Which lane to survey for road condition? TMH 9, 13, 18 and 22

A lane and wheelpath convention states which lane of a road, and which wheelpath (the strip of that lane a tyre follows), a road condition survey records. The South African Technical Methods for Highways (TMH) manuals 9, 13, 18 and 22 treat it differently. The visual assessment manual TMH 9 (Committee Draft Final, May 2016) concentrates the assessment of a multi-lane road on the worst lane, typically the slow lane (COTO, 2016b). Draft TMH 22 Part D (Road Asset Management Manual, March 2013) takes instrument data from the slow lane, in a wheelpath that depends on the parameter (COTO, 2013). A TMH 18 road-data file (Draft Version 4, October 2014, or Committee Draft 5.0, February 2016) records the lane in a code labelled optional yet required for lane-specific data, so an empty code loses the choice (COTO, 2014) (COTO, 2016i).

Key facts

• The South African visual assessment manual TMH 9 says, in Part A section A.5.1, that on a multi-lane road the assessment concentrates on the worst lane, typically the slow lane.
• Draft TMH 22 (road asset management) lists in Table D-8 the slow lane for instrument data: both wheelpaths for roughness and rut depth, the left wheelpath for macrotexture and deflection.
• The South African automated road condition manual TMH 13, Part A section A.4.3.5, leaves the lanes surveyed and the test positions, such as the left wheelpath, to the survey specification.
• TMH 18 road-data files (Draft Version 4, Committee Draft 5.0) label the lane code Optional yet mark it required for lane-specific data, so an empty code loses the lane.
• TMH 18 numbers road lanes from fast lane 1: the slow lane is P1 or S1 (primary or secondary direction) with one lane per direction, P2 or S2 with two.
• A visual rating, a roughness value, a rut depth and a deflection test can each describe a different lane or wheelpath of the same road.

The concept map

The connections in text

1. Instrument data: slow lane, wheelpath per parameter (TMH 22 Part D) → Visual rating: worst lane, full paved width (TMH 9 Part A): defers visual segments
2. Instrument data: slow lane, wheelpath per parameter (TMH 22 Part D) → Specification names lane and test position (TMH 13 Part A): planning to TMH 13
3. Visual rating: worst lane, full paved width (TMH 9 Part A) → Assessment form: no lane field (TMH 9 Part B): rating on form
4. Specification names lane and test position (TMH 13 Part A) → Roughness and rut: left and right columns (TMH 13 Parts C, D): names test position
5. Specification names lane and test position (TMH 13 Part A) → Texture and deflection: test positions (TMH 13 Parts E, F): names test position
6. Assessment form: no lane field (TMH 9 Part B) → Lane code: labelled optional (TMH 18): visual summary file
7. Roughness and rut: left and right columns (TMH 13 Parts C, D) → Lane code: labelled optional (TMH 18): lane code column
8. Texture and deflection: test positions (TMH 13 Parts E, F) → Lane code: labelled optional (TMH 18): lane code column
9. Lane code: labelled optional (TMH 18) → Stored record: lane kept or lost (TMH 22 Part D): lane kept or lost

Which lane and wheelpath do TMH 9, TMH 13, TMH 18 and TMH 22 name?

TMH 9 names the worst lane, TMH 22 Part D the slow lane with a wheelpath for each parameter, and TMH 18 gives the lane a code. TMH 13 (Committee Draft Final, May 2016) leaves both to the survey specification. TMH is a COTO (Committee of Transport Officials) series of road manuals. In the table, roughness is the unevenness of the surface, rut depth the depth of the permanent groove in a wheelpath, macrotexture the coarse texture of the surface and deflection the elastic movement of the surface under a load.

Manual and clause	Lane	Wheelpath	In the file
TMH 9 Part A, A.5.1	Worst lane, typically the slow lane	None set; full paved width	Lane code in the TMH 18 visual file
TMH 22 Part D, Table D-8	Slow lane for instruments; all lanes within segment for visual	Both for roughness and rut; left for texture and deflection	Stored per segment, 100 m or point
TMH 13, A.4.3.5 and Parts B to G	Set by the specification	Set by the specification	Typical columns: lane and direction; wheelpath values in Parts C to E
TMH 18, Table 6 and section 4.2	Lane code: direction, then a number from the fast lane (1)	Left and right columns; centre for texture	Lane code labelled optional; required for lane-specific data

Read across the rows: a visual rating, a roughness value, a rut depth and a deflection test can each describe a different strip of one road.

Which lane does a TMH 9 visual assessment rate?

TMH 9 Part A, section A.5.1, says that on a multi-lane road the assessment should concentrate on the worst lane, which typically is the slow lane. It also covers the full paved width, including paved shoulders (COTO, 2016b). The word "typically" makes the slow lane the usual case, not a rule.

Narrated film · 1:15

Which lane and wheelpath?

A map places TMH 9, TMH 13 and TMH 22 nodes that each carry the lane and wheelpath they use, converges them on one TMH 18 lane-code node, and ends on the callout 'Same road, different strips: record the lane.'

Read the film transcript

Which lane of a road does a survey record, and which wheelpath of that lane? A wheelpath is the strip of a lane that a tyre follows, and the South African TMH manuals pick different strips. Draft TMH 22 Part D takes instrument data from the slow lane, in a wheelpath that depends on the parameter. TMH 9 concentrates on the worst lane, typically the slow lane, but its flexible pavement form has no lane field. TMH 13 names neither, and leaves the lane and the test position to the survey specification. They converge on the TMH 18 lane code, labelled optional yet required for lane-specific data, so an empty code loses the lane. TMH 18 numbers lanes from the fast lane, so with two lanes each way the slow lane is P2 or S2. In this synthetic example the assessor rates the worst lane, P1, while the profiler runs in the slow lane, P2. Left and right roughness read three point two and four point six metres per kilometre, so their mean hides the gap. Four choices stay with the specification, so keep the lane code, what left means, and the clause that named each wheelpath. Same road, different strips, so record the lane on every value you keep. Next come the draft road manual TRH 12, on distress across the lane, and the COTO Standard Specifications, on testing new work.

TMH 9 sets no wheelpath for the rating; its distress descriptions mention wheelpaths as places where distress forms, such as rutting (COTO, 2016j). Table A.4, the list of road inventory items, has an optional item for the direction of assessment and none for the lane (COTO, 2016b). The Part B form for flexible pavements (section B.6, page B-54) has no lane field (COTO, 2016j). The guide on running a TMH 9 visual assessment covers the procedure.

Which lane and wheelpath does TMH 22 set for instrument data?

TMH 22 Part D, section D.5.4, lists the slow lane for every instrument record in Table D-8. The wheelpath depends on the parameter: both for roughness and rut depth, and the left wheelpath for macrotexture and deflection (COTO, 2013).

For visual assessment the table reads "all lanes within segment", with no wheelpath, while TMH 9 points to the worst lane of a multi-lane road. Section D.5.4 refers to TMH 9 for segment sizes, not for the lane, so the specification should say which wording a visual rating follows (COTO, 2013). The split is old: Draft TRH 22 (Technical Recommendations for Highways: Pavement Management Systems, 1994), section 4.5 and Table 4.3, lists all lanes for visual assessment and the slow lane for riding quality, deflection and rut depth (CSRA, 1994).

Which lane and wheelpath does TMH 13 set for an automated survey?

TMH 13 (Automated Road Condition Assessments) sets neither on its own. Part A, section A.4.3.5, says the survey specification should cover the facilities, such as slow and fast lanes and ramps, and the test positions, such as the left wheelpath (COTO, 2016d). Parts B, C, D and E list lane and direction among the typical section details of a data file (COTO, 2016c) (COTO, 2016a) (COTO, 2016e) (COTO, 2016f). The parts differ on the wheelpath:

• Roughness (Part C). The International Roughness Index (IRI) comes from the profile of one wheelpath; the half-car roughness index (HRI) averages the profiles of both travelled wheelpaths (Part A, section A.2.2.5). Part C lists left and right IRI as typical columns (COTO, 2016d) (COTO, 2016a).
• Rut depth (Part D). Typical columns hold a left value, or left and right values (section D.4.2), and a sidebar in section D.2.4 points to TMH 22 Part D for storing the average rut depth in each wheelpath (COTO, 2016e).
• Skid resistance and texture (Part E). Typical columns hold left and right wheelpath values (section E.4.3) (COTO, 2016f).
• Deflection (Part F). A test point carries its lane and its geometry, for example the wheelpath or the centre of a slab (section F.4.2) (COTO, 2016g).
• Imaging (Part G). The Class 1 collection lane is divided into five transverse zones, three named inside wheelpath, between wheelpaths and outside wheelpath (Table G.6) (COTO, 2016h).

What does a TMH 18 lane code carry, and what is lost when it is empty?

A TMH 18 lane code names the direction and lane position. The base-field table (Table 6, section 1.7) labels the field Optional, then says it is required when the data represent an individual lane, which is the case for most data sets; only data that are not lane specific leave it empty. Draft Version 4 and Committee Draft 5.0 word this the same way (COTO, 2014) (COTO, 2016i). An empty code therefore says the record is not lane specific, and the convention is gone.

Section 4.2 builds the code from direction (primary P or secondary S), a lane type left blank for travel lanes, and a lane number. The fast lane is 1 and numbers rise towards the slowest lane. The slow lane of TMH 22 is therefore P1 or S1 where each direction has one lane, and P2 or S2 where each has two.

The field appears in the visual condition summary files (sections 5.2 to 5.5), the combined instrument data (.cid) file (section 5.6) and the deflection file (section 5.7). The .cid file names the wheelpath in its columns (left, right and, for texture, centre); the deflection file has no column for the position within the lane. The guide on checking a TMH 18 file set before sending lists the lane-code check.

How can one road give records from different lanes and wheelpaths?

A visual rating and the instrument records of one road can describe different lanes. Synthetic example: every value below is invented. Synthetic Road R1 has two lanes in each direction, coded in the primary direction P1 (fast) and P2 (slow) in TMH 18 Committee Draft 5.0. In one segment the assessor judges P1 the worst lane and rates it, while the profiler runs in P2, as Table D-8 lists.

Record	Lane code	Strip	Synthetic value
Visual rating	P1	Whole lane, no wheelpath	Degree and extent on the form
Roughness (IRI)	P2	Left wheelpath	3.2 m/km
Roughness (IRI)	P2	Right wheelpath	4.6 m/km
Rut depth	P2	Left wheelpath	6.8 mm
Rut depth	P2	Right wheelpath	8.4 mm
Macrotexture (mean profile depth)	P2	Left wheelpath	0.8 mm

The mean of the two roughness values is (3.2 + 4.6) / 2 = 3.9 m/km, and the wheelpaths differ by 1.4 m/km. A left-only rut depth of 6.8 mm would hide the 8.4 − 6.8 = 1.6 mm by which the right wheelpath is deeper. The mean is not the HRI, which averages the two profiles before it calculates (COTO, 2016d).

In a Committee Draft 5.0 .cid row the five instrument values share the lane code P2 (COTO, 2016i). The visual row says P1 if its lane code is filled. If both codes are empty, the rows look like the same strip, with no warning that they differ.

What do the TMH lane and wheelpath conventions not decide, and what is commonly misread?

Four choices stay with the specification and the file:

• The lane of a visual rating on a multi-lane road. TMH 9 says the worst lane and TMH 22 Table D-8 says all lanes within the segment. The guide on specifying a road condition survey is where that choice is fixed.
• The meaning of left. Table D-8 and the TMH 18 columns say left; TMH 13 Part G says inside and outside. Part C, section C.5.3, asks the contractor for a definition sheet of codes and column names (COTO, 2016a), which should say left of what.
• The texture position. TMH 22 section D.4.4 allows left, right and/or centre, Table D-8 lists left, and TMH 13 Part E lists left and right (COTO, 2013) (COTO, 2016f).
• The TMH 18 version. The lane-code wording is identical, but the column names differ: AVG_IRI_L in Draft Version 4 and IRI_L_AVG in Committee Draft 5.0 (COTO, 2014) (COTO, 2016i).

Three misreadings follow:

• "Lane 1 is the slow lane." In TMH 18 the fast lane is 1.
• "Optional means the field can stay empty." It stays empty only for data that are not lane specific.
• "One IRI describes the lane." IRI belongs to a wheelpath, and an average of two hides which one was worse.

What should a survey record keep about the lane and wheelpath?

Keep these items with every delivery:

• the lane code on every record and, for a visual rating, the lane the assessor judged worst, because the TMH 9 Part B form has no field for it;
• the definition of the left, right and centre wheelpaths, and the direction of survey;
• the clause or table, such as TMH 22 Table D-8, that named the lane and wheelpath for each parameter;
• the TMH 18 version and the column names used.

The insight on survey acceptance evidence explains why such records show that data are usable; chainage, direction and wheelpath covers the location side.

Frequently asked

Which lane should a road condition survey use?

A road condition survey uses the lane its manual or specification names, and the manuals differ. The South African visual assessment manual TMH 9 says a multi-lane road assessment should concentrate on the worst lane, which typically is the slow lane. Draft TMH 22 Part D, Table D-8, lists the slow lane for roughness, rut depth, macrotexture and deflection data. The survey specification should name the lane for each record.

Is lane 1 the slow lane in a TMH 18 file?

In a TMH 18 file, lane 1 is the fast lane, not the slow lane. TMH 18, the South African road asset data exchange manual, numbers lanes from the fast lane as 1 and counts towards the slowest lane, in both Draft Version 4 and Committee Draft 5.0. The slow lane is therefore P1 or S1 where each direction has one lane, and P2 or S2 where each has two, with P for the primary direction.

Can the lane code be left empty in a TMH 18 file?

A TMH 18 lane code can be left empty only for data that are not lane specific. The base-field table of TMH 18, the South African road data exchange manual, labels the lane code Optional in Draft Version 4 and Committee Draft 5.0, yet says it is required when the data represent an individual lane, which is the case for most data sets. An empty lane code on a roughness record loses the lane.

Which wheelpath does TMH 22 use for roughness, rut depth and texture?

Draft TMH 22 Part D, the South African road asset management manual, lists in Table D-8 both wheelpaths for roughness and rut depth and the left wheelpath for macrotexture and deflection. Section D.4.4 describes macrotexture, reported as mean profile depth, for the left wheelpath, the right wheelpath and/or the centre, and TMH 13 Part E lists left and right, so the specification should name the position.

What is a wheelpath in a road condition survey?

A wheelpath is the strip of a lane that a tyre follows, so every lane has a left and a right wheelpath. In a road condition survey, roughness is calculated for each wheelpath, and rut depth and texture are recorded for them. The South African manuals say left and right in TMH 22 and in TMH 13 Parts C to E, and inside and outside in TMH 13 Part G.

Does TMH 13 say which lane an automated survey uses?

TMH 13, the South African manual for automated road condition assessments (Committee Draft Final, May 2016), sets neither the lane nor the wheelpath itself. Part A, section A.4.3.5, says the survey specification should cover the facilities included, such as slow and fast lanes and ramps, and the test positions, such as the left wheelpath. The specification therefore decides which strip an automated survey records.

Where this connects

Two manuals come next. Draft TRH 12 (1997), the South African Technical Recommendations for Highways (TRH) manual on flexible pavement rehabilitation, describes a detailed visual inspection that records where each distress lies across the traffic lane: on the shoulder, in the wheelpath or near the centre line (section 2.2.2.3) (Department of Transport, 1997). The COTO Standard Specifications, Chapter 20 (Draft Standard, October 2020), clause A20.1.5.5, describe the same instruments testing new work: roughness on every traffic lane and shoulder wider than 2.0 m, each wheelpath recorded separately and the two 100 m values averaged for judgement (COTO, 2020a). Last reviewed on 21 October 2026 against the editions named above.

References

1. COTO (2020a). Standard Specifications for Road and Bridge Works for South African Road Authorities. Draft Standard, October 2020. Original source.Sections, tables and clauses are cited inline in each guide
2. COTO (2016a). TMH13: Automated Road Condition Assessments. Part C: Roughness. Committee Draft Final, May 2016. Original source.Sections C.2–C.5; Table C.5 is for calibration-section selection
3. COTO (2016c). TMH13: Automated Road Condition Assessments. Part B: Positioning. Committee Draft Final, May 2016. Original source.Positioning and distance measurement guidance
4. 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
5. 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
6. COTO (2016e). TMH 13: Automated Road Condition Assessments. Part D: Rutting. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
7. COTO (2016f). TMH 13: Automated Road Condition Assessments. Part E: Skid Resistance and Texture. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
8. COTO (2016g). TMH 13: Automated Road Condition Assessments. Part F: Surface Deflection. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
9. COTO (2016h). TMH 13: Automated Road Condition Assessments. Part G: Imaging. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
10. 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
11. 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
12. COTO (2013). Draft TMH 22: Road Asset Management Manual. Draft, March 2013. Original source.Sections, tables and clauses are cited inline in each guide
13. COTO (2016j). TMH 9: Manual for Visual Assessment of Road Pavements. Part B: Flexible Pavements. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
14. CSRA (1994). Draft TRH22: Pavement Management Systems. Pretoria: Department of Transport, 1994. Original source.Technical Addendum A.3, equations A.3.1–A.3.3 and Table A.3.1, printed pp. 125–126; Section 6.2.2, printed pp. 55–56, for alternative-weight normalisation
15. 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

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