ASTM E1926-08(2021).pdf

Designation: E1926 − 08 (Reapproved 2021)

Standard Practice for Computing International Roughness Index of Roads from Longitudinal Profile Measurements1

This standard is issued under the fixed designation E1926; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

1. Scope

1.1 This practice covers the mathematical processing of

longitudinal profile measurements to produce a road roughness statistic called the International Roughness Index (IRI).

1.2 The intent is to provide a standard practice for comput-

ing and reporting an estimate of road roughness for highway pavements.

1.3 This practice is based on an algorithm developed in the

International Road Roughness Experiment sponsored by a number of institutions including the World Bank and reported in two World Bank Technical Papers (1, 2).2 Additional technical information is provided in two Transportation Re-search Board (TRB) papers (3, 4).

1.4 The values stated in SI units are to be regarded as the

standard. The inch-pound units given in parentheses are for information only.

1.5 This standard does not purport to address all of the

safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.

1.6 This international standard was developed in accor-

dance with internationally recognized principles on standard-ization established in the Decision on Principles for the Development of International Standards, Guides and Recom-mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

2. Referenced Documents

2.1 ASTM Standards:3

E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods

E867 Terminology Relating to Vehicle-Pavement Systems E950/E950M Test Method for Measuring the Longitudinal

Profile of Traveled Surfaces with an Accelerometer- Established Inertial Profiling Reference

E1082 Test Method for Measurement of Vehicular Response to Traveled Surface Roughness

E1170 Practices for Simulating Vehicular Response to Lon-gitudinal Profiles of Traveled Surfaces

E1215 Specification for Trailers Used for Measuring Vehicu-lar Response to Road Roughness

E1364 Test Method for Measuring Road Roughness by Static Level Method

E1656/E1656M Guide for Classification of Automated Pavement Condition Survey Equipment

E2133 Test Method for Using a Rolling Inclinometer to Measure Longitudinal and Transverse Profiles of a Trav-eled Surface

3. Terminology

3.1 Definitions:

3.1.1 Terminology used in this practice conforms to the

definitions included in Terminology E867.

3.2 Definitions of Terms Specific to This Standard:

3.2.1 International Roughness Index (IRI), n—an index

computed from a longitudinal profile measurement using a quarter-car simulation (see Practice E1170) at a simulation speed of 80 km/h (50 mph).

3.2.1.1 Discussion—IRI is reported in either metres per

kilometre (m/km) or inches per mile (in./mile). (Note—1 m/km = 63.36 in./mile.)

3.2.2 longitudinal profile measurement, n—a series of eleva-tion values taken at a constant interval along a wheel track.

3.2.2.1 Discussion—Elevation measurements may be taken

statically, as with rod and level (see Test Method E1364) or inclinometer (see Test Method E2133), or dynamically, as with an inertial profiler (see Test Method E950/E950M).

3.2.3 Mean Roughness Index (MRI), n—the average of the

IRI values for the right and left wheel tracks.

3.2.3.1 Discussion—Units are in metres per kilometre or

inches per mile.

1 This practice is under the jurisdiction of ASTM Committee E17 on Vehicle - Pavement Systems and is the direct responsibility of Subcommittee E17.33 on Methodology for Analyzing Pavement Roughness.

Current edition approved Feb. 1, 2021. Published February 2021. Originally approved in 1998. Last previous edition approved in 2015 as E1926 – 08 (2015).

DOI: 10.1520/E1926-08R21.

2 The boldface numbers given in parentheses refer to a list of references at the end of the text.

3 For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards volume information, refer to the standard’s Document Summary page on the ASTM website.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

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3.2.4 traveled surface roughness—the deviations of a sur-face from a true planar surface with characteristics dimensions that affect vehicle dynamics, ride quality, dynamic loads, and drainage, for example, longitudinal profile, transverse profile, and cross slope.

3.2.5 true International Roughness Index, n—the value of IRI that would be computed for a longitudinal profile measure-ment with the constant interval approaching zero.

3.2.6 wave number, n—the inverse of wavelength.

3.2.6.1 Discussion—Wave number, sometimes called spatial

frequency, typically has units of cycle/m or cycle/ft.

3.2.7 wheel track, n—a line or path followed by the tire of a road vehicle on a traveled surface.

4. Summary of Practice

4.1 The practice presented here was developed specifically

for estimating road roughness from longitudinal profile mea-surements.

4.2 Longitudinal profile measurements for one wheel track

are transformed mathematically by a computer program and accumulated to obtain the IRI. The profile must be represented as a series of elevation values taken at constant intervals along the wheel track.

4.3 The IRI scale starts at zero for a road with no roughness and covers positive numbers that increase in proportion to roughness. Fig. 1 associated typical IRI values with verbal descriptors from World Bank Technical Paper No. 46 (2) for roads with bituminous pavement, and Fig. 2 shows similar associations for roads with earth or gravel surfaces.

5. Significance and Use

5.1 This practice provides a means for obtaining a quanti-

tative estimate of a pavement property defined as roughness using longitudinal profile measuring equipment.

5.1.1 The IRI is portable in that it can be obtained from

longitudinal profiles obtained with a variety of instruments.

5.1.2 The IRI is stable with time because true IRI is based

on the concept of a true longitudinal profile, rather than the physical properties of a particular type of instrument.

5.2 Roughness information is a useful input to the pavement

management systems (PMS) maintained by transportation agencies.

5.2.1 The IRI for the right wheel track is the measurement

of road surface roughness specified by the Federal Highway Administration (FHWA) as the input to their Highway Perfor-mance Monitoring System (HPMS).

5.2.2 When profiles are measured simultaneously for both

traveled wheel tracks, then the MRI is considered to be a better measure of road surface roughness than the IRI for either wheel track.

NOTE 1—The MRI scale is identical to the IRI scale.

5.3 IRI can be interpreted as the output of an idealized

response-type measuring system (see Test Method E1082 and Specification E1215), where the physical vehicle and instru-mentation are replaced with a mathematical model. The units of slope correspond to accumulated suspension motions (for example, metres), divided by the distance traveled (for example, kilometres).

5.4 IRI is a useful calibration reference for response-type

systems that estimate roughness by measuring vehicular re-sponse (see Test Method E1082 and Specification E1215).

5.5 IRI can also be interpreted as average absolute slope of

the profile, filtered mathematically to modify the amplitudes associated with different wavelengths (3).

6. Longitudinal Profile Measurement

6.1 The longitudinal profile measurements can be obtained

from equipment that operates in a range of speeds from static to highway traffic speeds.

6.2 The elevation profile measuring equipment used to

collect the longitudinal profile data used in this practice must have sufficient accuracy to measure the longitudinal profile attributes that are essential to the computation of the IRI.

7. Computation of International Roughness Index (IRI)

7.1 This practice consists of the computation of IRI from an

algorithm developed in the International Road Roughness Experiment and described in the World Bank Technical Papers 45 and 46 (1, 2). Additional technical information provided in two TRB papers (3, 4).

7.2 A Fortran version of this algorithm has been imple-

mented as described in Ref (3).

7.2.1 This practice presents a sample computer program

“IRISMP” for the computation of the IRI from the recorded longitudinal profile measurement.

7.2.1.1 The computer program IRISMP is a general com-

puter program which accepts the elevation profile data set as input and then calculates the IRI values for that profile data set.

7.2.1.2 A listing of the IRISMP computer program for the

computation of IRI is included in this practice as Appendix X2.

7.2.1.3 A provision has been made in the computer program

listing (Appendix X2) for the computation of IRI from re-corded longitudinal profile measurements in either SI or inch-pound units.

7.2.2 The input to the sample IRI computer program is an

ASCII profile data set stored in a 1X,F8.3,1X,F8.3 Fortran format. In this format, the profile data appear as a multi-row, two column array with the left wheel path profile data points in Column 1 and the right wheel path points in Column 2. The profile data point interval is discretionary. However the quality of the IRI values computed by this algorithm is a function of the data point interval.

7.2.2.1 If the input to the IRI computer program is in SI

units, the elevation profile data points are scaled in millimetres with the least significant digit being equal to 0.001 mm.

7.2.2.2 If the input to the IRI computer program is in

inch-pound units, the elevation profile data points are scaled in inches with the least significant digit being equal to 0.001 in.

7.3 The distance interval over which the IRI is computed is

discretionary, but shall be reported along with the IRI results.

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7.4 Validation of the IRI program is required when it is

installed. Provision for the IRI program installation validation has been provided in this practice.

7.4.1 The sample profile data set TRIPULSE.ASC has been

provided in SI units in Appendix X2 for validation of the computer program installation.

7.4.2 Using the sample profile data set TRIPULSE.ASC as

input to the IRI computer program, an IRI value of 4.36 mm/m was computed for a profile data point interval of 0.15 m (0.5 ft) and a distance interval equal to 15 m of the profile data set in Appendix X2.

8. Report

8.1 Include the following information in the report for this

practice:

8.1.1 Profile Measuring Device—The class of the profile

measuring device used to make the profile measurement as defined in Test Method E950/E950M and Test Method E1364 shall be included in the report.

8.1.2 Longitudinal Profile Measurements—Report data from

the profile measuring process shall include the date and time of day of the measurement, the location of the measurement, the

FIG. 1 Road Roughness Estimation Scale for Paved Roads with Asphaltic Concrete or Surface Treatment (Chipseal)

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lane measured, the direction of the measurement, length of measurement, and the descriptions of the beginning and ending points of the measurement. The recorded wheel track (left, right, or both) must also be included.

8.1.3 IRI Resolution—The number of digits after the deci-

mal point depends on the choice of units. If the units are m/km, then results should be reported with two digits after the decimal

FIG. 2 Road Roughness Estimation Scale for Unpaved Roads with Gravel or Earth Surfaces

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point. If the units are in./mile, then the IRI results should be reported to a resolution of 0.1 in./mile.

9. Precision and Bias

9.1 The precision and bias of the computed IRI is limited by

the procedures used in making the longitudinal profile mea-surement. Guidelines for measuring longitudinal profile are provided in Test Method E950/E950M and Test Method E1364.

9.2 For the effects of the precision and bias of the measured profile on the computed IRI, see precision and bias in Appen-dix X1.

10. Keywords

10.1 highway performance monitoring system; HPMS; in-ternational roughness index; International Roughness Index;

longitudinal profile; pavement management systems; pavement roughness; PMS

APPENDIXES

(Nonmandatory Information)

X1. PRECISION AND BIAS

X1.1 Precision:

X1.1.1 The precision of the computed IRI is limited by the procedures used in making the longitudinal profile measure-ment. Guidelines for measuring longitudinal profile are pro-vided in Test Method E950/E950M and Test Method E1364.

X1.1.2 IRI precision depends on the interval between adja-cent profile elevation measures (see Test Method E950/E950M and Test Method E1364). Reducing the interval typically improves the precision. An interval of 0.3 m (12 in.) or smaller is recommended. For some surface types, a shorter interval will improve precision. More information about the sensitivity of IRI to the profile data interval is provided in Ref (3).

X1.1.3 IRI precision is roughly equivalent to the precision of the slope obtained from the longitudinal profile measurements, for distances ranging from approximately 1.5 m (5 ft) to about 25 m (80 ft). For example, a relative error on profile elevation of 1.0 mm over a distance of 10 m corre-sponds to a slope error of 0.1 mm/m, or 0.1 m/km (6.3 in./mi).

X1.1.4 IRI precision is limited by the degree to which a wheel track on the road can be profiled. Errors in locating the wheel track longitudinally and laterally can influence the IRI values, because the IRI will be computed for the profile of the wheel track as measured, rather than the wheel track as intended. These effects are reduced by using longer profiles.

X1.1.5 Computational errors due to round-off are typically about two orders of magnitude smaller than those due to limitations in the profile measuring process, and can be safely ignored.

X1.2 Bias:

X1.2.1 The bias of the computed IRI is typically limited by the procedures used in making the longitudinal profile mea-surement. Guidelines for measuring longitudinal profile are provided in Test Method E950/E950M and Test Method E1364.

X1.2.2 IRI bias depends on the interval between adjacent profile elevation measures. An interval of 0.3 m (12 in.) or smaller is recommended. Shorter intervals improve precision but have little effect on bias. More information about the sensitivity of IRI to the profile data interval is provided in Ref (3).

X1.2.3 Many forms of measurement error cause an upward bias in IRI. (The reason is that variations in profile elevation due to measurement error are usually not correlated with the profile changes.) Some common sources of positive IRI bias are: height-sensor round-off, mechanical vibrations in the instrument that are not corrected and electronic noise. Bias is reduced by using profiler instruments that minimize these errors.

X1.2.4 Inertial profiler systems (see Test Method E950/ E950M) include one or more filters that attenuate long wave-lengths (low wave numbers). If the cut-off wavelength is too short, then the IRI computed from the profile will have a negative bias. A cut off wavelength of 91.4 m/cycle (300 ft/cycle) is considered sufficiently long.

NOTE X1.1—Profiles obtained with static methods are generally not filtered, and therefore this source of bias is not relevant for them.

X1.2.5 The measures from some inertial profilers are pro-cessed during measurement to attenuate short wavelengths and prevent aliasing. The effect is to smooth the profile measure-ment. If a smoothing filter is used and it affects wavelengths longer than 1 m (3.3 ft), then the computed IRI will have a negative bias.

NOTE X1.2—If the profiler includes a smoothing filter that affects wavelengths shorter than 1 m (3.3 ft) and longer than 250 mm (10 in.), no more smoothing is required during the computation of IRI.

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X2. INTERNATIONAL ROUGHNESS INDEX COMPUTER PROGRAM

X2.1 Included in this appendix is the coding in Fortran language for a computer subroutine, SUBROUTINE IRI, (see Fig. X2.1), which calculates the International Roughness Index as prescribed by this practice. A sample main program is also included, which when executed, prompts the user for the name of a data file containing the profile data to be processed and the

FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index

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parameters needed by the subroutine to compute the IRI. The subroutine is called and returns the computed IRI values to the main program which then displays them.

X2.2 The sample program can process data files containing two profile tracks in either SI or inch-pound units. For SI data, the program assumes the input amplitudes are stored in millimetre units; if inch-pound, inches. For the sample program, the maximum length road section that can be pro-cessed is limited to 1058 sample pairs.

X2.3 The sample data file shown in Fig. X2.2 and Fig. X2.3 is in SI units (mm) and contains 101 profile data point pairs.

The tracks are identical. The recording interval for the data is

0.15 m.

FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index (continued)

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FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index (continued)

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FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index (continued)

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FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index (continued)

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FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index (continued)

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FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index (continued)

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FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index (continued)

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FIG. X2.1 Sample Fortran Program Using Subroutine IRI to Compute International Roughness Index (continued)

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NOTE 1—Elevations are metric units (mm). The profile consists of identical right and left wheel tracks, each consisting of zero elevations everywhere except the triangular ‘pulse’ from 0.6 to 3.0 m peaking at 20.0 mm. The interval between elevations is 0.15 m and the total length is 15 m. This data set may be used as a test of the user’s implementation of IRI standard computation.

FIG. X2.2 Sample Load Profile Input Data Set, TRIPULSE.ASC

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REFERENCES

(1) Sayers, M. W., Gillespie, T. D., Queiroz, C. A. V., “The International Road Roughness Experiment,” World Bank Technical Paper , Number 45, 1986.

(2) Sayers, M. W., Gillespie, T. D., Paterson, W. D. O., “Guidelines for Conducting and Calibrating Road Roughness Measurements,” World Bank Technical Paper, Number 46, 1986.

(3) Sayers, M. W., “On the Calculation of IRI from Longitudinal Profile,” Transportation Research Record, Vol 1501, Transportation Research Board, 1995, pp. 1–12.

(4) Sayers, M. W., “Two Quarter-Car Models for Defining Road Rough-ness: IRI and HRI,” Transportation Research Record, Vol 1215, 1989,

pp. 165–172.

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FIG. X2.3 Input/Output for RNSMP sample program using data input file ’TRIPULSE.ASC’

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