Smoothness

Introduction

From their car to the pavement they drive on, drivers today value a smooth ride. And that begins with the roadway. Asphalt provides that—a smooth, continuous surface that adds to pavement longevity and requires less maintenance than rougher roads.1 In fact, the Federal Highway Administration (FHWA) has determined that pavement smoothness is a key factor in ensuring satisfaction for road users.2


Before and after: Potholes in I-264 in Norfolk

Key Pavement Technologies

What makes asphalt a better, smoother pavement choice? To begin with, asphalt pavements are constructed with a mixture of materials that are continuously paved for a smooth surface and without the need for transverse expansion joints every few yards, which disrupt the ride. But it goes further than that.

Because of how asphalt pavements are placed and designed, as well as their intrinsically flexible nature, they can provide and maintain a greater level of smoothness than other paving options over time.3 Smoothness is an indicator of quality construction and a road that is built smooth is more likely to remain smoother longer.4 A well-engineered foundation and pavement structure are important to ensure a road is smooth and long lasting. With a Perpetual Pavement design, long-life asphalt pavements are built to ensure that the structure lasts indefinitely. Maintenance is simply a matter of milling the surface and applying an asphalt overlay every 12 to 15 years, or longer.5 These periodic improvements underline asphalt’s ease of maintenance and ultimately contribute to the pavement’s consistent smoothness.

Asphalt overlays, including thin overlays, can also be used to restore performance to older pavements, including concrete pavements.6 Even when it may be necessary to place an overlay over very rough pavements, an asphalt overlay can restore and maintain a smooth ride.7,8 Best of all for the driving public, the overlay provides the experience and performance of a brand-new road.

Emerging Research

In an effort to ensure roads are smooth for drivers, the industry is working with partners to research new and improved pavement technologies.

Life Cycle Solutions constructed a user-friendly interface for FHWA Long-Term Pavement Performance (LTPP) pavement roughness data as measured by the International Roughness Index (IRI). This database, the IRI Explorer, provides an increased understanding of pavement maintenance cycles and road condition impacts on drivers, as well as a means to quantify a road’s drivability. The IRI Explorer also quantifies the environmental benefits of smooth pavements in terms of fuel savings and greenhouse gas (GHG) emission reductions associated with pavement construction, use, maintenance, and end-of-life.

Engineers at the National Center for Asphalt Technology (NCAT) are researching how technical advances in asphalt pavement mix design can improve pavement performance and increase a pavement’s lifespan.

Drivability

A smooth roadway not only provides drivers with peace of mind, it also increases vehicle fuel efficiency and decreases vehicle wear and tear. Contrary to some recent claims that pavement rigidity is a major contributing factor to vehicle fuel economy, the FHWA WesTrack Tests quantified the relationship between smooth pavements and improved fuel economy.9 Smoother pavements lead to lower fuel consumption — 4.5 percent lower in the WesTrack Tests.9 And smoother pavements help reduce wear and tear on vehicles, reducing maintenance costs for motorists.10 Well maintained pavements can also improve productivity by making travel and the delivery of goods and services more efficient.11

While manufacturers make strides in improving an automobile’s fuel economy, transportation agencies, researchers, and engineers are concurrently working to refine and build smooth roadways that do the same. All told, Americans burn 175 billion gallons of fuel driving approximately 3 trillion miles a year. If roads across the nation were smoother and maintained in good condition, approximately 4 percent of the fuel consumed could be saved, reducing annual vehicle fuel consumption by about 7 billion gallons — the equivalent of taking more than 10 million vehicles off the road every year.12 This would be a significant, tangible step in conserving fuel, our natural resources and benefiting the environment.

Sources

  1. Smith, K.L., L. Titus-Glover, L.D. Evans (2002). Pavement Smoothness Index Relationships, Final Report (FHWA-RD-02-057). Federal Highway Administration, McLean, Virginia. 

  2. FHWA (2002). Tech Brief: Help with Converting Pavement Smoothness Specifications (FHWA-RD-02-112). Federal Highway Administration, McLean, Virginia. 

  3. WSDOT (2012). “WSDOT Pavement Roughness (IRI) Report: 2010” in WSDOT Pavement Notebook. Washington State Department of Transportation, Olympia, Washington. 

  4. Perera, R.W., and S.D. Kohn (2002). Issues in Pavement Smoothness: A Summary Report, NCHRP Web Document 42 (Project 20-51[1]). TRB, National Research Council, Washington, D.C. 

  5. Pavement Interactive (2007). “Perpetual Pavements.” 

  6. Khazanovich L., R. Lederle, D. Tompkins, J.T. Harvey, & J. Signore (2012). Guidelines for the Rehabilitation of Concrete Pavements Using Asphalt Overlays FHWA TPF-5(149) Final Report. 

  7. FHWA (2000). Tech Brief: Performance Trends of Rehabilitated AC Pavements (FHWA-RD-00-165). Federal Highway Administration, McLean, Virginia.

  8. FHWA (1998). Tech Brief: Reducing Roughness in Rehabilitated Asphalt Concrete (AC) Pavements (FHWA-RD-98-149). Federal Highway Administration, McLean, Virginia. 

  9. Sime, M., S.C. Ashmore, and S. Alavi (2000). Tech Brief: WesTrack Track Roughness, Fuel Consumption, and Maintenance Costs (FHWA-RD-00-052). Federal Highway Administration, McLean, Virginia.

  10. TRIP (2012). Bumpy Roads Ahead: America's Roughest Rides and Strategies to Make our Roads Smoother. 

  11. Shatz, H.J., K.E. Kitchens, S. Rosenbloom, and M. Wachs (2011). Highway Infrastructure and the Economy: Implications for Federal Policy. RAND Corporation, Santa Monica, California. 

  12. APA (2011). Smoothness Matters. Asphalt Pavement Alliance, Lanham, Maryland.