On hill roads, what is the primary purpose of providing parapet walls (wheel-guard walls) along the roadway—especially where a retaining wall or valley edge exists?

Introduction / Context:
In hill-road engineering, parapet or wheel-guard walls are low masonry or concrete upstands placed along edges and retaining walls. They serve as physical checks to keep vehicles from straying onto hazardous margins.

Given Data / Assumptions:

• Road located on hillside with retaining/valley side edges.
• Parapet wall provided along the edge.
• Focus on safety function, not earth retention.

Concept / Approach:
Parapet walls are safety appurtenances: their duty is to discourage wheels from climbing onto or beyond the retaining wall and to warn or guide drivers near edges. They are not designed for holding back soil (that is a retaining wall's job) nor to stabilize slopes (that is a geotechnical measure).

Step-by-Step Solution:

Verification / Alternative check:
Design manuals categorize parapets/kerbs/barriers as safety features; they do not contribute significantly to slope or earth retention.

Why Other Options Are Wrong:
(a) Retaining backfill is a retaining wall function; (b) preventing hill slides is a geotechnical stabilization task; (d) contradicts established safety purpose.

Common Pitfalls:
Confusing parapets with retaining walls; assuming parapets provide substantial structural retention.

Final Answer:
To prevent the wheels of vehicles from mounting the retaining wall and to reduce the risk of vehicles going off the edge