Space Requirements
Roundabouts that are designed to accommodate vehicles larger than passenger cars or small trucks typically require more space than conventional intersections. However, this may be more than offset by the space saved compared with turning lane requirements at alternative intersection forms. The key indicator of the required space is the inscribed circle diameter. A detailed design is required to determine the space requirements at a specific site, especially if more than one lane is needed to accommodate the entering and circulating traffic. This is, however, another case in which the use of assumptions and approximations can produce preliminary values that are adequate for planning purposes.
One important question is whether or not the proposed roundabout will fit within the existing property lines, or whether additional right-of-way will be required. Four examples have been created to demonstrate the spatial effects of comparable intersection types, and the assumptions are summarized in Exhibit 3-14. Note that there are many combinations of turning volumes that would affect the actual lane configurations and design storage lengths. Therefore, these examples should not be used out of context.
As can be seen in Exhibit 3-15 through Exhibit 3-18, roundabouts typically require more area at the junction than conventional intersections. However, as capacity needs increase the size of the roundabout and comparable conventional (signalized) intersection, the increase in space requirements are increasingly offset by a reduction in space requirements on the approaches. This is because the widening or flaring required for a roundabout can be accomplished in a shorter distance than is typically required to develop left turn lanes and transition tapers at conventional intersections.
As can be seen in Exhibit 3-18, flared roundabouts offer the most potential for reducing spatial requirements on the approaches as compared to conventional intersections. This effect of providing capacity at the intersections while reducing lane requirements between intersections, known as “wide nodes and narrow roads,” is discussed further in Chapter 8.