Pavement design traffic – cumulative standard axles for divided highway An existing 2-lane NH section is to be widened and strengthened as a divided highway. Current traffic in one direction is 2500 commercial vehicles (CV) per day. Construction duration is 1 year. Design CBR of subgrade = 5%. Given: annual traffic growth rate for CV = 8%, vehicle damage factor (VDF) = 3.5 standard axles per CV, design life = 10 years, and traffic distribution factor = 0.75. Compute the cumulative standard axles (msa) for design.
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A35
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B37
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C65
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D70
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E28
Answer
Correct Answer: 35
Explanation
Introduction:IRC pavement design requires estimating the cumulative number of standard axle load repetitions (in millions of standard axles, msa) over the design life. This combines base-year commercial traffic, growth rate, lane/distribution factors, and vehicle damage factor (VDF). Accurate calculation ensures appropriate crust thickness for the given subgrade CBR.
Given Data / Assumptions:
- Base-year CV in one direction A = 2500 CV/day.
- Growth rate r = 8% per year; design life n = 10 years.
- Traffic distribution factor F = 0.75 (critical design lane share).
- Vehicle damage factor VDF = 3.5 standard axles per CV.
- 365 days per year; growth from the time of opening to traffic (construction duration accounted as per base year).
Concept / Approach:
Use the geometric growth factor for cumulative CV over n years: GF = ((1 + r)^n − 1) / r. Then total standard axles N (not in millions) equals 365 * A * GF * F * VDF. Divide by 10^6 to express in msa, and round suitably for design.
Step-by-Step Solution:
Compute growth factor: (1.08)^10 ≈ 2.1589 → GF = (2.1589 − 1) / 0.08 ≈ 14.486.Daily CV base: A = 2500; annual factor: 365.N = 365 * 2500 * 14.486 * 0.75 * 3.5 ≈ 34.7 × 10^6 standard axles.Express as msa: ≈ 35 msa (rounded).Verification / Alternative check:
Reasonableness: For moderate base traffic and 8% growth, values in the few tens of msa are typical, aligning with the 35 msa result.
Why Other Options Are Wrong:
37 is an overestimation from rounding up inputs; 65 and 70 double-count directions or omit distribution; 28 underestimates growth or VDF.
Common Pitfalls:
Forgetting the lane distribution factor; using both-direction traffic instead of one direction for divided highways; not converting to msa.
Final Answer:
35