TECHBRIEF Relating Ride Quality and

Structural Adequacy for

Pavement Rehabilitation and

Management Decisions

FHWA Publication No.: FHWA-HRT-12-046

FHWA Contact: Larry Wiser, HRDI-30, (202) 493-3079, 

larry.wiser@dot.gov

This document is a technical summary of the Federal Highway 

Administration (FHWA) report, Relating Ride Quality and Structural 

Adequacy for Pavement Rehabilitation/Design Decisions (FHWAHRT-12-035).(1)

This TechBrief discusses the relationship between pavement ride 

quality and structural adequacy. It presents highlights from a literature search and findings from the analysis and comparison of ride

quality and structural adequacy data from the Long-Term Pavement 

Performance (LTPP) program. Although a viable relationship between 

the two parameters could not be established, the lack of correlation 

is considered of value to pavement management system (PMS) 

practitioners, as it indicates that good ride quality does not mean 

good structural support or vice-versa. This becomes an important 

consideration for those who want to base performance measures on 

ride quality indicators.

Background

State highway agencies spend billions of dollars each year on transportation infrastructure assets to meet legislative, agency, and public 

expectations. Pavements are a major component of those assets, and 

pavement rehabilitation is one of the most critical, costly, and complex 

elements because it preserves pavements to extend their service life 

and, more importantly, to improve motorist safety and satisfaction and 

save public tax dollars.

PMSs are at the heart of the rehabilitation decisionmaking process. 

Earlier PMS generations considered ride quality and distress as direct 

results of the American Association of State Highway Officials Road 

Test. With advances in technology, PMSs started to use distress (cracking, rutting, etc.) and longitudinal roughness (typically the International 

Roughness Index (IRI)) as key performance indicators in the decisionmaking process. Both are important indicators that merit emphasis 

within the PMS process, but they are not the only ones. Structural 

adequacy, for example, is another important indicator for making 

rational pavement investment decisions, and many State highway 

agencies are incorporating deflection testing as part of their routine 

PMS activities.

Although ride quality and structural adequacy are key performance 

indicators, the relationship between them has been a topic of frequent and continuing discussion in the pavement community. To 

date, an accepted and widely used relationship has not been identified.

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

http://www.fhwa.dot.gov/

research/tfhrc/programs/

infrastructure/pavements/ltpp/

The Long-Term Pavement Performance 

(LTPP) program is a large research project

for the study of in-service pavements 

across North America. Its goal is to extend 

the life of highway pavements through 

various designs of new and rehabilitated 

pavement structures, using different materials and under different loads, environments, subgrade soil, and maintenance 

practices. LTPP was established under the 

Strategic Highway Research Program and 

is now managed by the Federal Highway 

Administration.
2

Recognizing the benefits of such a relationship,

FHWA sponsored a study to identify the relationship between ride quality and structural adequacy

(if any) using the LTPP data. This was done to develop 

a mechanism to include both ride and structural 

adequacy values within the context of current 

network-level PMS practices. This TechBrief presents 

the major findings and conclusions from the study.

Literature Search

The purpose of the literature search was to collect, 

review, and synthesize available information related 

to ride quality and structural adequacy in support of 

pavement rehabilitation and design decisions. A total 

of 62 references were identified, but only 16 of them 

were considered relevant to the study. The majority 

(62 percent) of these references were from LTPPrelated studies, and the remainder (38 percent) were 

from universities or highway agencies. In addition, 

the majority (69 percent) of these references were 

articles, and the remainder (31 percent) were reports. 

Table 1 shows the distribution of the 16 references 

by topic area.

Although a couple of studies indicated a potential 

relationship between structural response and pavement performance, neither of them established a 

direct relationship. Several studies were successful 

in relating ride quality to pavement condition or 

distress. In fact, the roughness models in the 

American Association of State Highway and Transportation Officials (AASHTO) Mechanistic-Empirical 

Pavement Design Guide use distress as an input to 

predict roughness.(2) Similarly, structural response 

has been used to predict certain distresses. While 

it is evident that many factors influence pavement 

performance in different ways, a strong correlation 

between the three different performance measures 

(i.e., ride quality, pavement distress, and structural adequacy) has yet to be established such that 

measurement of any one performance indicator

is adequate for understanding the structural and 

functional condition of the pavement.

Data Review and Analysis

The best source of data to accomplish the study 

objective was the LTPP program, which was established to explain how pavements perform and why 

they perform as they do. In particular, the use of data 

associated with Specific Pavement Studies (SPS)-1 

(new hot mix asphalt (HMA) pavements), SPS-2 

(new jointed concrete pavements), and SPS-5 

(rehabilitation of existing HMA pavements) project 

sections were considered particularly relevant, as 

they capture performance and the factors influencing

it throughout the entire pavement life cycle. The 

pavement sections selected for use in the study 

were grouped into the following categories:

• Group 1: Seven SPS-1 sections included 

pavements with a wide range in IRI values 

changing over time.

• Group 2: Four SPS-1 sections exhibited significant decreases in structural adequacy over time.

• Group 3: Four SPS-1 sections included different 

base types exhibiting structural differences. 

• Group 4: Three SPS-5 sections exhibited significant increases in IRI since overlay.

• Group 5: Three SPS-2 sections exhibited significant increases in IRI over time.

A number of parameters were investigated to select 

an appropriate ride quality indicator, but IRI was 

identified as the best one for the study. More 

specifically, continuous IRI plots using a 25-ft base 

length were used to look at how IRI varied with 

distance along the section. Such plots showed the 

distribution of roughness along the section, with 

any point on a 25-ft base length continuous IRI plot 

showing the average IRI of a 25-ft-long segment 

centered at that point. Only the right wheel path 

was considered in the study as it matched the falling

weight deflectometer (FWD) test path. Similarly, 

many parameters were considered to select an 

appropriate structural adequacy indicator. Of these, 

Table 1. Distribution of literature search references by type and topic.

Type/Topic

Relationship 

Between Ride 

and Structural 

Adequacy

Factors 

Influencing 

Pavement 

Performance

Structural- and 

Functional-Based 

Approaches 

for Pavement 

Evaluations

Relationship 

Between Ride 

and Distress Total (Percent)

Report/guideline 2 1 0 2

5 

(31 percent)

Article/

presentation 0 6 1 4

11 

(69 percent)

Total

2 

(13 percent)

7 

(43 percent)

1 

(6 percent)

6 

(38 percent)

16 

(100 percent)
3

the AASHTO effective pavement structural number

(SN) for flexible pavements and effective concrete 

slab thickness (Deff) for rigid pavements were 

selected. FWD deflection data were used to estimate 

both SN and Deff. 

Continuous IRI and SN or Deff plots were developed 

to evaluate and compare the relationship between 

changes in ride quality and structural adequacy. 

These changes were evaluated using the initial and 

latest profile survey and FWD test data available 

in the LTPP database for each section.(3) The resulting 

plots were then normalized to visualize the percent 

changes in IRI and SN or Deff. Figure 1 illustrates 

these plots for section 010102 in Alabama, which is 

one of the group 1 sections. As shown, there was 

a significant increase in IRI over the last 300 ft of 

the section when compared to the first 200 ft of the 

section. The decrease in SN for the last 300 ft of the 

section was also greater, but the decrease was only 

slightly higher than that for the first 200 ft. Also, a 

slight decrease in SN was noted at the 300-ft station, 

which was within the limits that showed the highest 

increase in IRI for this section.

Figure 2 shows the relationship between average percent change in IRI and SN observed in the 

sections of groups 1 through 4. While significant 

IRI changes were noticed at localized areas within 

most sections, the observed changes in SN at these 

localized areas were not significantly different than 

those over the remainder of the section. Consequently, an IRI to SN relationship was not apparent 

and/or did not exist. For the group 5 sections, Deff

was used as the structural adequacy parameter. Similar to the flexible pavement sections, no 

relationship was observed between changes in IRI 

and changes in effective slab thickness. 

Supplemental analyses were conducted to validate 

the study findings. The evaluation of maintenance and 

repair activities indicated that limited maintenance

had been performed on the sections, and, with few 

exceptions, this did not affect the findings. The review 

of IRI and FWD time history data confirmed that 

the use of the initial and latest survey/test dates to 

characterize the change in IRI and SN or Deff over 

time appeared reasonable. Also, an assessment of 

concrete pavement warping and curling showed that 

significant changes in IRI can occur with changes in 

slab shape, which has no relationship with the structural adequacy of the pavement.

Summary

This TechBrief summarizes the major findings from a 

study to identify the relationship (if any) between ride 

quality and structural adequacy or ride deterioration 

and structural adequacy using the LTPP pavement 

performance data. Major conclusions from the study 

are as follows:

• Continuous IRI and SN or Deff plots were 

generated to see if a viable relationship between 

ride quality and structural capacity relationship 

could be identified. Such a relationship was not 

observed in the sections and data investigated. 

It is possible that other researchers will pursue 

alternate approaches that will yield a reliable 

relationship.

• The lack of correlation found in the study is 

considered of value to PMS practitioners, as it 

indicates that good ride quality does not mean 

good structural adequacy or vice-versa. This is 

an important consideration for those who want 

to base performance measures on ride quality 

indicators. 

• While a relationship would be expedient for 

PMS applications, a fundamental understanding 

of the differences in factors causing structural 

deterioration and roughness makes it unlikely 

to find a simple relationship, particularly one 

excluding most other factors. 

Figure 1. Normalized SN and IRI plot for section 010102 in Alabama. SN Effective (percent)

0

100

200

300

400

500

600

700

800

Stations (ft)

0 50 100 150 200 250 300 350 400 450 500

6/21/1995 SN 4/29/2005 SN 8/25/1994 IRI 5/4/2005 IRI

IRI (percent)

0

100

200

300

400

500

600

700

800
4

Researchers—This study was performed by Fugro Consultants, Inc., in Austin, TX.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is 

being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from FHWA Product Distribution Center by e-mail to 

report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.

dot.gov/research.

Key Words—Ride quality, Structural adequacy, Structural capacity, Pavement management systems, 

Pavement rehabilitation, Pavement design decisions.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation in 

the interest of information exchange. The U.S. Government assumes no liability for the use of the information 

contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks 

or manufacturers’ names appear in this report only because they are considered essential to the objective of 

the document.

Quality Assurance Statement—The Federal Highway Administration (FHWA) provides high-quality 

information to serve the Government, industry, and public in a manner that promotes public understanding. 

Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure 

continuous quality improvement.

june 2012 FHWA-HRT-12-046

HRDi-30/06-12(700)E

• Although pavement functional and structural 

performances are not independent of each 

other, they are not related in a one-to-one 

manner that can easily be implemented 

within a network-level PMS. It is hypothesized 

that a strong relationship between these 

two performance indicators will require the 

inclusion of many other variables, potentially 

undoing its usefulness for PMS.

References

1. Rada, G.R., Perera, R., and Prabhakar, V. (2012). 

Relating Ride Quality and Structural Adequacy 

for Pavement Rehabilitation/Design Decisions, 

Report No. FHWA-HRT-12-035, Federal Highway 

Administration, Washington, DC.

2. American Association of State Highway and 

Transportation Officials. (2008). MechanisticEmpirical Pavement Design Guide, A Manual of 

Practice, Interim Edition, Washington, DC.

3. Turner-Fairbank Highway Research Center. How 

to Get LTPP Data, Federal Highway Administration, Washington, DC. Obtained from: http://

www.fhwa.dot.gov/research/tfhrc/programs/

infrastructure/pavements/ltpp/getdata.cfm.

Figure 2. Relationship between percent change in IRI and SN. Percent Change in SN (percent)

-40

-30

-20

-10

0

10

20

30

40

Percent Change in IRI (percent)

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325