--- title: "Design traffic from surveys, with design-method examples" subject: "Transportation Engineering II" chapter: 2 key: "semester_7/transportation_engineering_ii:design-traffic-from-surveys-examples" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/design-traffic-from-surveys-examples site: BCT Engineering --- # Design traffic from surveys, with design-method examples Transportation Engineering II · Chapter 2: Highway Pavement - Asked in: 2070 Chaitra, 8 marks - Video: [Lecture 4, How to estimate design traffic from traffic survey data? All details and calculations](https://www.youtube.com/watch?v=yJSkpMrpCGA) — Transportation Engg Talks | Satish IITR - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/design-traffic-from-surveys-examples - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2 ## Question Explain how design traffic is calculated from the data obtained from traffic surveys. Give at least three different examples in various design methods. ## Short answer Design traffic begins with classified volume counts to get commercial vehicles per day, projected to opening year. Axle load survey yields vehicle damage factor using the fourth power law. Cumulative standard axles over design life N = (365[(1+r)^n-1]/r)*A*D*F. Examples: IRC:37 computes msa; AASHTO uses ESALs with load equivalency factors; Asphalt Institute uses design lane ESAL. ## Answer 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which vehicles are used for structural pavement design from a classified count? A. All vehicles including cars B. Only two-wheelers C. Only buses D. Commercial vehicles above 3 t laden weight Answer: D. Passenger cars cause negligible damage, so only commercial vehicles (above 3 t laden weight) are used. (Key point: Commercial vehicles only) 2. What does the axle load survey give for design traffic? A. The vehicle damage factor (VDF) from converting axle loads to standard axles B. The growth rate of traffic C. The lane distribution factor D. The ADT by vehicle type Answer: A. Weighing axles gives the load spectrum, which is converted by the fourth power law or load equivalency factors into the VDF. (Key point: Vehicle damage factor) 3. In Example 1 (2000 CVPD, r = 7.5 %, 2 years to completion), what is traffic A at completion? A. 2000 x 2 = 4000 CVPD B. 2000 x (1.075)^2 = 2311 CVPD C. 2000 x 0.5 = 1000 CVPD D. 2000 / (1.075)^2 = 1731 CVPD Answer: B. A = P(1+r)^x = 2000(1.075)^2 = 2311 CVPD. (Key point: IRC:37 example) 4. In AASHTO-93 how is W18 built compared with the IRC method? A. Cars are counted at full weight B. Each vehicle class is multiplied by its load equivalency factor and by directional and lane factors and growth factor C. Only the subgrade CBR is used D. Traffic is classified into CVPD classes A-G Answer: B. AASHTO sums n_i x LEF_i x 365 x D_D D_L x growth factor; classing by CVPD is the IRC:58 rigid approach. 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