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.

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:

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