Standards reference

TMH 13: Road Roughness and the International Roughness Index (IRI)

IRI is expressed in metres per kilometre (m/km) and is computed by running the standardised "Golden Car" quarter-car simulation over a road's measured longitudinal profile; South African guidance rates roads below 2.5 m/km as Very Good, below 4.0 m/km as Good, below 6.0 m/km as Fair, and 6.0 m/km or higher as Poor. (Sayers, Gillespie and Queiroz, 1986)

Last reviewed: July 2026 · Verified against ASTM E1926 and COTO TMH guidance

Table 1. IRI condition classes and typical interpretation
IRI (m/km)	Condition class	Typical interpretation
< 2.5	Very Good	Routine maintenance only
2.5 – 4.0	Good	Preventive maintenance candidate
4.0 – 6.0	Fair	Rehabilitation candidate
≥ 6.0	Poor	Priority rehabilitation or resurfacing

Figure 1. IRI condition scale from 0–8 m/km, with the 3.6 m/km worked example marked.

IRI = 1/L · ∫₀^L/V |ż_s − ż_u| dt

where L = profile length (km); V = 80 km/h simulation speed; żs, żu = sprung / unsprung mass vertical velocities of the Golden Car quarter-car model (Sayers, 1995).

Figure 2. The Golden Car quarter-car model: sprung mass (ms) and unsprung mass (mu) connected by suspension (ks, cs) and tyre stiffness (kt) over the measured road profile.

Worked example — IRI for a 2.0 km survey run

Survey run length L = 2.0 km, simulated at V = 80 km/h per ASTM E1926.
Golden Car simulation over the measured profile accumulates 7.2 m of relative suspension motion.
IRI = accumulated motion ÷ length = 7.2 / 2.0 = 3.6 m/km.

Result: IRI = 3.6 m/km → Good (2.5 – 4.0 band). Under TRH 12 this is then checked against the road's category band.

How RoadSense computes IRI

RoadSense derives IRI from accelerometer and profilometer telemetry (.xyp/.xyd survey files) captured during a road survey run, applying the (ASTM International, 2021) computation method to the longitudinal profile to produce a standardised IRI value per chainage segment. Results are aggregated per road segment, banded against TRH 12 category thresholds, visualised on a georeferenced map, and exported in TMH 18 exchange format for submission to road authorities and asset management systems.

What IRI is considered good?

An IRI below 2.5 m/km is rated Very Good, and below 4.0 m/km is rated Good; roads in these bands typically require only routine or preventive maintenance, while Fair (4.0–6.0 m/km) and Poor (≥ 6.0 m/km) roads are candidates for rehabilitation or resurfacing, subject to the road category thresholds set out in TRH 12.

How is IRI calculated?

IRI is calculated by reconstructing the road's longitudinal profile from accelerometer and distance/GPS data, then running that profile through the Golden Car quarter-car simulation at a standard 80 km/h reference speed; the accumulated simulated suspension travel divided by the profile length produces the IRI value in m/km, per ASTM E1926.

What is the difference between TMH 13 and TRH 12?

TMH 13 defines the roughness measurement methodology and the IRI metric itself, while TRH 12 defines how those IRI values should be interpreted for pavement management — setting condition bands per road category (A–D) and confidence percentile; TMH 18 is a separate exchange format standard used to package and submit IRI and other condition data between systems.

References

ASTM International (2021) ASTM E1926-08(2021): Standard Practice for Computing International Roughness Index of Roads from Longitudinal Profile Measurements. West Conshohocken, PA: ASTM International.
Committee of Transport Officials (COTO) (2016) TMH 9: Manual for Visual Assessment of Road Pavements. Pretoria: Department of Transport.
Department of Transport (1997) TRH 12: Flexible Pavement Rehabilitation Investigation and Design. Pretoria: Committee of State Road Authorities.
Sayers, M.W. (1995) 'On the calculation of International Roughness Index from longitudinal road profiles', Transportation Research Record, 1501, pp. 1–12.
Sayers, M.W., Gillespie, T.D. and Queiroz, C.A.V. (1986) The International Road Roughness Experiment: Establishing Correlation and a Calibration Standard for Measurements. World Bank Technical Paper No. 45. Washington, DC: The World Bank.

For the full set of survey standards RoadSense reports against, see the standards library, or request access to discuss a TMH 13 roughness survey for your network.