Widening at transitions: when a single-lane carriageway transitions to a two-lane carriageway, the width change is introduced through a taper approximately equal to which of the following?

Introduction / Context:
Transitions from single-lane to two-lane carriageways must be smooth to minimize abrupt steering and speed changes. The taper ratio (longitudinal length to lateral widening) guides field implementation, ensures driver comfort, and maintains safety, especially on undivided rural roads where overtaking and speed variance are common.

Given Data / Assumptions:

• Transition from one-lane to two-lane carriageway.
• Need a practical range of taper values rather than a single rigid number.

Concept / Approach:
Recommended tapers reflect expected operating speeds and sight conditions. Too short a taper causes sudden lateral shifts; too long a taper consumes unnecessary length without added benefit. A range of about 1:15 to 1:20 (length:offset) is commonly adopted for rural highway transitions, providing a manageable compromise across traffic mixes and speeds.

Step-by-Step Solution:
Define total widening required (from single-lane to two-lane standard). Multiply the lateral widening by the chosen taper ratio to get taper length. Adopt the range 1 in 15 to 1 in 20 to accommodate local speed and site constraints.

Verification / Alternative check:
Field practice and geometric design references consistently specify this taper range, with finer adjustments made for posted speed, sight distance, and presence of intersections or access points near the transition zone.

Why Other Options Are Wrong:

• 1 in 10: Too abrupt for typical speeds; can surprise drivers.
• 1 in 15 or 1 in 20 alone: Acceptable individually but the recommended answer is a range, not a single fixed ratio.
• 1 in 25: Overly long for general practice, unless dictated by special constraints.

Common Pitfalls:

• Starting the taper too close to a crest curve or intersection, reducing driver expectancy and sight distance.

Final Answer:
1 in 15 to 1 in 20.