Foundation of Rodel

Capacity prediction in roundabout engineering has come a long way since the first circular intersections at the beginning of the 20th century. Arguably the most significant advancement in roundabout design technology emerged from a series of capacity and safety studies performed in the 1970s and 80s by the Transport and Road Research Laboratory (TRRL).

A privatized company known as the Transportation Research Laboratory (TRL) today, the former "TRRL" was then a public agency tasked with advancement of traffic research throughout the United Kingdom. After the introduction of the offside priority (yield-at-entry) rule in 1966, the lab embarked upon the most comprehensive and detailed series of experiments and observations on roundabout operations in history.

Professor Kimber's Equations

TRL research included 35 experimental track layouts and 11,000 minutes of "at-capacity" data (see At-Capacity Operations) of nearly one million vehicles at 86 field sites. In a comprehensive paper incorporating the findings of these studies (The Traffic Capacity of Roundabouts, Kimber, TRL, 1980) Professor Kimber describes 6 independent geometric parameters that influence roundabout capacity and operations. Kimber described a series of equations using these parameters that could be used for capacity estimation of a given roundabout design. Kimber described a series of equations using these parameters that are now used worldwide for capacity estimation of a given roundabout design.

"Kimber's Equations" now serve as the foundation of UK roundabout design and of Rodel capacity estimation. Read more about the research and the geometric variables in our Capacity section.