating costs for road users as pavements are kept in smoother condition. | Increased environmental impact of materials production and construction when treatments are more frequent; reduced environmental impact due to less fuel use. Benefit can be offset if vehicle speeds increase because of improved smoothness. | Emphasis on maintaining high-volume routes in smoother condition may improve economic efficiency and average road user cost, but may result in neglect of lower volume routes depending on funding levels. | |
| Minimize pavement roughness due to utility cuts through regulation, construction practice enforcement and better planning. | Reduced pavement maintenance costs. Increased enforcement costs. Reduced vehicle operating costs for road users. | Reduced environmental impact due to less fuel use when poorly repaired utility cuts cause roughness, particularly on high traffic volume routes. | Improved economic efficiency. Improved urban aesthetics. | |
| Reduce Fuel Use Due to Macrotexture (where impact is significant) | Avoid high positive macrotexture on routes with high heavy truck traffic volumes at slow speeds while maintaining safety. | May result in less use of some low-cost maintenance treatments with high positive macrotexture over the life cycle on high-volume heavy truck routes. | Reduced environmental impact due to less fuel use on high traffic volume heavy truck routes. | Improved economic efficiency, reduced tire wear. Potential for increased crashes due to reduced surface friction if also high speed traffic. |
| Calibrate and Validate Models for Fuel Use Due to Structural Responsiveness to Vehicle Loading (use them once research is completed) | Perform research to calibrate and validate models for vehicle fuel use as a function of pavement structural responsiveness to vehicle loading. Calibration requires experiments that characterize responsiveness of pavement sections and then measure fuel use on same sections. Calibrated models can be used to determine where structural responsiveness is significant and develop appropriate strategies based on those results. | Calibrated models will permit evaluation of alternative structures considering traffic, climate and other variables which will allow consideration of both road user and agency costs versus environmental benefits for designs. | Optimization may reduce environmental impact due to less fuel use, particularly on high truck traffic volume routes in certain climates. | Optimization may improve economic efficiency particularly on high truck traffic volume routes. |
See Chapter 6 (.pdf) of the Reference Document for more details.
References
Anderson, D. A., R. S. Huebner, J. R. Reed, J. C. Warner, and J. J. Henry. 1998. Improved Surface Drainage of Pavements. NCHRP Web Document 16. Transportation Research Board, Washington, DC.
Chupin, O., J. M. Piau, and A. Chabot. 2013. "Evaluation of the Structure-Induced Rolling Resistance (SRR) for Pavements Including Viscoelastic Material Layers." Materials and Structures. Vol. 46, No. 4. Springer Netherlands.
Flintsch, G., E. de León, K. McGhee, and I. Al-Qadi. 2002. "Pavement Surface Macrotexture Measurement and Applications." Transportation Research Record 1860. Transportation Research Board, Washington, DC.
Flugge, W. 1975. Viscoelasticity. Berlin-Heidelberg-New York. Springer-Verlag, New York, NY.
Jackson, R., J. R. Willis, M. Arnold, and C. Palmer. 2011. Synthesis of the Effects of Pavement Properties on Tire Rolling Resistance. Report No. 11-05. National Center for Asphalt Technology, Auburn, AL.
Panagouli, O. K. and A. B. Kokkalis. 1998. "Skid Resistance and Fractal Structure of Pavement Surface." Chaos, Solutions and Fractals. Vol. 9, No. 3. Elsevier, Philadelphia, PA.
Sandberg, U. 1997. Influence of Road Surface Texture on Traffic Characteristics Related to Environment, Economy and Safety: A State-of-the-Art Study Regarding Measures and Measuring Methods. VTI notat 53A-1997. VTI (Swedish National Road and Transport Research Institute), Linkoping, Sweden.