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Louisiana/Arkansas/Mississippi were in hurricane-induced power outage for more than           a week. HDR indicated standb...
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Transcript of "Ddi"

  1. 1. interchange is essential to the Strategic Intermodal System (SIS), one of thethirteen (13 ) along I-75 through Manatee and Sarasota counties.The FDOT is planning a major reconfiguration of this University Parkway I-75interchange, an alteration with major regional impact on our mobility, evacuationcapability, and economic vitality. There, the FDOT has been planning a DDI, a“Diverging Diamond Interchange.”FDOT consultant HDR presented its DDI concept to the MPO CAC and TAC at theirmeetings this month. We have posted that presentation to . However, that FDOT presentation is not on your MPOagenda Monday.Should DDI come up Monday, given the significance of the I-75 UniversityParkway interchange, it may be helpful to have at hand some questions andpoints that came up at the CAC, and some that could have, in understanding thisunique relatively untested design:  Though this FDOT SIS project is long into the planning, HDR did not have a projection of capital and operating cost. SIS $ have priority over $ for Sarasota-Manatee MPO priorities. Could MPO project funding suffer ?  The FDOT’s consultant indicated no alternatives to the DDI concept were considered:  FDOT is currently preparing a manual stating “roundabouts should be considered in the context of new interchanges and interchange modifications.” Virginia, Georgia, and other DOTs mandate this practice.  Sarasota Manatee MPO has on record its policy requiring roundabout study for intersection changes (#2004-09).  FHWA Guidance: study roundabout alternatives when using federal funds.  HDR was unaware of the DDR alternative (Diverging Diamond Roundabout) … see Institute of Traffic Engineers “Variations on the DDI Concept” Figure 8  DDI intersections are signalized. DDI is a power-reliant interchange placed here where hurricane/tornado prone -- knowing the recent reality that wide areas of
  2. 2. Louisiana/Arkansas/Mississippi were in hurricane-induced power outage for more than a week. HDR indicated standby power generators could be used, adding to cost. A roundabout is not power dependent.  Safety. A DDI has more conflict points (14) than a modern roundabout’s (8)…i.e. DDI has a higher crash rate. Signalized ongoing timing adjustments unable to keep up moment to moment to changing volumes can incent red light running … see FHWA Memorandum regarding Value of Statistical Life ($6 million per fatality prevented). Most states use modern roundabouts at interchanges not only for better lower-cost mobility, but, as well, to relieve dangerous backup from the at-grade intersection into high speed interstate lanes (as happens today at our I-75 interchanges). Does DDI?  FDOT has never constructed a DDI. Nor has the FDOT ever constructed a roundabout interchange. All FDOT interchanges today are signalized. DDI use is recent in a few states (Missouri/Kentucky/Indiana,e.g.). Modern roundabouts at interchanges are used by most states and have a longer experience history: notably Wisconsin/Colorado/Arizona/NewYork/Washington/California/NorthCarolina, etc… thirty examples are collected at a CAC rep & a student paying attention ….Rod WarnerMPO CACA Sarasota County Rep3648 Tangier TerraceSarasota, Florida, 34239941 924 2400Fax: 942 924 1650PS. Re the US41 Multimodal Corridor adopted in the MPO’s 2035 LRTP, as a resource of USA mobility best practicewe regularly update these sister websites… from Palmetto to North Port) Sarasota Bayfront & North Trail) of Sarasota Mobility).