Roundabouts: Do you want to Supersize that? Carla P. Anderson, P.E., Traffic Engineering Associate, Kansas Department of Transportation, Topeka, Kansas, and Chair, APWA Road Safety Subcommittee; Todd W. Thalmann, P.E., Assistant Vice President, TranSystems, Kansas City, Missouri
ver the last few years roundabouts have popped up at intersection locations throughout the United States, and Kansas is no exception. This popularity can likely be attributed to the reduction of crash severity and improved traffic flow that roundabouts exhibit when compared to signalized or stopcontrolled intersections. In Kansas roundabouts have been incorporated at several rural intersections between high-speed roadways (over 55 mph) to mitigate higher-than-average crash history. Interchanges were considered at some of these locations. The Kansas experience has shown that roundabouts address the safety issues quite well and can obviously be constructed for considerably less dollars than an interchange. Define Oversize/Overweight Vehicles Oversize/Overweight vehicles, sometimes referred to as SuperLoads, can make it difficult to determine what size design vehicle to use for analyzing movements through a roundabout. The challenge becomes finding ways to accommodate these large loads, many of which are related to the wind energy industry in Kansas, while still maintaining the safety and function of the roundabout design for the majority of the traveling public. Oversize/Overweight vehicles, or SuperLoads, mean different things in different states. However, in most states it means a vehicle that requires a permit to operate on the state’s highways. For example, in Kansas a SuperLoad is defined as a vehicle 66 APWA Reporter
July 2012
requiring a bridge analysis because it is transporting a non-divisible load that exceeds 150,000 pounds gross vehicle weight. The transport can move into the SuperLoad category based on various axle loadings as well triggered by single axles with over 22,000 pounds on up to quad-axles with more than 65,000 pounds.
accommodate an Oversize/Overweight vehicle, the designer will need to consider the low clearance of the Oversize/Overweight for the design movements. This is critical at those points in the roundabout where the actual load, not just the wheels, encroach on the truck apron and curbs.
In Indiana, the Indiana Department of Transportation (INDOT) defines a SuperLoad as any vehicle (plus its load), which exceeds 16 feet in width, 15 feet in height, 110 feet in length or weighs more than 108,000 pounds.
A larger Inner Circle Diameter (ICD), wider approach widths, and wider circulatory roadway are other accommodations that could be useful to consider. As with all roundabouts these individual components must be evaluated in a holistic way to make sure the general population is provided with a safe roundabout that also accommodates the Oversize/ Overweight. To do this, it is essential that the Oversize/Overweight design vehicle be defined early in the design process, so appropriate roundabout geometry and its related operating characteristics can be established for all users.
Roundabout Design to Accommodate the Oversize/ Overweight Vehicle While these state guidelines help define Oversize/Overweight vehicles that require permits, the primary concern with these vehicles as related to roundabout design is not weight, but length. Trucks with long loads may have difficulty traversing through many roundabout designs without some special accommodation. Ordinary truck apron designs may not be sufficient to accommodate Oversize/Overweight vehicles, but an increased truck apron width can accommodate off-tracking of long vehicles in a roundabout. A balance between the circulatory roadway width and the truck apron width must be struck since too much truck apron can reduce perceived deflection of approaching drivers. Many Oversize/Overweight haulers have very low ground clearance as well. So when a roundabout is being evaluated for its ability to
Pooled Fund Project “Accommodating Oversized/ Overweight Vehicles at Roundabouts” The Kansas Department of Transportation is currently involved in a pooled fund project being conducted by Kansas State University for “Accommodating Oversized/ Overweight Vehicles at Roundabouts.” The project is to complete the research described in the NCHRP 2012 C-14 research project and is being conducted by Eugene R. Russell, Professor Emeritus and E. Dean Landman, Adjunct Professor in the Department of Civil Engineering
