Reference
From a classified count to design traffic in msa
A flexible pavement is designed for one number: the cumulative standard axles it will carry over its design period. A traffic count supplies one input to that number. Here are the formula, the guideline’s own example, what the count must contain, and what no count can supply.
The equation
Equation 4.5, term by term
IRC:37-2018 is the current edition on IRC’s publications list of 28 September 2026. Its standard axle is an 80 kN single axle with dual wheels; design traffic is the number of those the pavement must carry, in million standard axles (msa).
| Term | What it is | From the count? |
|---|---|---|
| A | Commercial vehicles per day in the year construction is completed: directional on a divided carriageway, two-way otherwise | Yes, as P, carried forward by eq. 4.6 |
| D | Lateral distribution factor, set by the carriageway | No |
| F | Vehicle damage factor: standard axles per commercial vehicle | No: an axle load survey |
| n | Design period: 20 years for National Highways, State Highways and Urban Roads; 15 years for other roads; above 300 msa, long-life or at least 30 years | No |
| r | Annual growth rate, set per category of commercial vehicle by IRC:108, and at least 5% | Not from one count |
| Carriageway | D | Of |
|---|---|---|
| Single-lane | 1 | Both directions, in full |
| Intermediate lane, 5.5 m | 0.75 | Two-way total |
| Two-lane two-way | 0.50 | Two-way total |
| Four-lane single carriageway | 0.40 | Two-way total |
| Dual two-lane | 0.75 | Each direction |
| Dual three-lane | 0.60 | Each direction |
| Dual four-lane | 0.45 | Each direction |
Worked example
The guideline’s own example: 131 msa
Annex II.3 designs a four-lane divided carriageway carrying 5,000 commercial vehicles a day, two-way, in the year construction is completed, growing at 6% for 20 years, with a VDF of 5.2 in both directions. These are the code’s illustrative inputs, not survey data.
Built in stages, the first 10 years carry 46.9 msa (printed 46.9); because the bituminous layer must keep 40% of its life at overlay, stage one is designed for 46.9 ÷ 0.60 = 78 msa.
- 1Directional traffic5,000 × 0.5 = 2,500 (the example assumes an even split)
- 2DistributionD = 0.75 for each direction of a dual two-lane carriageway
- 3Growth[(1 + 0.06)^20 − 1] ÷ 0.06 × 365 = 13,426.7
- 4First-year axles per day2,500 × 0.75 × 5.2 = 9,750
- 5Design traffic13,426.7 × 9,750 = 130.9 million; IRC prints 131 msa
What the count must contain
Five things the count has to get right
Every one of these changes A, and N is proportional to A: on the example, a count 5% low gives 130.9 × 0.95 = 124.4 msa.
- Seven days, 24 hours
- IRC:37-2018 bases present traffic on “the seven-day 24-hour traffic volume count”. It does not ask for AADT.
- Each direction separately
- On a divided carriageway A is directional; a count replaces the example’s even split with the measured one. Where axle loads differ by direction, VDF is evaluated for each.
- Commercial vehicles, in the groups the axle load survey uses
- Growth rates are set for each category of commercial vehicle, and MoRTH’s sample DPR reports VDF for LCV, 2-axle, 3-axle, MAV and bus. Equation 4.5 is worked per group and added.
- The date
- The count gives P, and equation 4.6 carries it forward x years to the year construction ends.
- The two-way total
- The indicative VDF table is read on two-way commercial vehicles per day, so the band a road falls in comes from the two directions added.
Classes
Which of our 19 classes are commercial vehicles
IRC:37 draws the line by weight: “Only the commercial vehicles having gross vehicle weight of 3 tonnes or more are considered for the structural design of pavements.” It names no vehicle types. A camera sees shape and axles, not weight, so the mapping below is ours; state the judgement rows as assumptions in the design report. The nearest IRC list, in IRC:SP:72-2015 for low-volume rural roads, names heavy trucks, full-sized buses, tractor-trailers, mini buses and pick-up vans.
| Class | Commercial? | Basis |
|---|---|---|
| Bus | Yes | IRC:SP:72-2015 lists full-sized buses; MoRTH’s sample DPR has a bus VDF |
| Truck 2-Axle, Truck 3-Axle | Yes | VDF groups “2-axle” and “3-axle” in MoRTH’s sample DPR |
| Truck 4-Axle, Truck 5-Axle, Truck 6-Axle, Truck MAV | Yes | VDF group “MAV” |
| Truck Articulated | Yes | Grouped by its axle count — our judgement |
| LCV | Judgement | MoRTH’s sample DPR has an LCV VDF; the class name carries no weight |
| Mini LCV | Judgement | SP:72 counts pick-up vans; a camera cannot weigh one |
| Mini Bus | Judgement | SP:72 lists mini buses |
| Tractor Trolley | Judgement | SP:72 lists tractor-trailers |
| Tractor | Judgement | SP:72 names tractor-trailers, not tractors alone |
| Other | Check the video | A residual class: look at the verification video |
| Two Wheeler, Three Wheeler, Car / Jeep / Van, Cycle, Cycle Rickshaw | No | SP:72: bicycles and motor cycles are “of little consequence”; the rest are light passenger vehicles — our judgement |
The judgement is not a rounding error
In the 35-minute run whose output we publish, the clear classes counted 126 commercial vehicles. Adding the judgement classes gives 303, which is 303 ÷ 126 = 2.4 times as many.
That is 35 minutes of software output, not a seven-day count, so no msa is computed from it. It shows why the mapping belongs in the design report.
What the count cannot supply
VDF needs a weigh pad, not a camera
IRC:37-2018 arrives at VDF by axle load surveys on existing roads for at least 24 hours in each direction, on vehicles chosen at random, loaded or empty. Each axle’s load is converted by a fourth power over a reference load: 65 kN for a single axle with single wheels, 80 kN with dual wheels, 148 kN for a tandem and 224 kN for a tridem.
MoRTH’s DPR terms of reference ask for two normal days in both directions, normally on axle load pads, with a VDF for each truck type, and add a floor: “if the calculated VDF is found to be below the national average, then national average shall be used”. They do not say where that national average is published, and we did not find it.
The indicative table is a fallback with two conditions, both required: “Where information on the axle loads is not available and the proportion of heavy vehicles using the road is small”. It is not a default for a national highway.
| CV/day | Rolling / plain | Hilly |
|---|---|---|
| 0–150 | 1.7 | 0.6 |
| 150–1,500 | 3.9 | 1.7 |
| More than 1,500 | 5.0 | 2.8 |
| Below 3,000 CV/day | 20% |
|---|---|
| 3,000 to 6,000 CV/day | 15%, at least 600 vehicles |
| Above 6,000 CV/day | 10%, at least 900 vehicles |
Older numbers
Check which code a number came from
IRC:81-1997, the Benkelman-beam overlay guideline, is still listed, and its traffic numbers differ from IRC:37-2018’s for new pavements. A figure carried over from an overlay design will not match.
The 20 msa floor is gone too. Until 1 February 2020, IRC’s two-, four- and six-laning manuals said a flexible pavement’s design traffic “shall not be less than 20 msa”; the amendments replaced that with IRC:37-2018’s design periods. Service and slip roads stay at ten million standard axles under the four-laning manual.
| IRC:37-2018 | IRC:81-1997 | |
|---|---|---|
| Indicative VDF, plain, above 1,500 CV/day | 5.0 | 4.5 |
| Indicative VDF, plain, 150–1,500 | 3.9 | 3.5 |
| Two-lane two-way distribution | 0.50 | 0.75 |
Where Playcraft fits
What we ship, what we build on demand, what we refuse
Shipped: Playcraft ATCC counts traffic from recorded video by direction, in 15-minute intervals, with a per-vehicle log and an annotated verification video. Its schedule separates 2-, 3-, 4-, 5- and 6-axle trucks and MAV: that is P, by direction and by class.
What our accuracy covers. We publish 95.0% by volume and 92.1% by weighted class against 16,560 hand-counted vehicles, 60% of them two-wheelers. That says little about trucks: of the 6 classes with a published figure, bus (92.7% on 150) is the only clearly commercial one, and the other 13 classes, every truck class among them, had 189 hand-counted vehicles between them. Where commercial vehicles decide the design, check those classes on your own footage.
Built on demand: cumulative msa is not part of the shipped product; we build it on demand from your count, with VDFs from your axle load survey or Table 4.2 where its conditions hold.
Refused: we do not run axle load surveys. See also AADT from a count and the traffic surveys a DPR asks for.
Sources
- IRC:37-2018, Guidelines for the Design of Flexible Pavements (Fourth Revision)Indian Roads Congress · cl. 4.1–4.6; Tables 4.1, 4.2; Annex II.3, II.7 · November 2018
- IRC:SP:72-2015, Guidelines for the Design of Flexible Pavements for Low Volume Rural RoadsIndian Roads Congress · cl. 3.1.2, 3.4.3 · 2015
- IRC:81-1997, Guidelines for Strengthening of Flexible Road Pavements Using Benkelman Beam Deflection TechniqueIndian Roads Congress · cl. 5.4.2, 5.4.3; Table 4 · 1997
- Notifications No. 31–33: amendments to IRC:SP:73, SP:84 and SP:87Indian Roads Congress · Indian Highways, February 2020; effective 1 February 2020 · February 2020
- Revised RFP for consultancy services for preparation of Detailed Project ReportsMoRTH · Circular 130.247, Terms of Reference cl. 4.9.5; Appendix VIII sample DPR summary · 29 September 2025
Next step
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