IRC 37:2018 designs flexible pavements against cumulative standard axles — million standard axles, or MSA — over the design period, typically twenty years. One standard axle is 80 kN. Everything about the pavement section follows from that single number and the subgrade CBR.
The design traffic is N = 365 × A × [(1+r)^n − 1] / r × VDF × LDF × 10⁻⁶, where A is the AADT of commercial vehicles at opening, r the annual growth rate, n the design life, VDF the vehicle damage factor and LDF the lane distribution factor.
The vehicle damage factor is where designs quietly diverge. It converts a mixed commercial fleet into standard axles, and IRC 37 ranges it from about 1.5 for low-volume plain-terrain roads to 5.5 for high-volume plain-terrain national highways. Choosing 2.5 where the traffic warrants 4.5 is not a rounding error; the damage relationship is a power law, so it changes the required section outright.
The subgrade side is simpler and just as often fudged. The minimum is 5 per cent CBR at 97 per cent of maximum dry density, and the design CBR is not the average of the test results — it is the value that 90 per cent of the tested values equal or exceed. Using the mean instead of that 90th-percentile rule designs the road for its best patch of subgrade.
With those two inputs the catalogue does the rest: IRC 37 tabulates total thickness and the layer breakdown for subgrade CBR from 2 to 10 per cent and design traffic from 1 to 150 msa. The catalogue is not the shortcut some treat it as — it is the answer, provided the two numbers you entered it with were honest.