Segment vs uniform section vs reporting interval on a road pavement

An assessment segment, a reporting interval and a uniform section are three lengths of one road pavement, and an assessment segment is not a uniform section. An assessment segment is the length of road for which one visual rating is recorded. A reporting interval is the length over which an instrument value is averaged and stored. A uniform section is a length of road pavement whose condition and structure are similar enough to be treated as one unit. The South African visual assessment manual TMH 9 (Committee Draft Final, May 2016) sets out the first of these (COTO, 2016b). The road asset management manual TMH 22 (Draft, March 2013) lists the second (COTO, 2013). Draft TRH 12 (1997), the flexible pavement rehabilitation manual, describes the third (Department of Transport, 1997).

Key facts

• An assessment segment is the length of road with one recorded rating, and TMH 9 Part A, clause A.3, recommends that the road authority predefine segments across its whole network.
• TMH 9 Part A, clause A.3, states that its visual assessment is not meant to identify uniform sections of distress, so an assessment segment is not a uniform section.
• A reporting interval is the length over which an instrument value is averaged and stored, and TMH 22 clause D.5.4 lists one for each data type.
• A uniform section is a length of road pavement with similar condition and structure, and SAPEM Chapter 10 clause 5.1.2 and Draft TRH 12 clause 2.2.4 describe it.
• The investigating engineer sets uniform sections from as-built, visual, instrument and deflection evidence read together, so their boundaries need not coincide with segment or reporting-interval boundaries.
• Visual segments, instrument intervals and deflection points share one road, so each record is kept at its own length and placed by overlap on one kilometre datum.

What is a TMH 9 assessment segment?

An assessment segment is the length of road for which one assessment rating is recorded. TMH 9 Part A defines it in its glossary, and TMH 22 Part B uses the same definition for a visual or an instrument rating (COTO, 2016b) (COTO, 2013). TMH (Technical Methods for Highways) and TRH (Technical Recommendations for Highways) are companion series of South African road manuals, now issued under the Committee of Transport Officials (COTO).

The road authority sets the segment, not the assessor. TMH 9 clause A.3 recommends that the authority predefine segments over its complete network (COTO, 2016b). The road-data exchange standard TMH 18, in Draft Version 4 (October 2014) and Committee Draft 5.0 (February 2016), repeats the standard lengths in clause 5.1 (COTO, 2014) (COTO, 2016i).

A segment rating is an average. Clause A.5.1 has the assessor mark an average degree and extent for each distress at the end of the segment (COTO, 2016b), so it cannot say where in the segment the distress sits.

What is a reporting interval for roughness and other instrument data?

A reporting interval is the length over which an instrument measurement is averaged and stored as one value. It differs from the test interval, the spacing at which values are collected. TMH 22 clause D.5.4 lists a reporting interval for each data type in Table D-7 and sets it against the test interval in Table D-8; deflection is stored per test point (COTO, 2013). See taking a profiler run to a register value for one roughness record.

The specification sets the interval, not the pavement. The automated road condition assessment manual TMH 13 (Committee Draft Final, May 2016), Part A, clause A.4.3.7, asks the survey specification to state the averaging procedure and the reporting segment length (COTO, 2016d). Clause A.2.2.4 recommends averaging international roughness index (IRI) values over 100 m sections for interpretation (COTO, 2016d). TMH 22 adds that raw data are stored at the lowest collected interval, so the finer record survives averaging (COTO, 2013).

In TMH 18 files a combined instrument data (.cid) record carries START_KM and END_KM. A normalised deflection bowl record carries one SURVEY_KM (clauses 5.6 and 5.7 of Draft Version 4 and of Committee Draft 5.0) (COTO, 2014) (COTO, 2016i).

Narrated film · 1:22

Three lengths of one road

Three figures place Synthetic Road S8 on its chainage: the two assessment segments, the three uniform sections with the segment boundary marked, and one 100 m record that straddles a section boundary.

Read the film transcript

Why does one road pavement carry three different lengths at once? In TMH 9, the South African visual assessment manual, an assessment segment is the road length with one rating. A reporting interval is the length over which an instrument value is averaged. A uniform section is a length of road pavement with similar condition and structure. The road authority and the specification fix the first two beforehand, but the engineer finds the third afterwards. Take a synthetic road, three point six kilometres, with two assessment segments meeting at kilometre thirteen point eight. The engineer finds three uniform sections, with boundaries at kilometres thirteen point one five and fourteen point six. The segment boundary falls inside the second section, so each segment's one rating covers two sections. A segment can stand in for a section only while the other evidence inside it stays similar. A hundred metre record can straddle a boundary too, here at kilometre thirteen point one five. The stored ten metre values can be split there, but the hundred metre average cannot. These six steps are our reading, not a clause: one road definition, each record at its own length, joined by overlap. TMH 9 says its assessment is not meant to identify uniform sections, so an assessment segment is not a uniform section. Next, see how TMH 18 road-data files record these lengths, and how draft TRH 12 uses uniform sections.

What is a uniform section in a road pavement?

A uniform section is a length of road pavement for which the condition and structure are similar, so that one set of rehabilitation options suits all of it. The South African Pavement Engineering Manual (SAPEM), Chapter 10 (2nd edition, October 2014), defines it in a sidebar to clause 5.1.2: one design serves the whole section (SANRAL, 2014b). Draft TRH 12, clause 2.2.4, takes all the information gathered on the road into account to divide it into uniform pavement sections (Department of Transport, 1997). SAPEM Chapter 14 (2nd edition, October 2014), clause 4.4.2, says treatments stay the same once a section is allocated (SANRAL, 2014f). See how SAPEM is organised.

The engineer sets the section, after the evidence is in. Draft TRH 12 clauses 1.4.1 and 2.1 warn that a length flagged at network level cannot be assumed uniform in its needs (Department of Transport, 1997). The section is a finding from as-built, visual, instrument and deflection evidence read together, and SAPEM Chapter 10 clause 5.1.2.1 describes dividing each lane into uniform sections (SANRAL, 2014b). The insight on one candidate boundary shows the arithmetic of a single split.

How do a segment, a reporting interval and a uniform section compare?

An assessment segment and a reporting interval are fixed before the road is surveyed. A uniform section is a result. The table adds the road link, which TMH 22 defines as uniform in road type, cross-section, construction, district and traffic (COTO, 2013). A road link is a network unit, not a uniform section in the TRH 12 sense.

Term	Who sets it	Length basis	Used for	Where described
Assessment segment	Road authority, in its network definition	Fixed in advance by pavement type and road setting	One visual rating at network level	TMH 9 A.3, A.7; TMH 22 B.1.3
Reporting interval	Survey specification and data standard	Fixed per data type; per point for deflection	Roughness, rut, texture and deflection records	TMH 22 D.5.4; TMH 13 A.4.3.7; TMH 18 5.6, 5.7
Uniform section	Investigating engineer, from combined evidence	Variable: ends where condition or structure changes	Project-level investigation and design	TRH 12 2.2.4; SAPEM 10 5.1.2; SAPEM 14 4.4.2
Road link	Road authority	Between significant intersections or boundaries	Network management	TMH 22 B.1.3

Why is a TMH 9 assessment segment not a uniform section?

TMH 9 says as much: clause A.3 states that it is "not the purpose of the assessment to identify uniform sections of distress" (COTO, 2016b). Three reasons follow. The boundary is fixed before anyone looks at the road, the rating is an average over the length, and the predefined lengths exist to keep one assessment consistent with the next.

A segment can stand in for a uniform section only while the other evidence at the same kilometre positions stays similar inside it; once structure, roughness, deflection or distress changes markedly, it stops being usable. TMH 9 clause A.2.1 asks for more detail on extent and location at project level (COTO, 2016b), and SAPEM Chapter 10 clause 5.1.2.1 says project-level visual segments are normally shortened (SANRAL, 2014b). Draft TRH 12 Appendix 1, Table 14, recommends basic unit lengths and notes that several are usually combined into one uniform pavement section (Department of Transport, 1997). See taking a visual rating to a rehabilitation shortlist for the next hand-off.

How do the three lengths line up on one synthetic road?

Synthetic example; every value is invented. Synthetic Road S8 is a rural flexible pavement, one lane, from km 12.00 to km 15.60 (3.6 km). The authority defines two assessment segments of 1.8 km: A1 from km 12.00 to km 13.80 and A2 from km 13.80 to km 15.60. Roughness is reported per 100 m, giving 36 records, with raw 10 m values stored. Falling weight deflectometer (FWD) tests every 250 m from km 12.00 give 15 points, the last at km 15.50. The engineer then proposes uniform sections U1 from km 12.00 to km 13.15, U2 from km 13.15 to km 14.60 and U3 from km 14.60 to km 15.60.

U1 is 1.15 km, U2 is 1.45 km and U3 is 1.00 km, a total of 3.60 km. A1 overlaps U1 for 1.15 km and U2 for 0.65 km; A2 overlaps U2 for 0.80 km and U3 for 1.00 km. No interior segment boundary coincides with a section boundary. Only 63.9 % of A1 lies in U1 (1.15 divided by 1.80); the other 36.1 % lies in U2, so A1's single rating describes both sections.

The boundary at km 13.15 falls inside the 100 m record from km 13.10 to km 13.20, which straddles U1 and U2. The stored 10 m values can be split there; the 100 m average cannot. The deflection points fall five in U1, six in U2 and four in U3.

How do you align visual, instrument and deflection records on one kilometre datum?

A kilometre datum is the zero point and direction from which every kilometre value is measured. The manuals state parts of an alignment rule in different places; the six steps below are the RoadSense reading of them, not a clause of any manual.

1. Fix one road definition: the same road identifier, section, start kilometre and direction of increasing kilometres for every record.
2. Match the geographic information system (GIS) road line to the direction of the kilometre distances, and keep both surveyed lengths and official kilometre positions where they differ. TMH 22 clause B.1.4 sets out both (COTO, 2013).
3. Check survey direction against that definition. TMH 13 Part B, clause B.4.3, lists survey direction and linear distance references among its data checks (COTO, 2016c); the insight on chainage, direction and wheelpath covers a reversal.
4. Keep each record at its own length: a segment stays a segment, an interval an interval, a deflection a point.
5. Join by overlap on the datum, treating a point as an interval of zero length.
6. Check that no value falls outside the road's start and end kilometres: clause 5 of TMH 18 Draft Version 4 and of Committee Draft 5.0 says such values are discarded (COTO, 2014) (COTO, 2016i).

What do the manuals not decide, and what is commonly misread?

TMH 9 does not decide uniform sections, and TMH 22 lists reporting intervals but not where a uniform section ends. Draft TRH 12 and SAPEM Chapters 10 and 14 say what a uniform section is for; SAPEM Chapter 10 adds that delineation is often based on a cumulative difference approach (SANRAL, 2014b), but each boundary stays an engineering judgement. None of the clauses cited here gives a rule for carrying a rating from one length to another.

Three misreadings recur.

• "A segment rating describes a uniform stretch." It is an average over a predefined length (TMH 9 A.3, A.5.1).
• "A 100 m record is a uniform section." It is a reporting unit set by the specification, and its boundaries follow the grid.
• "Shorter segments give uniform sections." They give finer ratings on a finer grid; uniformity is still a judgement.

What should you keep when all three lengths are in one record?

• The road definition: identifier, section, start kilometre, direction, and the manual edition behind each length.
• Each assessment segment with start and end kilometre, date, assessor and rating as recorded (COTO, 2016b).
• Each instrument record with start and end kilometre, interval, device, lane and raw finer values, and each deflection result as a point with kilometre, date and lane.
• Each uniform section with its boundaries, the evidence behind each and the records inside it.
• The overlap table and the TMH 18 version (Draft Version 4 or Committee Draft 5.0) of any data file.

Frequently asked

What is the difference between an assessment segment and a uniform section?

An assessment segment is the length of road for which one visual rating is recorded, and the road authority fixes it before the survey. A uniform section is a length of road pavement whose condition and structure are similar enough for one design, and the engineer sets it from combined evidence. Their boundaries need not coincide, and the South African visual assessment manual TMH 9 says its assessment is not meant to identify uniform sections.

What is a reporting interval for roughness?

A reporting interval is the length over which an instrument value, such as the international roughness index (IRI) of a road pavement, is averaged and stored as one record. The road asset management manual TMH 22 lists a reporting interval for each data type in clause D.5.4, and TMH 13 Part A recommends interpreting IRI averaged over 100 m sections. The interval is a reporting choice and does not show where the pavement changes.

Can a TMH 9 assessment segment be used as a uniform section for a road pavement?

A TMH 9 assessment segment can stand in for a uniform section of road pavement only while the other evidence at the same kilometre positions stays similar inside it. TMH 9 Part A, clause A.3, says its assessment is not meant to identify uniform sections, and a segment rating is an average. Draft TRH 12 and SAPEM Chapter 10 expect the engineer to divide the road into uniform sections from combined as-built, visual, instrument and deflection evidence.

What is a uniform section of road pavement?

A uniform section is a length of road pavement whose condition and structure are similar enough for one design to serve the whole of it. The engineer sets it after the evidence is in. SAPEM Chapter 10 (2nd edition, October 2014) defines it in a sidebar to clause 5.1.2, and Draft TRH 12 (1997) clause 2.2.4 uses all the information gathered on the road to divide it into uniform pavement sections.

How do you align visual ratings, instrument records and deflection points on one road?

Place every record on one road definition: the same road identifier, section, start kilometre and direction of increasing kilometres. Keep each record at its own length, so a visual segment stays a segment, an instrument record stays an interval and a deflection result stays a point. Join them by overlap on the kilometre datum, and record uniform sections as separate results with their reasons.

Which editions of the manuals does this guide use?

The South African road manuals TMH 9 and TMH 13 are Committee Draft Final, May 2016. TMH 22 is a draft dated March 2013. TMH 18 is read in Draft Version 4 (October 2014) and Committee Draft 5.0 (February 2016). Draft TRH 12 is dated 1997, and SAPEM Chapters 10 and 14 are the 2nd edition, October 2014. Confirm the edition a client names before relying on any clause.

Where this connects

The two manuals to read next are TMH 18 and Draft TRH 12. TMH 18 writes segments, intervals and points into exchange files. Draft TRH 12, read with SAPEM Chapter 10, turns uniform sections into an investigation and a design.

Last reviewed on 21 October 2026 against TMH 9, TMH 13, TMH 18, TMH 22, Draft TRH 12 and SAPEM Chapters 10 and 14, in the editions cited.

References

1. COTO (2016b). TMH9: Manual for Visual Assessment of Road Pavements. Part A: General. Committee Draft Final, May 2016. Original source.Section A.2: degree and extent
2. COTO (2013). Draft TMH 22: Road Asset Management Manual. Draft, March 2013. Original source.Sections, tables and clauses are cited inline in each guide
3. COTO (2016d). TMH 13: Automated Road Condition Assessments. Part A: General. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
4. COTO (2014). TMH 18: Road Asset Data Electronic Exchange Formats. Draft Version 4, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
5. COTO (2016i). TMH 18: Road Asset Data Electronic Exchange Formats. Committee Draft 5.0, February 2016. Original source.Sections, tables and clauses are cited inline in each guide
6. Department of Transport (1997). TRH12: Flexible Pavement Rehabilitation Investigation and Design. Draft TRH12, Pretoria: Department of Transport for COLTO. Original source.Chapter 1: rehabilitation design process; investigation, evaluation, analysis and economic appraisal
7. SANRAL (2014b). South African Pavement Engineering Manual, Chapter 10: Pavement Design. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
8. SANRAL (2014f). South African Pavement Engineering Manual, Chapter 14: Post-Construction. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
9. COTO (2016c). TMH13: Automated Road Condition Assessments. Part B: Positioning. Committee Draft Final, May 2016. Original source.Positioning and distance measurement guidance

Use the manual edition and project requirements that apply to your work. Worked examples use synthetic values and are not site-specific design advice.