In highway engineering history, who first proposed a basic empirical formula for determining pavement thickness in design practice?

Introduction / Context:
Pavement design evolved from early empirical rules toward modern analytical–mechanistic methods. Before layered elastic theory and fatigue-based limit states became common, engineers relied on basic empirical formulas that related anticipated wheel loads to an appropriate slab or surface thickness for safe performance. This question checks historical attribution for the earliest widely cited basic pavement thickness formula in practice-focused literature.

Given Data / Assumptions:

• The question seeks the pioneer associated with the first basic pavement thickness rule-of-thumb.
• Context is early 20th-century road construction when empirical relationships guided slab and surface thicknesses.
• No numerical computation is required—only correct attribution.

Concept / Approach:
Identify the historical contributor commonly credited in traditional exam texts for introducing a basic thickness relationship that predated more refined approaches (e.g., Westergaard's theory for rigid pavements and later flexible pavement design charts). Among the provided names, Kelly is typically cited as the earliest proposer of a simple thickness formula in many standard MCQ compilations and survey references used in competitive exams.

Step-by-Step Solution:
Review early empirical contributors listed in classic highway texts.Match the name most consistently referenced as the first to suggest a basic thickness rule.Select Kelly as the historically recognized proposer for the earliest empirical thickness relation.

Verification / Alternative check:
Later contributors such as Goldbeck are associated with specific rigid pavement thickness expressions, while Spangler is more linked to soil–structure and pipe deflection work; hence, Kelly aligns best with the earliest general thickness rule attribution in exam-oriented literature.

Why Other Options Are Wrong:

• Goldbeck: known for a later rigid-pavement thickness expression, not the earliest general formula.
• Spangler: more prominent in soil mechanics and flexible pipe behavior, not the first pavement thickness rule.
• Picket: not the standard attribution for the first basic thickness formula in common references.

Common Pitfalls:

• Confusing 'first' proposer with more widely used later formulas (e.g., Goldbeck).
• Assuming modern mechanistic–empirical methods replace historic attributions for exam questions—they often still test the historical naming.

Final Answer:
Kelly