12.2.2 Design Values

Design values selected for a particular project must satisfy and preferably exceed the values shown below. Selected design values should be consistent and compatible with the prevalent design features on the existing off-system roadway.
  • Minimum Design Speed:
     Meet or improve conditions that are typical on the remainder of the roadway.
  • Vertical Curvature, Minimum K values:
     Meet or improve conditions that are typical on the remainder of the roadway.
  • Horizontal Curvature:
     Meet or improve conditions that are typical on the remainder of the roadway.
  • Minimum Superelevation:
     Meet or improve conditions that are typical on the remainder of the roadway.
  • Maximum Grades:
     Meet or improve conditions that are typical on the remainder of the roadway.
  • Minimum Structure Width, Nominal Face to Nominal Face of Rail:
     A minimum width of 24-ft may be used when ALL following conditions apply:
    • Approach roadway width is 24-ft or less;
    • The county or local entity has no near-term plans to widen the approach roadway; and
    • There is no major near-term development anticipated.
    If any of the conditions above are not met, a minimum width of 28-ft is required. If the existing approach roadway width is greater than 28-ft then the existing approach roadway width would apply.
  • Bridge End Guard Fence:
    • Minimum Conditions – Transition consistent with current applicable Roadway Design Standards.
    • If an intervening roadway or driveway prevents usual placement of guard fence, a guard fence radius may be used consistent with specific application in the applicable Roadway Design Standard.
  • Approach Roadway Width:
    • For minimum length of 50-ft adjacent to the bridge end, the approach roadway width should match the clear roadway width across the structure plus additional width to accommodate approach guard fence.
    • An appropriate transition (minimum length 50-ft) to the existing roadway width should be made in the sections of approach roadway located at the federal project extremities.
    • If roadway surfacing is included, a minimum roadway width should be used for the 50-ft roadway section adjacent to the bridge.
  • Traffic Control:
    • When provided, and to the extent feasible, detours should, at a minimum, match existing road design features. Design details for detours should be shown in the plans and on the preliminary layouts.
    • Traffic control devices should be in conformance with the
      TMUTCD
      , and details should be included in the plans.
    • See
      Chapter 23
      for more information.