$40 M Double Diamond Interchange

A Double Diamond Interchange (DDI) is under construction at the intersection of I-95 & SR-2oo in Yulee, at a cost of $39,980,000, before the normal cost overruns.  Historically 82% of highway projects over $5 M have cost overruns.  FDOT held an open house Thursday (2/23/17).  In promotional information FDOT claims:

This will be a significant improvement in safety, since long left turns crossing traffic lanes to enter or exit the Interstate will be eliminated. Additionally, the DDI promotes added efficiency with only two clearance intervals (the time for traffic signals to change from green to yellow to red) instead of the six or more found in other interchange designs.

The supporting safety information is an analysis of the performance of a DDI at the intersection of I-44 & Route 13 in metropolitan Springfield, Missouri.

Looking at the subject of safety first.  “Lt. Renee Graham, Nassau County Sheriff’s Office Special Operations, said the current interchange at I-95 and S.R. 200/A1A “has a high crash rate,” adding that she hopes that the DDI would result in fewer crashes.”

Let me digress for a moment.  As an engineer it was drilled into me to NEVER use the word hope always uses either expect or plan.  Also high crash rateis a subjective term, I would hope NCSO has data.

The report (op cit) cites a prior 5 year (2004 – 2008)  prior accident average of 114 incidents and a post (Aug 2009 – Aug 2010) incident rated of 56, a 46% reduction.  It would be interesting to see a 5 year post analysis (Aug 2009 – Aug 2014) analysis, but evidently no one has done the analysis.

John Stack (vice chair Nassau County Planning and Zoning Board) raised the option of a Flyover Interchange, which he, and I, expect would be a safer alternative, if safety is the major concern.

The efficiency of the DDI is often cited, but the data does not support any significant improvement.

ddi_eff
These results (op cit) are based on a computer model, not measured data.  The total AM traffic volume was 2095 vehicles per hour entering/exiting I-44 and a PM volume of 1890 vehicles per hour.  It would be interesting to see the corresponding data FDOT has for our interchange.  Downtown Springfield vs Nassau County, it should be a significant difference.

There is a 8% improvement in AM total delay and 12% increase in PM total delay.  Likewise AM total stops decreased by 7% and PM total stops increase 11%.  Aggregated the delay time increased 3.5%.  So much for improved efficiency.

 

 

 

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