Evolution of Design
The Contraflow Left (CFL) interchange concept was first implemented in the 1960s at the intersection of RT 7 with US 441 in Sunrise, Florida. The design replaced a Tight Diamond interchange that was failing due to the number of signal phases. A narrow overpass and business development tight to the arterial right-of-way prohibited the possibility of adding adequate opposing left-turn bays at the interchange approaches. Therefore the contraflow lanes, which are built within the intersection, were introduced. Originally the design was to be an interim solution until funds were made available to reconstruct the interchange. However, the design worked well for nearly 20 years until the bridge was rebuilt in the 1980s. Today, there are at least two other known CFL interchanges still in operation in Florida, one of which was originally constructed as a CFL.
Design & Operations
The CFL design is a variation on the Diamond interchange, and is best used as an alternative to a Tight Diamond interchange. All movements in a CFL interchange are the same as those of a typical Diamond interchange configuration except for left turns from the cross street. Cross street left turns move over into left turn storage lanes (separated from the cross street through lanes by raised median) approximately 300 feet prior to the first ramp intersection. From this left-turn storage lane, vehicles move past the first signal and into contraflow lanes within the interchange, before making the turn onto the ramp. These special lanes run in the opposite direction from the adjacent through-lanes, and overlap within the interchange. The CFL design reduces the number of signal phases compared to a Tight Diamond interchange from four to three by allowing the two opposing left-turn movements to be made during the same signal phase. The same requirements for additional intersection clearance time exist in the CFL design as with both SPUI and Tight Diamond interchange designs. The greatest design benefit of the CFL is the reduction in the need for left-turn storage, as most left turns do not have to be stored in the area between the two interchange signals.
Studies & Research
There are no known studies comparing the operations of the CFL to Diamond or other interchanges. The CFL interchange has the greatest potential for application at Diamond interchanges with tight ramp spacing and/or where left-turn demand exceeds storage capacity between the ramps. The design may also be attractive where right-of-way is limited along the cross street and additional left-turn storage is desired. The CFL interchange design may not be as operationally effective where the left-turn volumes are significantly imbalanced.
To date, all known Contraflow Left Interchanges are designed where the arterial or freeway crosses over the cross street, but similar reductions to the bridge width can be expected if the cross street is the elevated roadway. The Florida Department of Transportation recommends that the contraflow lane extend at least 300 feet prior to the first ramp intersection, but actual design should consider expected queue lengths. In the development of the RT 7 interchange design, there were concerns that drivers might be confused by design geometrics and enter the wrong way into the contraflow lanes. Special attention was paid to signing and marking for the project. In 20 years of operations, there was never an accident caused by a motorist entering the wrong way into the contraflow lanes. Accident rates at the RT 7 interchange were reduced compared to the Tight Diamond conditions by reducing stop-and-go traffic on the cross street and reducing ramp queues.