Compared with constructing a single large highway tunnel, what are the principal advantages of providing a pair of parallel highway tunnels carrying separated traffic streams?

Introduction / Context:
Highway tunnel planning often prefers twin (bi-directionally separated) bores rather than one large bi-directional bore. This configuration impacts safety, operations, maintenance, and, in many cases, total cost when life-cycle considerations are included. Understanding these advantages guides alignment and staging decisions.

Given Data / Assumptions:

• Each bore typically carries one traffic direction.
• Cross-passages provide emergency egress and maintenance access.
• Cost, safety, and operational flexibility are being compared.

Concept / Approach:

Two smaller bores can reduce excavation difficulty in poor ground, improve ventilation, and allow safer traffic segregation. Construction staging is more flexible, and one bore can remain open while the other is repaired, resurfaced, or retrofitted. Crash severity is reduced by eliminating head-on conflicts.

Step-by-Step Solution:

Verification / Alternative check:

Case histories show that life-cycle costs (including ventilation power and incident management) are often favorable for twin-bore solutions, especially in urban settings with high ADT and stringent safety requirements.

Why Other Options Are Wrong:

• Any single stated advantage alone understates the holistic benefits; 'All the above' captures the full rationale.

Common Pitfalls:

• Comparing only initial excavation cost without accounting for safety and O&M.
• Ignoring evacuation and emergency response logistics that are simpler in twin bores with cross-passages.

Final Answer:

All the above.