Double Crossover Diamond Interchange

Double Crossover Diamond Interchange

A document from the Turner-Fairbank Highway Research Center of the US Federal Highway Administration describes a new concept in freeway interchanges, the Double Crossover Diamond Interchange.

The novelty here is that the overpass operates with traffic on the left, in the USA, reducing the number of conflicts and signal phases.

Figure 1  (a photo) in the PDF document and Figure 3 (a diagram) show how pedestrian traffic might be accommodated, except that Figure 3 does not show the pedway in the median that is shown in Figure 1. Apparently, in Figure 3, there would be sidewalks along the outside instead.

Figure 2 shows no pedestrian accommodation, and there is an unsignalized two-lane on-ramp at the lower right.

The high barriers adjacent to the median in Figure 1 screen glare from the wrong side of headlamp beams.

The word “bicyclists” appears in the introduction, but the document does not address accommodation of bicyclists.

Figure 1 does not show shoulder/bike lane width to facilitate on-road bicycle movements. Bicyclists might merge left of the off ramp and enter a bikeway/pedway in the median immediately after the crossover, reducing width requirements for the overpass. But then they still must pass the traffic island and cross the on ramp after the freeway, or else enter the traffic island and use the set-back crosswalk on its far side — a circuitous route. Using the pedestrian route for the whole way is even more so.

As the crossovers have to be signalized anyway, signalization of the on ramps would also improve flow on the overpass. Alternatively, the on ramps might be placed at a sharp angle with a small-radius turn, to slow entering traffic. If the on ramps are treated in one of these ways, if there is sufficient width before and after the bridge, and if there is a central bikeway/pedway, bicycle travel could be quite convenient — though snow clearance issues in winter could require bicyclists to ride across the overpass in the roadway, and also make travel difficult for pedestrians.

Another possibility would be a bicycle/pedestrian underpass at each of the four ramp locations, with sidewalks along the outside of the overpass. As the ramps already must be elevated, adding these underpasses would not substantially increase the cost of the project.

An “upside-down” version of this intersection is also conceivable, with the freeway above the road that crosses it. Sight lines would not be as long; the crossing in an underpass would be noisy and relatively dark; bicycle/pedestrian grade separations, if used, would be overpasses instead, and more expensive.