In the geometric design of traffic rotaries (roundabouts), the width of the circulating carriageway (rotary) should be taken as which of the following practical rules of thumb?

Introduction / Context:
Roundabout (rotary) design requires adequate circulating width to accommodate entering streams, weaving, and lane changes. A common sizing cue links the circulating width to the largest approach width with a margin to allow safe weaving.

Given Data / Assumptions:

• Multiple approach roads with varying widths.
• Design aims for safe weaving and capacity with conventional passenger traffic mix.
• No channelization beyond standard splitter islands.

Concept / Approach:
A pragmatic approach is to provide a circulating width at least equal to the widest approach plus one extra lane for weaving and storage. This aligns with typical roundabout design guidance that scales circulating width to entry width and expected weaving volumes.

Step-by-Step Solution:

Verification / Alternative check:
Detailed weaving-section analysis (weaving width and length) refined by expected flows will confirm or adjust this initial sizing.

Why Other Options Are Wrong:
(a) and (d) are arbitrary; (b) underestimates weaving needs and can cause conflicts.

Common Pitfalls:
Ignoring the weaving requirement; making the rotary too narrow, which elevates crash risk and reduces capacity.

Final Answer:
Equal to the width of the widest road plus one additional traffic lane