According to the U.S. Highway Research Board (HRB), what is the practical lane width commonly adopted for high-speed traffic operations?

Introduction / Context:Lane width directly affects capacity, operating speed, and safety. The HRB (now TRB) conventions and associated design practices commonly treat 3.6 m (approximately 12 ft) as the practical standard lane width for high-speed facilities, balancing driver comfort, lateral clearance, and lane discipline.

Given Data / Assumptions:

• Application: high-speed traffic conditions.
• Goal: practical, widely applied lane width.
• HRB/US practice context (12 ft ≈ 3.66 m, rounded to 3.6 m for design tables).

Concept / Approach:Wider lanes offer improved lateral clearance and reduce encroachments into shoulders. The 3.6 m width supports heavy vehicles and higher speeds, lowering side-swipe risks and improving comfort compared with narrower urban lanes (e.g., 3.0–3.3 m) used where right-of-way is constrained.

Step-by-Step Solution:Convert 12 ft to metric: 12 ft ≈ 3.66 m.Recognize practical metric design value: 3.6 m.Select 3.6 m as the HRB practical lane width.

Verification / Alternative check:Capacity models typically reference 3.6 m lanes for base conditions; narrower lanes incur adjustments (reduced free-flow speed and capacity).

Why Other Options Are Wrong:

• 2.7–3.3 m: used on low-speed/urban roads or constrained contexts.
• 4.5 m: too wide for a single lane; introduces lane discipline issues.

Common Pitfalls:

• Assuming 3.0 m lanes are adequate for all contexts—heavy traffic at speed benefits from 3.6 m.

Final Answer:3.6 m.