Road roughness – definition of roughness index The roughness index of roads is best described as which of the following?
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ASize of the surface aggregate used on the pavement
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BNumber of visible patches on the pavement per kilometre
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CCumulative vertical surface deformation per unit horizontal distance
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DType of the road surface (rigid vs. flexible)
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EAverage rut depth only
Answer
Correct Answer: Cumulative vertical surface deformation per unit horizontal distance
Explanation
Introduction / Context:Roughness affects ride quality, vehicle wear, and operating costs. Maintenance prioritization and pavement management systems rely on consistent measures of roughness.
Given Data / Assumptions:
- Roughness index is a metric derived from measurement devices (e.g., bump integrators, response-type meters) or profilers.
- Expressed as unevenness per unit distance (e.g., mm/km).
Concept / Approach:Roughness is the accumulated vertical deviations of the pavement surface from a reference profile over a given length. It integrates many defect types—patches, undulations, ruts, and waves—into a single index, rather than counting any one defect only.
Step-by-Step Solution:
Measure longitudinal profile or device displacement response.Sum vertical deviations over a test length.Normalize by the test length to obtain index (e.g., mm per km).Verification / Alternative check:International Roughness Index (IRI) correlates a standardized quarter-car response to the measured profile, also representing cumulative deviation per distance.
Why Other Options Are Wrong:Aggregate size and surface type do not quantify roughness. Counting patches or rut depth alone ignores the many other components of unevenness.
Common Pitfalls:Comparing roughness indices from unlike devices without calibration; using spot repairs to lower index without addressing structural causes.
Final Answer:Cumulative vertical surface deformation per unit horizontal distance