How smooth should a newly paved road be? The answer depends on several factors, including the road’s material and geographic location.
Most people agree that smooth roads are preferable to rough roads. Smoother roads provide a more pleasant driving experience, are safer, require less maintenance, and cost vehicle owners less money. However, the exact level of smoothness required to meet construction specifications varies from state to state.
Let’s begin with a crash course in road profiling, smoothness, and ride quality. Several indices are used to rate pavement smoothness. In all cases, the first step is to collect accurate data. Then the index is calculated, both for overall smoothness and localized roughness. Overall smoothness looks at average smoothness values over longer segments of the road, typically 0.1 mile or 528 feet. Localized roughness identifies specific locations that stand out and may need corrective action. These locations are typically referred to as “hot spots.”
Currently used roughness indices include:
- Profilograph Index — The Profilograph Index (PrI) data is collected with a profilograph. A blanking band is generally applied to the profilograph trace, and the PrI is based on the scallops outside of that band.
- Ride Number — Ride Number (RN) estimates the Mean Panel Rating, or public opinion of ride quality on a scale of 0 to 5. It is computed using profile data from a valid profiler.
- International Roughness Index — The International Roughness Index (IRI) is calculated using a mathematical model that predicts vehicle response to a “true profile” of the pavement’s surface in the wheelpaths. Profiles used to compute the IRI are collected with any valid profiler (inertial profiler, inclinometer-based device, rod-and-level, etc.).
Variations of IRI:
- Mean Roughness Index (MRI) is the average of the IRI of the left wheelpath and the IRI of the right wheelpath.
- Half-car Roughness Index (HRI) is effectively the IRI of a profile created from a point-by-point average of the left profile and right wheelpath profiles.
With a variety of indices to choose from, what are state departments of transportation (DOTs) specifying as smoothness requirements, how are pavement profiles collected, and what qualifies for incentive or disincentive pay? To find out, pavement engineering firm The Transtec Group recently conducted a study of current state-of-the-practice for pavement smoothness specifications in the United States for the Southeast Transportation Consortium (STC) and the Louisiana Transportation Research Center (LTRC). According to the project’s final report, this study aimed “to document and summarize ongoing and completed research related to pavement smoothness, best construction practices for achieving required pavement smoothness, agency specifications/criteria for IRI-based pavement smoothness, technologies and practices for IRI collection and processing, and educational and training practices for agency and contractor personnel.”
The comprehensive research included:
- conducting a literature search on pavement smoothness-related topics,
- surveying states along with Canadian provinces through the American Association of State Highway and Transportation Officials (AASHTO) Subcommittee on Materials,
- collecting current smoothness specifications from all states, and
- developing recommendations for achieving smooth pavements through construction best practices.
The survey of state representatives of the AASHTO Subcommittee on Materials resulted in responses from 35 states plus the Canadian province of Ontario. While beneficial, additional information was required to provide a more complete picture of practices nationwide. This was accomplished by gathering and reviewing U.S. state smoothness specifications. The results’ scope was limited to new construction or full-depth reconstruction, with the understanding that most agencies have different requirements for rehabilitation projects encompassing overlays (and mill and overlays/inlays) for asphalt pavement and diamond grinding for concrete pavement. Any state with an IRI-based specification, whether a full specification or special specification/special provision, is included in the summary.
Pavement smoothness specifications have evolved significantly during the last decade. States are increasingly transitioning from profilograph-based smoothness specifications to specifications based on IRI. Currently, 78 percent (up from 66 percent in 2009) of asphalt pavement specifications are IRI-based and 46 percent (up from 21 percent in 2009) of concrete pavement specifications are IRI-based.
As shown in Figure 1, most states use IRI-based specifications for asphalt pavement. Nine states call for the use of the profilograph index, and only two work with ride number. Of those states with IRI-based indices, more than half (59 percent) base construction acceptance on MRI, three states use HRI, and the rest use IRI.
For concrete pavements, smoothness specifications are more evenly split between IRI and PrI. As shown in Figure 2, 46 percent of states use a form of IRI in concrete specifications, and 42 percent call for PrI. Six states do not have concrete smoothness specifications. As with asphalt, of those states with IRI-based indices, more than half (57 percent) base construction acceptance on MRI. Colorado is the only state that bases acceptance of concrete pavements on HRI.
What it’s worth
DOTs prescribe certain levels of smoothness in pavement specifications and often penalize or provide bonuses to contractors depending on the smoothness they provide. Figure 3 summarizes IRI and MRI pay adjustment thresholds for concrete and asphalt pavement. Note the following:
- The values for incentive pay adjustment represent the minimum, maximum, and average upper limits for when incentive payment is given.
- The values for full pay are the minimum, maximum, and average values for both the lower limit (LL) and upper limit (UL) of full payment.
- The values for disincentive pay adjustment are the minimum, maximum, and average values for both the lower limit (i.e., value at which disincentives would be applied) and upper limit (i.e., value at which disincentive can still be applied rather than requiring correction) for disincentives.
- The values for correction are the minimum, maximum, and average values at which agencies require corrective action before acceptance.
Other notable findings from the report:
- Of the states with IRI-based specifications, 85 percent provide incentive and disincentive pay adjustments for asphalt pavements, while 78 percent provide these adjustments for concrete pavements.
- There is still a fairly wide range of IRI thresholds for incentives, disincentives, full pay, and correction, and no general consensus on what thresholds are most appropriate.
- There is a wide range of pay adjustments for pavement smoothness, with the majority of states applying pay adjustments on a dollar amount-per-lot basis versus a percentage of the contract price.
- Although most states have localized roughness provisions, there are a variety of localized roughness methodologies used and no general consensus as to which is best. Methods include continuous IRI, fixed interval IRI, profile moving average, profilograph simulation, and straightedge only.
- Several states permit a flat dollar amount pay deduction for areas of localized roughness in lieu of correction.
Putting it into practice
The United States hosts a wide range of smoothness specifications. However, regardless of location or material, a few construction practices can help to improve pavement smoothness. For contractors aiming to construct smooth pavements, the following best practices are recommended:
Plan and communicate. Think through the entire paving operation. Make contingency plans in case issues arise with equipment, weather, or materials. Communicate plans to everyone involved and hold pre-paving meetings.
Use quality materials in construction and ensure material quality and consistency from batch to batch, along with a steady, uninterrupted supply of material.
Continually monitor your work. Whenever practical, smoothness numbers should be checked on a daily basis so corrections can be made. FHWA’s ProVAL profile viewing and analysis software (www.roadprofile.com) is a useful tool.
Additional best practices specific to asphalt pavements and concrete pavements are shared in the project’s final report.
Jennifer Rutledge is a project manager at Transtec.
David Merritt, P.E., also a Transtec project manager, served as co-principal investigator for the project, Best Practices for Achieving and Measuring Pavement Smoothness, A Synthesis of State-of-Practice. Austin, Texas-based The Transtec Group (www.thetranstecgroup.com) provides engineering in pavement design, design-build, public-private partnerships, construction, research, pavement surface testing, pavement software development, and technology implementation.