Design traffic from surveys, with design-method examples

Design traffic is the number of load repetitions the pavement must carry in its design life. It is built from survey data in these steps:

1. Traffic volume count (classified, 3-7 days, 12-24 h) gives the ADT/AADT by vehicle type; only commercial vehicles (above 3 t laden weight) are used for structural design (passenger cars are ignored).
2. Growth - the count is projected with the growth rate $r$ (from past trends, regression or economic data) to the year of opening: $A=P(1+r)^x$.
3. Axle load survey (at weigh stations, 24 h) gives the axle load spectrum; each axle is converted to standard axles by the fourth power law or load equivalency factors, giving the vehicle damage factor (VDF).
4. Lane and direction distribution - the share of traffic in the design lane.
5. Cumulative load over design life $n$:

$$N_s=\frac{365\,[(1+r)^n-1]}{r}\,A\,D\,F$$

Example 1 - IRC:37-2012 flexible pavement (msa)

Count 2000 CVPD, $r=7.5\%$, 2 years to completion, $n=15$ years, VDF = 3.5, two-lane road ($D=0.5$): $A=2000(1.075)^2=2311$ CVPD; $N_s=365\times26.118\times2311\times0.5\times3.5/10^6=38.6$ msa. This design msa and the subgrade CBR are used to read the thickness from the catalogue.

Example 2 - AASHTO-93 (design ESALs, $W_{18}$)

Each vehicle class is multiplied by its load equivalency factor (LEF) for the structural number and terminal serviceability, then by directional ($D_D$ = 0.5) and lane ($D_L$ = 0.8-1.0) factors and growth: $W_{18}=\sum n_i\,\text{LEF}_i\times365\times D_DD_L\times\text{growth factor}$. For example, 100 trucks per day with LEF 1.2, $D_DD_L$=0.45, growth factor 27.15 (8 percent, 15 years): $W_{18}=100\times1.2\times365\times0.45\times27.15=0.535\times10^6$.

Example 3 - Asphalt Institute method (design lane ESAL)

Now daily ESAL both directions 1850 and design lane factor 0.45 (4 lanes): $ESAL=1850\times0.45\times365\times\dfrac{(1.06)^{15}-1}{0.06}=7.07\times10^6$ (construction period neglected). This ESAL and the subgrade modulus $M_R=10\,CBR$ are used with the AI chart.

Example 4 - IRC:58 rigid pavement (CVPD classes)

The design CVPD at the end of construction (e.g. $1050(1.075)^3=1304$) decides the traffic class (E: 450-1500 CVPD) and the thickness adjustment; the design wheel load (5100 kg) and the axle-load spectrum are used in the fatigue check of the slab.