Designation: E1926 − 08 (Reapproved 2015)Standard Practice forComputing International Roughness Index of Roads fromLongitudinal Profile Measurements1This standard is issued under the fixed designation E1926; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the mathematical processing oflongitudinal profile measurements to produce a road roughnessstatistic 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 highwaypavements.1.3 This practice is based on an algorithm developed in TheInternational Road Roughness Experiment sponsored by anumber of institutions including the World Bank and reportedin two World Bank Technical Papers (1, 2).2Additionaltechnical 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 thestandard. The inch-pound units given in parentheses are forinformation only.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE867 Terminology Relating to Vehicle-Pavement SystemsE950 Test Method for Measuring the Longitudinal Profile ofTraveled Surfaces with an Accelerometer EstablishedInertial Profiling ReferenceE1082 Test Method for Measurement of Vehicular Responseto Traveled Surface RoughnessE1170 Practices for Simulating Vehicular Response to Lon-gitudinal Profiles of Traveled SurfacesE1215 Specification for Trailers Used for Measuring Vehicu-lar Response to Road RoughnessE1364 Test Method for Measuring Road Roughness byStatic Level MethodE1656 Guide for Classification of Automated PavementCondition Survey EquipmentE2133 Test Method for Using a Rolling Inclinometer toMeasure Longitudinal and Transverse Profiles of a Trav-eled Surface3. Terminology3.1 Definitions:3.1.1 Terminology used in this practice conforms to thedefinitions included in Terminology E867.3.2 Definitions of Terms Specific to This Standard:3.2.1 International Roughness Index (IRI), n—an indexcomputed from a longitudinal profile measurement using aquarter-car simulation (see Practice E1170) at a simulationspeed of 80 km/h (50 mph).3.2.1.1 Discussion—IRI is reported in either metres perkilometre (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 ofelevation values taken at a constant interval along a wheeltrack.3.2.2.1 Discussion—Elevation measurements may be takenstatically, as with rod and level (see Test Method E1364)orinclinometer (see Test Method E2133), or dynamically, as withan inertial profiler (see Test Method E950).3.2.3 Mean Roughness Index (MRI), n—the average of theIRI values for the right and left wheel tracks.3.2.3.1 Discussion—Units are in metres per kilometre orinches per mile.3.2.4 traveled surface roughness—the deviations of a sur-face from a true planar surface with characteristics dimensionsthat affect vehicle dynamics, ride quality, dynamic loads, anddrainage, for example, longitudinal profile, transverse profile,and cross slope.1This practice is under the jurisdiction of ASTM Committee E17 on Vehicle -Pavement Systems and is the direct responsibility of Subcommittee E17.33 onMethodology for Analyzing Pavement Roughness.Current edition approved May 1, 2015. Published July 2015. Originally approvedin 1998. Last previous edition approved in 2008 as E1926 – 08. DOI: 10.1520/E1926-08R15.2The boldface numbers given in parentheses refer to a list of references at theend of the text.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at

[email protected] For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.5 true International Roughness Index, n— the value ofIRI 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 spatialfrequency, typically has units of cycle/m or cycle/ft.3.2.7 wheel track, n—a line or path followed by the tire ofa road vehicle on a traveled surface.4. Summary of Practice4.1 The practice presented here was developed specificallyfor estimating road roughness from longitudinal profile mea-surements.4.2 Longitudinal profile measurements for one wheel trackare transformed mathematically by a computer program andaccumulated to obtain the IRI. The profile must be representedas a series of elevation values taken at constant intervals alongthe wheel track.4.3 The IRI scale starts at zero for a road with no roughnessand covers positive numbers that increase in proportion toroughness. Fig. 1 associated typical IRI values with verbaldescriptors from World Bank Technical Paper No. 46 (2) forroads with bituminous pavement, and Fig. 2 shows similarassociations for roads with earth or gravel surfaces.5. Significance and Use5.1 This practice provides a means for obtaining a quanti-tative estimate of a pavement property defined as roughnessusing longitudinal profile measuring equipment.5.1.1 The IRI is portable in that it can be obtained fromlongitudinal profiles obtained with a variety of instruments.5.1.2 The IRI is stable with time because true IRI is basedon the concept of a true longitudinal profile, rather than thephysical properties of a particular type of instrument.5.2 Roughness information is a useful input to the pavementmanagement systems (PMS) maintained by transportationagencies.5.2.1 The IRI for the right wheel track is the measurementof road surface roughness specified by the Federal HighwayAdministration (FHWA) as the input to their Highway Perfor-mance Monitoring System (HPMS).5.2.2 When profiles are measured simultaneously for bothtraveled wheel tracks, then the MRI is considered to be a bettermeasure of road surface roughness than the IRI for either wheeltrack.NOTE 1—The MRI scale is identical to the IRI scale.5.3 IRI can be interpreted as the output of an idealizedresponse-type measuring system (see Test Method E1082 andSpecification E1215), where the physical vehicle and instru-mentation are replaced with a mathematical model. The unitsof slope correspond to accumulated suspension motions (forexample, metres), divided by the distance traveled (forexample, kilometres).5.4 IRI is a useful calibration reference for response-typesystems 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 ofthe profile, filtered mathematically to modify the amplitudesassociated with different wavelengths (3).6. Longitudinal Profile Measurement6.1 The longitudinal profile measurements can be obtainedfrom equipment that operate in a range of speeds from static tohighway traffic speeds.6.2 The elevation profile measuring equipment used tocollect the longitudinal profile data used in this practice musthave sufficient accuracy to measure the longitudinal profileattributes 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 analgorithm developed in the International Road RoughnessExperiment and described in the World Bank Technical Papers45 and 46 (1, 2). Additional technical information provided intwo 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 recordedlongitudinal profile measurement.7.2.1.1 The computer program IRISMP is a general com-puter program which accepts the elevation profile data set asinput and then calculates the IRI values for that profile data set.7.2.1.2 A listing of the IRISMP computer program for thecomputation of IRI is included in this practice as Appendix X2.7.2.1.3 A provision has been made in the computer programlisting (Appendix X2) for the computation of IRI from re-corded longitudinal profile measurements in either SI orinch-pound units.7.2.2 The input to the sample IRI computer program is anASCII profile data set stored in a 1X,F8.3,1X,F8.3 Fortranformat. In this format, the profile data appear as a multi-row,two column array with the left wheel path profile data points inColumn 1 and the right wheel path points in Column 2. Theprofile data point interval is discretionary. However the qualityof the IRI values computed by this algorithm is a function ofthe data point interval.7.2.2.1 If the input to the IRI computer program is in SIunits, the elevation profile data points are scaled in millimetreswith the least significant digit being equal to 0.001 mm.7.2.2.2 If the input to the IRI computer program is ininch-pound units, the elevation profile data points are scaled ininches with the least significant digit being equal to 0.001 in.7.3 The distance interval over which the IRI is computed isdiscretionary, but shall be reported along with the IRI results.E1926 − 08 (2015)27.4 Validation of the IRI program is required when it isinstalled. Provision for the IRI program installation validationhas been provided in this practice.7.4.1 The sample profile data set TRIPULSE.ASC has beenprovided in SI units in Appendix X2 for validation of thecomputer program installation.7.4.2 Using the sample profile data set TRIPULSE.ASC asinput to the IRI computer program, an IRI value of 4.36 mm/mwas 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 inAppendix X2.8. Report8.1 Include the following information in the report for thispractice:8.1.1 Profile Measuring Device—The Class of the profilemeasuring device used to make the profile measurement asdefined in Test Method E950 and Test Method E1364 shall beincluded in the report.8.1.2 Longitudinal Profile Measurements—Report data fromthe profile measuring process shall include the date and time ofday of the measurement, the location of the measurement, theFIG. 1 Road Roughness Estimation Scale for Paved Roads With Asphaltic Concrete or Surface Treatment (Chipseal)E1926 − 08 (2015)3lane measured, the direction of the measurement, length ofmeasurement, and the descriptions of the beginning and endingpoints 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 decimalpoint. If the units are in./mile, then the IRI results should bereported to a resolution of 0.1 in./mile.9. Precision and Bias9.1 The precision and bias of the computed IRI is limited bythe procedures used in making the longitudinal profile mea-surement. Guidelines for measuring longitudinal profile areprovided in Test Method E950 and Test Method E1364.9.2 For the effects of the precision and bias of the measuredprofile on the computed IRI, see precision and bias in Appen-dix X1.FIG. 2 Road Roughness Estimation Scale for Unpaved Roads with Gravel or Earth SurfacesE1926 − 08 (2015)410. Keywords10.1 highway performance monitoring system; HPMS; in-ternational roughness index; International Roughness Index;longitudinal profile; pavement management systems; pavementroughness; PMSAPPENDIXES(Nonmandatory Information)X1. PRECISION AND BIASX1.1 Precision :X1.1.1 The precision of the computed IRI is limited by theprocedures used in making the longitudinal profile measure-ment. Guidelines for measuring longitudinal profile are pro-vided in Test Method E950 and Test Method E1364.X1.1.2 IRI precision depends on the interval between adja-cent profile elevation measures (see Test Method E950 and TestMethod E1364). Reducing the interval typically improves theprecision. An interval of 0.3 m (12 in.) or smaller is recom-mended. For some surface types, a shorter interval willimprove precision. More information about the sensitivity ofIRI to the profile data interval is provided in Ref (3).X1.1.3 IRI precision is roughly equivalent to the precisionof the slope obtained from the longitudinal profilemeasurements, for distances ranging from approximately 1.5 m(5 ft) to about 25 m (80 ft). For example, a relative error onprofile 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 awheel track on the road can be profiled. Errors in locating thewheel track longitudinally and laterally can influence the IRIvalues, because the IRI will be computed for the profile of thewheel track as measured, rather than the wheel track asintended. These effects are reduced by using longer profiles.X1.1.5 Computational errors due to round-off are typicallyabout two orders of magnitude smaller than those due tolimitations in the profile measuring process, and can be safelyignored.X1.2 Bias:X1.2.1 The bias of the computed IRI is typically limited bythe procedures used in making the longitudinal profile mea-surement. Guidelines for measuring longitudinal profile areprovided in Test Method E950 and Test Method E1364.X1.2.2 IRI bias depends on the interval between adjacentprofile elevation measures. An interval of 0.3 m (12 in.) orsmaller is recommended. Shorter intervals improve precisionbut have little effect on bias. More information about thesensitivity of IRI to the profile data interval is provided in Ref(3).X1.2.3 Many forms of measurement error cause an upwardbias in IRI. (The reason is that variations in profile elevationdue to measurement error are usually not correlated with theprofile changes.) Some common sources of positive IRI biasare: height-sensor round-off, mechanical vibrations in theinstrument that are not corrected and electronic noise. Bias isreduced by using profiler instruments that minimize theseerrors.X1.2.4 Inertial profiler systems (see Test Method E950)include one or more filters that attenuate long wavelengths(low wave numbers). If the cut-off wavelength is too short,then the IRI computed from the profile will have a negativebias. A cut off wavelength of 91.4 m/cycle (300 ft/cycle) isconsidered sufficientl