Highway alignment and geometry: identify the incorrect statement about providing straight, horizontal, and vertical elements on highways.

Introduction / Context:
Highway alignment and geometric design must reconcile safety, comfort, topography, and economy. Real-world roads rarely run as perfect straight lines for long distances; instead, they combine tangents with horizontal and vertical curves to achieve safe sight distance, drainage, and earthwork balance.

Given Data / Assumptions:

• The statements compare straight alignments with the use of horizontal and vertical curves.
• Standard practice follows geometric design standards (e.g., sight distance, super-elevation, grade).
• No special constraints such as tunnels or bridges override normal design logic.

Concept / Approach:
Horizontal curves (circular with transition) connect tangents and control lateral acceleration, while vertical curves (summit/valley) smoothly join grades to maintain stopping-sight distance and riding comfort. Continuous straight lines are neither typical nor desirable over varied terrain because they would often violate these requirements.

Step-by-Step Solution:

Verification / Alternative check:
Review any national geometric design manual: you will find minimum radii for horizontal curves, criteria for summit/valley curves, and transition lengths, confirming that curves are essential features.

Why Other Options Are Wrong:
(b) True: horizontal curves are standard. (c) True: vertical curves join grades. (d) True: both types are routinely combined on most alignments.

Common Pitfalls:
Assuming long straight stretches are “safer.” Excessively long tangents can reduce driver alertness and may not meet topographic constraints or sight-distance needs at crests/sags.

Final Answer:
Highways are always constructed in straight line