Road roughness – definition of roughness index The roughness index of roads is best described as which of the following?

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:

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