Everything You Always Wanted To Know About The IRI But Were Affraid To Ask

Everything You Always Wanted to Know about the IRI,

But Were Afraid to Ask!

by

Thomas D. Gillespie, Ph. D.

The University of Michigan Transportation Research Institute

Presented at the

Road Profile Users Group Meeting

September 22-24, 1992

Lincoln, Nebraska

Everything You Always Wanted to Know about the IRI,

But Were Afraid to Ask!

by

Thomas D. Gillespie, Ph. D.

The University of Michigan Transportation Research Institute

Abstract

The International Roughness Index has become the standard scale on which road

roughness information is reported both here in the United States [1]* and in many countries

of the world. Procedures for determining the IRI are well developed and reliable, yet many

users are unaware of the history of its development and the physical significance of this

measure of roughness. This paper describes the history of roughness measurement from

which the IRI evolved, and explains the physical meaning of the index.

Roughness Measurement in the Past

“Ever since roads and highways have been constructed, the people who use them have

been keenly aware of the relative degrees of comfort or discomfort experienced in

traveling” [2]. The evidence that remains today from the paved roads of the Roman Empire

(see Fig. 1) suggests that roughness must have been a concern for chariot travel. Even in

the 1800s, high-speed travel in this country by stage coach had a reputation for rigor

directly resulting from roughness of the roadway.

Fig. 1 Photograph of an early Roman road [2]

* Numbers in brackets refer to references at end.

2

With the introduction of the gasoline-powered motor vehicle at the turn of the Twentieth

Century, more people had access to means of high-speed personal travel and the travel

speeds rapidly surpassed the limits practical with horses. The increase in speed placed even

greater premium on building and maintaining roads with a smooth travel surface.

Those early years saw the first rudimentary attempts to measure the roughness

properties of a road. A sliding straightedge, known as the “Viagraph,” (see Fig. 2) was one

of the first methods to measure roughness by recording the deviation at the center point of

the straightedge [2]. The measurement response of the straightedge is indicated by the gain

shown in the figure. Long wavelengths (low wavenumbers) produced no response,

whereas the gain approached unity at wavelengths equal to or less than the base length of

the straightedge. Interestingly enough, the roughness measured by this device was reported

in feet of deviation per mile, with 15 feet/mile (180 inches/mile) recommended as the

standard for construction.

0 0.5 1.0 1.5 2.0

0

0.5

1.0

1.5

2.0

Wavenumber x Baselength (Cycles/baselength)

Gain

SLIDING STRAIGHTEDGE

Fig. 2 Response of the sliding straightedge road roughness measuring device

With the obvious disadvantages of the effort to move the sliding straightedge and the

wear and tear that resulted, it was not surprising to see development of the rolling

straightedge (see Fig. 3). This device had its own unique response to roughness that was

different from the sliding straightedge, characterized by the fact that it recorded every bump

three times—once when the front wheel passed over, a second time when the measuring

wheel passed over, and a third time when the rear wheel passed over. Because the

straightedge contacted the road surface at three points, bumps of certain wavelengths

recorded at twice amplitude, while others did not record at all. Thus the rolling straightedge

"tuned" to certain wavelengths of roughness in the road, while ignoring others.

To overcome this problem the rolling concept was subsequently improved by adding an

array of wheels to establish a reference plane from which to measure deviations (see Fig.

4), and remains with us today memorialized as the Profilograph. Bogey attachments for the

array of wheels averaged the elevation of all points under the wheels, and roughness was

measured as deviation of the center wheel from this reference. With a large number of

wheels the response approaches the theoretical limit shown in the figure.

3

0 0.5 1.0 1.5 2.0

0

0.5

1.0

1.5

2.0

Wavenumber x Baselength (Cycles/baselength)

Gain

ROLLING STRAIGHTEDGE

Fig. 3 Response of the rolling straightedge road roughness measuring device

0 0.5 1.0 1.5 2.0

0

0.5

1.0

1.5

2.0

Wavenumber x Baselength (Cycles/baselength)

Gain

PROFILOGRAPH

Fig. 4 Response of the Profilograph

With the variation in response properties for each of these measurement devices, it is

clear that progress was not being made toward consensus on a universal and standardized

measure of road roughness.

By the 1920s highway engineers recognized that roughness properties in a road of

greatest importance were those responsible for causing vibrations of motor vehicles. The

“Via-Log” developed by the State of New York evidenced this thinking by measuring the

suspension travel of a passenger car as an indication of the roughness level. The first

devices recorded the suspension motion, but were soon modified to sum the motion on a

mechanical counter and measure an "inches/mile" statistic.

Over the next decades the difficulty of obtaining consistent measurements by this

method, due to the variations in dynamics of motor vehicles, led to the attempt to

“standardize” the vehicle. The Bureau of Public Roads (BPR) Roughometer (later adapted

in similar form as the Bump Integrator by the Transport and Road Research Laboratory in

4

England) was born in 1941. The Roughometer was a single-wheel trailer (see Fig. 5) in

which all dimensions, mass properties, and tire and suspensions properties were

standardized in an effort to achieve comparable performance on all devices.

Hitch

Spring

Integrator Dampers

Cable

Wheel

Fig. 5 The BPR Roughometer

One other important roughness measurement device developed at the time of the

AASHO Road Test [3] was the CHLOE (an acronym formed from the first letters of the

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