Design of a Modern Roundabout
Roundabouts must be designed to accommodate all intersection users (pedestrians, bikes, cars, etc.) users of an intersection, yet provide slow uniform traffic speeds to maximize capacity and safety within the constraints of cost and land availability.
The relationship of roundabout capacity to geometry has been described using the following six geometric design parameters:
- Entry Width, E.
- Flare Length, L’.
- Half Width, V.
- Entry Radius, R.
- Entry Angle, ø.
- Inscribed Circle Diameter, D or ICD.
These parameters are developed in The Traffic Capacity of Roundabouts by R.M. Kimber and are used in the computer programs ARCADY and RODEL.
The empirical equations are the result of extensive research of delays and queing done in the 1970s. Data was taken at the entries of 86 roundabouts in the U.K. that were experiencing sustained at-capacity operation. The research was revisited in 1997 at 35 roundabouts in the U.K. with such good agreement with the original equations that no changes were made.
Similarly, the relationship of roundabout safety to geometry is described in Accidents at 4-Arm Roundabouts by G. Maycock and R.D. Hall. The article shows the relationship of roundabout safety and the geometric parameters of entry width, circulating width, angle between arms, and entry path radius (or deflection).
Roundabouts should be designed with sufficient entry width and flare to ensure capacity demands can be met, yet provide enough deflection to limit fastest-path speeds to a safe range of values. These values are dependent on site context, and the relationship between entering and circulating traffic flows, and should minimize the potential for entry-circulating crashes and approach (rear-end and single vehicle) crashes.
Roundabout layouts also need to be checked for accommodation of the design vehicle, for pedestrian refuge widths on the splitter islands, and for adequate sightlines.
Mini-roundabouts and multi-lane roundabouts require further design refinement. In particular, multi-lane roundabouts must be designed to avoid vehicle path overlap on the entries and exits while still maintaining acceptable fastest-path speeds.
Other design considerations:
- Pedestrian crossings (marked or unmarked)
- Bicycle treatments
- Vertical profile
- Signage and pavement marking
Good introductory design information can be found in the Roundabout Design Guidelines and the FHWA publication Roundabouts: An Informational Guide. However, it should be cautioned that adherence to guidelines will not always ensure a good layout, which remains the responsibility of the designer.