Standards reference

TRH 12 Pavement Condition Bands: IRI & Rut Depth

TRH 12 classifies measured pavement condition into Sound, Warning and Severe bands — rut depth Sound <10 mm / Warning 10–20 mm / Severe >20 mm uniformly across road categories A–D, while IRI bands are set per category at the 50th–95th confidence percentiles (Department of Transport, 1997).

Last reviewed: July 2026 · Verified against TRH 12 band definitions
Table 1. TRH 12 rut-depth condition bands (uniform across categories A–D)
Rut depth (mm)BandAction signal
< 10SoundRoutine monitoring
10 – 20WarningProgramme investigation
> 20SeverePriority intervention

Interactive classifier: set a measured rut depth (0–25 mm) and IRI (0–8 m/km), pick a road category (A–D) and confidence percentile, and read per-indicator Sound / Warning / Severe bands plus a worst-of-two combined verdict. Rut thresholds (10 / 20 mm) are fixed for every category; IRI thresholds tighten as road importance rises, so a fixed roughness can be Sound on a Category D road yet Severe on a Category A road.

Figure 1. TRH 12 condition classifier for measured rut depth and IRI, illustrating category-invariant rut bands against category-dependent IRI thresholds and their worst-of-two combined condition verdict.
IRIp = value at rank ⌈ p/100 · n ⌉ of the sorted segment IRIs
where p = confidence percentile (TRH 12 evaluates 50th–95th); n = number of subsegments in the assessment length. The percentile IRI, not the mean, is compared against the category band.

Worked example — banding a Category B section

  1. A 2 km section yields 20 subsegment IRI values.
  2. Sorted, the 90th-percentile rank = ⌈0.90 × 20⌉ = 18th value → IRI₉₀ = 4.9 m/km.
  3. 4.9 m/km sits in the Warning region of the Category B IRI band at that percentile.
  4. Rut for the same section: 95th-percentile 17 mm → Warning (10–20 mm).
Both indicators in Warning → the section enters programme-level investigation rather than emergency intervention.

How RoadSense applies TRH 12

RoadSense bands measured IRI (computed to ASTM E1926, see TMH 13) and rut depth automatically per segment and confidence percentile as survey data is processed, rendering Sound/Warning/Severe classifications directly on georeferenced maps and exporting them in TMH 18 exchange format for submission to road authorities and asset management systems.

What rut depth is considered severe?

Under TRH 12, a rut depth greater than 20 mm is classified as Severe, 10–20 mm falls into the Warning band, and depths below 10 mm are classified as Sound; this scale applies uniformly across all road categories, unlike IRI.

What are TRH 12 condition bands?

TRH 12 condition bands are a three-tier classification — Sound, Warning and Severe — used to describe pavement condition indicators such as IRI (roughness) and rut depth, giving road authorities a consistent basis for prioritising maintenance across a network.

How does road category affect IRI bands?

TRH 12 sets IRI condition-band thresholds separately for road categories A through D, since higher-category strategic routes are held to stricter ride-quality expectations than lower-volume roads, and evaluates those bands across confidence percentiles from the 50th to the 95th to capture both typical and worst-case roughness on a section.

References

  • Department of Transport (1997) TRH 12: Flexible Pavement Rehabilitation Investigation and Design. Pretoria: Committee of State Road Authorities.
  • 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.
  • 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 TRH 12 condition survey for your network.