Crossover Roundabouts: An Alternative Interchange Design

The double crossover diamond (DCD) (also known as the diverging diamond interchange) was first introduced conceptually to engineers and planners in the United States less than a decade ago. The concept has been spreading rapidly since that time; as of July 2013, DCDs had been constructed in 19 locations in eight states and had been studied in countless other projects nationwide. Nearby frontage roads pose operational challenges for all interchanges, and DCD geometry intensifies this problem. Reverse curvature for crossover points reduces the storage length between ramps and nearby frontage roads in DCDs. The crossover roundabout concept was conceived as a potential solution to this challenge. Instead of a signalized crossover point (as in a DCD), a five-legged roundabout was proposed for the intersecting arterial, frontage road, and ramps. A crossover roundabout would function essentially like any other roundabout except that it would adjoin an arterial roadway with travel directions flipped on one side. The operational and safety benefits of typical roundabouts are expected to apply to crossover roundabouts; no new driver behaviors must be learned. Variations of crossover roundabout interchanges are possible: one crossover roundabout could adjoin half of a DCD, or two crossover roundabouts could form a complete interchange. Interchanges with two crossover roundabouts would look like typical double-roundabout diamond interchanges with the arterial road directions flipped between the roundabouts. Crossover roundabouts are not required to contain frontage roads to function. Microsimulation and cost estimations have shown the concept to have merit; crossover roundabouts can combine the best attributes of DCDs and roundabouts.