International Roughness Index (IRI): Scale and Math

The international roughness index, or IRI, is how road agencies put a number on smoothness. Instead of asking drivers how a road feels, it measures the actual up-and-down shape of the pavement along a wheel path and runs that shape through a standard model of a car's suspension. The result is used to grade highways, set repair priorities, pay contractors for smooth new paving and report condition to the federal government.

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Quick facts

What it is: a roughness statistic computed from a measured road profile.
Units: meters per kilometer (m/km) or inches per mile (in/mi).
Higher is rougher. A perfectly flat profile would score zero.
U.S. federal cutoffs: under 95 in/mi Good, 95 to 170 Fair, over 170 Poor.

Where IRI came from

Before IRI, roughness was measured with many different devices, and most of them read the motion of a particular car or trailer. Two vans could drive the same road and report different numbers, and the same van could drift over the years as its springs and tires aged. World Bank research in the 1980s set out to fix that with one scale that any accurate profile could be converted to. The answer was IRI.

The older American yardstick was the present serviceability rating, a 0 to 5 score that came from panels of people riding test tracks at the AASHO Road Test. Researchers have published equations that link PSR and IRI, but those equations depend heavily on the data behind them.

How the number is computed

Measure the profile. A device records the elevation of the pavement along a wheel path at close spacing.
Simulate a quarter-car. Software drives a standard model of one wheel, one spring, one damper and a share of the car body over that profile at 80 km/h (about 50 mph).
Add up the suspension travel. The total up-and-down motion between the body and the axle is summed over the distance.
Divide by distance. The total motion divided by the length driven gives the average rectified slope. Expressed in the right units, that is the IRI.

Because the car is simulated rather than real, any accurate profile gives the same IRI. That is the whole point. In the U.S., the federal rules point to AASHTO R 43 for how IRI is calculated from profile data.

Unit conversion

One meter per kilometer is one part in a thousand. A mile is 63,360 inches, so 1 m/km equals 63.36 in/mi.

Example (made-up numbers): a road reported at 2.0 m/km is 2.0 × 63.36 = 126.7 in/mi. Under the federal cutoffs that is Fair. A freshly paved section at 0.9 m/km is about 57 in/mi, which is Good.

The federal good, fair and poor scale

IRI (in/mi)	Rating
Under 95	Good
95 to 170	Fair
Over 170	Poor

These ratings come from the national pavement performance rules for the Interstate and National Highway System. A section only earns an overall Good rating if IRI, cracking and rutting (or faulting on concrete) are all Good. Two or more Poor ratings make the section Poor overall. Where the posted speed is under 40 mph, the rules let states report the older serviceability rating at specific locations instead.

Tools that measure roughness

Rod and level survey. Accurate but slow, only practical for short stretches.
Walking profilers. A dipstick-style tool with two feet about 12 inches apart is walked down the line, reading the height difference at each step. Often used to check other equipment.
Profilographs. A long frame with a center wheel that traces bumps against the frame. Common for acceptance testing of new concrete paving, not for network surveys.
Response-type meters. Sensors that read axle motion in a particular vehicle. Cheap, but results depend on that vehicle and drift over time.
Inertial profilers. Vans or trucks with laser height sensors and accelerometers that rebuild the true profile at highway speed. This is how most state networks are measured today.

What changes IRI

Anything that makes the wheel path go up and down: patches that sit high or low, potholes, wide cracks, shoving, settlement over utility trenches, faulted joints on concrete and frost heave. Construction quality matters too. A paver that stops and starts, uneven material delivery or poor joint work can bake bumps into a brand-new mat, which is why many agencies offer smoothness bonuses or penalties on new paving.

Using IRI with other condition data

IRI tells you how a road rides, not why. Agencies pair it with a visual distress survey such as the pavement condition index, and with structural testing when deeper failure is suspected. A road with rising IRI and growing cracks is a candidate for a mill and overlay. A road with a good ride and light cracking is often better served by preservation treatments that slow the decline.

Does IRI apply to a driveway?

Not formally. Driveways are too short and too slow for the quarter-car model to mean much. But the idea carries over. A driveway that feels rough usually has a base problem, poor drainage or uneven compaction, and our page on why a new asphalt driveway looks rough covers the surface side of that. If you are planning an overlay, the asphalt calculator estimates the tons.

Questions people ask

What is the international roughness index?

It is a standard number for how rough a road's longitudinal profile is. It is computed by simulating a quarter of a car driving the measured profile and adding up how much its suspension moves per distance traveled. Higher numbers mean a rougher ride.

What units does IRI use?

Metric reports use meters per kilometer or millimeters per meter. U.S. highway agencies report inches per mile. One meter per kilometer equals 63.36 inches per mile.

What IRI is considered good?

For federal performance reporting in the U.S., a section under 95 inches per mile is rated Good, 95 to 170 is Fair and over 170 is Poor. Those cutoffs apply to all pavement types.

Who developed IRI?

It came out of World Bank road research in the 1980s, which needed one roughness scale that different countries and instruments could share.

How is IRI measured?

Most agencies use inertial profilers mounted on vans that combine laser height sensors with accelerometers. Slower tools such as rod-and-level surveys and walking profilers are used for short sections and to check the vans.

Does a low IRI mean the pavement is in good shape?

Not by itself. IRI describes ride quality. A smooth road can still have cracking or a weak base. That is why federal rules combine IRI with cracking and rutting or faulting to rate overall condition.

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