Distribution of extra widening on curves Where should the full amount of extra pavement width on a horizontal curve be achieved along a layout with transition curves?

Introduction / Context:
Extra widening on curves is provided to accommodate off-tracking of long vehicles and lateral clearance at speed. On alignments with transition curves, the widening must vary smoothly.

Given Data / Assumptions:

• Curve layout includes transitions leading into and out of the circular arc.
• Extra width is linearly developed along transitions.
• Full extra width is needed over the tightest curvature (circular arc).

Concept / Approach:
Because the radius is smallest on the circular arc, that is where the greatest off-tracking and lateral clearance demand occur. The widening should be zero at tangent, increase along transitions, and reach the full value at the middle of the circular arc for a symmetric layout.

Step-by-Step Solution:

Verification / Alternative check:
Standard curve widening diagrams show triangular widening ramps on transitions peaking at mid-arc; this ensures comfort and uniform lateral clearance.

Why Other Options Are Wrong:
Beginning or centre of transition provides only partial widening; beginning of circular curve captures only half of the requirement.

Common Pitfalls:
Applying full widening abruptly at the start of transition; ignoring symmetry when different transition lengths exist.

Final Answer:
Centre of the circular curve.