Circulatory Width is linked to Entry Path Width - it should throw a warning if both of this values are inconsistent.
CD116 Para 3.6 provides a minimum and maximum width. “The width of the circulatory carriageway for normal or compact roundabouts shall be between 1.0 and 1.2 times the maximum entry width, excluding any overrun area.” VT knows what the maximum entry width is, so this is a simple calculation.
In the United States, the Federal Highway Administration (FHWA) provides guidelines for roundabout design in the "Roundabouts: An Informational Guide" (NCHRP Report 672). Here are some typical standards from this guide:
Entry Path Width:
- For single-lane entries, the entry width typically ranges from 4.2 to 6.0 meters.
Circulatory Width:
- For single-lane roundabouts, the recommended circulatory width is generally between 6.0 and 7.2 meters. This width allows for safe vehicle circulation while maintaining appropriate speeds and deflection.
Given these guidelines:
- If the entry path width is 6 meters, a circulatory width of 9 meters is indeed larger than typical recommendations for single-lane roundabouts.
- A circulatory width closer to 6.0 to 7.2 meters would be more standard and likely preferable for a single-lane roundabout.
The larger 9-meter circulatory width could be justified in scenarios where larger vehicles frequently use the roundabout or if it is designed to accommodate potential future expansion to a multi-lane roundabout.
Conclusion: Based on typical FHWA standards, a circulatory width of 9 meters would generally be considered too large for a single-lane roundabout with an entry path width of 6 meters. A circulatory width in the range of 6.0 to 7.2 meters would be more standard and in line with typical U.S. practices.
