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Road Note 31 Flexible Pavement Design: Design Traffic, Subgrade & Catalogue Structures

Step 1: heavy vehicles and axle loads → equivalent standard axles (ESA) and the traffic class. Step 2: subgrade CBR → subgrade class. Step 3: capping → foundation class. Step 4: every Road Note 31 catalogue structure for your traffic and foundation, compared by thickness and by cost with your own rates.

Version: 2026-10-29 09:40Reference: Road Note 31, 5th edition (TRL / FCDO / ReCAP, 2023), Chapters 2, 3, 4, 8 & 9Author: Shahid Pervaiz
👋 You are looking at sample data so you can see what this tool does. Change any value, or click New Project to start your own.

Project

Design traffic (Chapter 2)

Design life, growth and design lane

Growth periodYearsGrowth rate (% a year)
Design life = sum of the years. RN31 recommends 15–20 years for flexible pavements (Section 2.1.4).

How do you want to describe the heavy traffic?

One row per vehicle type and load state (laden and empty as two rows). Enter the daily number in each direction in the count year and the load on each axle group. Vehicles with an unladen weight under 3,000 kg are ignored (Section 2.1). The vehicle types follow Table 2-1; new rows start at the Table 2-4 standard load of each group (Pakistan types at the NHA legal limits).

#Vehicle (Table 2-1) & descriptionDir. 1 per dayDir. 2 per dayAxle groups & loadsESA / vehicleESA / day dir. 1ESA / day dir. 2
Legal axle load limits:
Axle group codes and standard loads (Tables 2-2 and 2-4)
CodeAxle groupAxlesTable 2-4 standard load = 1 ESA

Equation 2-5: EF = (P / SA)n. With each axle separately the group load is shared equally by its axles and each axle is compared with 80 kN (8,160 kg); with each axle group the whole group is compared with its Table 2-4 load. RN31 recommends the Table 2-4 standard axles where more than 25% of the trucks run on super-single (wide-base) tyres (Key point 6). The equation applies up to 13 t per axle (Section 2.3.2).

ESA per day in the count year from an axle load survey analysis (e.g. TRL Overseas Road Note 40 or the RN31 spreadsheet), for each direction.

Damage exponent n (Table 2-3)

Subgrade (Chapter 3)

Presumptive CBR values when there is no test data (Table 3-5)
MaterialUSCSExcellent to good drainageFair to poor drainage

Expansive (reactive) subgrade

Foundation class and capping (Section 8.2, Keys 2 & 3)

The catalogues are built on four foundation classes. Subgrades S3 to S6 are foundation classes F1 to F4 as they are; S1 and S2 must be capped to at least F1, and any subgrade can be capped to a higher class to thin the upper layers. Every route below is used in the comparison in Step 4.

Pavement structures from the catalogues (Chapter 9)

Costs (your rates, in place)

CodeMaterialRateUnit

Leave a rate blank if you don’t know it: options are then ranked by total thickness instead of cost.

Life-cycle cost (Chapter 12)

Options for this traffic and subgrade

Charts to consider
Untick charts that do not suit the project, e.g. surface-treated charts where the agency requires an asphalt wearing course. The ranking, life-cycle cost and report then cover only the ticked charts; charts that are valid but unticked are listed as not considered.
Design philosophy of the charts (Road Note 31)

Selected design

Changing a layer material (Section 8.6, Equation 8-1)

Where one material is scarce and another plentiful (for example sub-base material scarce but roadbase material plentiful), a catalogue layer can be replaced by a thinner layer of a stronger material: hc = h2 × a2 / a1, with the layer coefficients of Table 11-2. The new material must still meet the specification of the layer it replaces.

Method, assumptions & references

  • Design traffic. Equivalence factor EF = (P/SA)n (Equation 2-5) with the Table 2-3 exponent; ESA per day in each direction from the daily counts × seasonal factor F × mean EF per vehicle; the design-lane loading from Table 2-5; cumulative ESA by Equation 2-6, summed year by year when there are several growth periods. Traffic class from Key 1 (T1 < 0.3 … T10 50–80 MESA; T10 designs are long-life and used for all higher traffic). A value on a class boundary goes to the higher class.
  • Exponent per chart. Table 2-3 gives n by roadbase/sub-base: granular/granular 4 (Charts A1, B, C1, F), granular/cemented 3 (Charts A2, C2, D), cemented/cemented 4.5 (used here for Chart A3, whose cemented roadbase sits on a granular or hydraulically-modified sub-base; Table 2-3 has no cemented/granular row), bituminous/granular and bituminous/cemented 4. Choose one value for every chart to override.
  • Subgrade. Design CBR = lower 10th percentile of the soaked CBR results of a uniform section (5th percentile for trunk roads), PERCENTILE.INC method; at least 10 results; coefficient of variation below 30% (Section 3.3.6). Subgrade class from Table 3-9. Cover over reactive subgrades from Table 3-8.
  • Foundation. Foundation classes F1–F4 (Key 2, Table 8-1) and the capping options of Key 3. Foundation class F5 is a performance design proved by trials and is not covered. Performance foundation designs (Section 8.2.3) are checked on site with FWD/LWD surface modulus.
  • Structures. Thicknesses are read from the Chapter 9 catalogues (Charts A1, A2, A3, B, C1, C2, D for main roads; Chart F for low volume roads below 3 MESA, by LVR1/LVR2 and effective subgrade class). Chart cells are reproduced as printed; only the layer substitutions stated in the chart notes (40 mm AC/HRA for DBST, GS1 for GS2, triple surface treatment) are offered. Chart E (structural numbers) and rigid pavements (Chapter 10) are not included.
  • Costs. Quantity per km = thickness × carriageway width × 1,000 m (surface treatments in m²). Construction cost only, at your rates. RN31 Chapter 12 describes whole-of-life economic comparison.
  • Reference: Road Note 31, A guide to the structural design of surfaced roads in tropical and sub-tropical regions, 5th edition, TRL, 2023 (FCDO / ReCAP). Always read the notes of the chart you use, and check the design against your road agency’s requirements.

How to use the Road Note 31 Flexible Pavement Design tool

The tool follows Road Note 31 (5th edition): design traffic, then subgrade class, then foundation class, then the structure catalogues. Everything runs in your browser; nothing is uploaded.

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Start with the sample. The page opens with sample data so you can see what the tool does. Load Sample Data brings it back; New Project clears it.
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Step 1 — design traffic. Enter the years from the count to opening, the seasonal factor and the cross-section (Table 2-5 decides whether the design uses both directions or the heavier one). Add one or more growth periods; their years add up to the design life. Then describe the heavy traffic:
  • Vehicle mix & axle loads: + Add vehicle, pick a Table 2-1 vehicle or a Pakistan truck type, enter the daily number in each direction and the load on each axle group. Laden and empty trucks are separate rows. Choose whether equivalence factors are worked out per axle (80 kN) or per axle group (Table 2-4).
  • ESA per day by direction: when your axle load survey is already analysed.
  • Design traffic (MESA) known: type the design traffic directly.
The damage exponent n comes from Table 2-3 for each chart, or one value for all. The result is the design traffic in MESA and the traffic class (T1–T10, and the low-volume class when below 3 MESA).
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Step 2 — subgrade. Paste the soaked CBR results of one uniform section: the tool gives the 10th (or 5th) percentile design CBR, the coefficient of variation and the subgrade class S1–S6. Or type a design CBR. Enter the swell if the soil may be expansive.
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Step 3 — foundation. The table lists every way to reach a foundation class: the subgrade as it is (S3–S6) and the capping options of Key 3.
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Step 4 — structures. Pick the road type, the surfacing for Charts A1–A3, the Chart F axle-load type and the climate-resilient variant. Type your rates (per m³; surface treatments per m²), the width and the currency. The options table lists each chart that covers your traffic class with its best foundation route (tick Show every foundation route to see all), sorted by cost, or by thickness until every rate is filled. Use selects an option.
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Life-cycle cost and choosing an option (Chapter 12, Section 8.4). Set the analysis period (20–30 years), the discount rate (about 6%), the reseal interval for surface-treated options and the overlay thickness and interval for asphalt options (your assumptions), plus optional routine maintenance and residual value. With every rate filled, the table under the options gives each option’s life-cycle cost in present value and the options are ranked by it. The box Choosing between the options lists what Road Note 31 says to weigh as well: materials available, climate and drainage, construction capability, traffic certainty, life-cycle cost and other criteria. Every listed option is a valid design; the ranking is a screening, not a recommendation. Use Charts to consider (or Asphalt-surfaced only) to leave out charts that do not suit the project; they are then listed as not considered.
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Step 5 — selected design. The cross-section, layer table, quantities and cost per km, the chart notes, the material specifications and the full catalogue chart with your cell highlighted.
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Step 6 — changing a material. Equation 8-1 converts a catalogue layer to an equivalent thickness of another material with the Table 11-2 layer coefficients.
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Vehicles from a spreadsheet. Export Vehicles (.csv) shows the format; edit it and use Import Vehicles (.csv) (replaces the list). Columns: Class (lt2, mt3, … pk22, c), Description, Dir1PerDay, Dir2PerDay, AxleGroups (e.g. SAST 5.5; TADT 22, in the load unit shown on the page).
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Save your work. Save Project (.json) downloads all inputs; Load Project opens them again. With Autosave in this browser ticked the page also remembers your last inputs on this computer.
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Print or report. Print / Save as PDF prints this page. Generate Report builds a design report (traffic, subgrade, foundation, options, selected design and specifications) in a new tab, or inside the page if pop-ups are blocked.
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Inputs are never uploaded. Thicknesses are the catalogue values. Read the notes of the chart you use and check the design against Road Note 31 and your road agency’s requirements.

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