I-75 and SR 884 (Colonial Boulevard) Interchange

Client: FDOT District One

Location: Lee County, Florida

Start Date: 2012

Completion Date: 2016 (estimated)


The interchange of I-75 and Colonial Boulevard needs to be improved to accommodate future traffic growth due to development in the vicinity of the interchange and is anticipated to enhance overall safety, capacity, and emergency evacuation within the county.

The purpose of the study is to re-evaluate the original preferred alternative, a Single Point Urban Interchange (SPUI), for improved operations to meet future traffic needs. In 2011, FDOT widened I-75 to six lanes and widened the existing bridges over Colonial Boulevard. Furthermore, Lee County widened Colonial Boulevard to six lanes in 2012. Implementing a SPUI would require replacement of the I-75 bridges over Colonial Boulevard. In order to salvage the newly widened I-75 bridges, FHWA suggested to FDOT a reassessment of the interchange. American completed the re-evaluation and determined that a Diverging Diamond Interchange (DDI) Configuration with Continuous Flow Intersection (CFI) at Ortiz Avenue and Superstreet at Forum Boulevard is the best solution for this location. Design will begin in late 2014.

The DDI interchange alternative salvages the bridges, eliminates on and off-ramp conflicts with through lanes, and improves signal spacing.

A CFI was developed for the Ortiz Avenue intersection. A CFI is a unique design where some or all of the left turn movements begin before crossing the intersection. The major advantage of the CFI is left turning traffic can be eliminated as a critical movement for the signal timing. This is especially useful for the Ortiz Avenue intersection, where the AM peak for the design year is projected to be more than 1,300 left turning vehicles from westbound Colonial Boulevard to southbound Ortiz Avenue. Another advantage is fewer signal phases, which can reduce the cycle length and allow more green time for the through movements.

A Superstreet was developed for the Forum Boulevard intersection. A Superstreet is an innovative intersection design that reduces signal phases and improves traffic flow for the major direction. In a Superstreet, the major roadway, in this case Colonial Boulevard, is allowed to make all the movements directly at the minor roadway, in this case Forum Boulevard. However, from the minor roadway, traffic will be forced to make a right turn onto the major roadway with no direct through or left turn movements allowed. To complete those movements, traffic will need to make a U-turn at a median opening east and west of the intersection and then return to the minor roadway intersection to complete the desired movement. All movements are signalized in the Superstreet. Operationally, there are two major advantages of the Superstreet. First, all traffic signals have just two phases. This allows a higher percentage of green time for each phase as well as a reduction of the overall cycle length. The second advantage is that signals in the eastbound direction are independent from signals in the westbound direction. This results in each direction acting as one-way streets that can have “perfect” signal progression.

The advantages of having a CFI and Superstreet on the opposite sides of the DDI include reduction of signal phases at each intersection, resulting in a shorter cycle length. The one-way roadway characteristics of the Superstreet also complement the one-way roadway characteristics of the DDI. This allows for the maximum green bandwidth possible between all intersections in each direction.

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Project Category: Diverging Diamond Interchange