7.2 Design Considerations for Turnouts and Grades

  • Turnouts are widened, unobstructed shoulder areas where slow-moving vehicles can turn out of the through lane so quicker vehicles may pass. They are particularly valuable on two-lane highways in mountainous areas.
  • Turnout designs should inform drivers that a turnout is present, allow vehicles to enter at safe speeds, give trailing vehicles an opportunity to pass, and provide enough sight distance of the lane behind a vehicle to allow it to safely merge back into the through lane. Table 7.1 provides design guidelines.
Table 7.1 Turnout Design Elements
Topic Guidance
Turnout Entry Signs
  • Install signs that notify motorists a downstream turnout is available, specify legal requirements for turnout use, and indicate where a turnout begins.
  • Ideally, install two signs: an advance sign and a sign at the turnout entrance.
  • Install signs 500 – 800 ft upstream of turnouts.
  • Use MUTCD sign R4-14 to let drivers know slow-moving vehicles should move to the right.
Sight Distance
  • When locating turnouts, provide enough sight distance to the entrance so vehicles can decelerate to a safe entry speed.
  • If possible, do not locate a turnout on or near a horizontal or vertical curve that limits sight distance in either direction. Available sight distance should be at least 1,000 ft on the approach to the turnout.
Entry Speeds
  • Approach speeds are contingent on grade and horizontal alignment. Turnouts on downgrades with gentle curves should be longer than turnouts on steeper grades with persistent curvature.
  • If possible, do not locate turnouts on fill slopes or drop-offs, especially on the outside of curves. This can discourage their use.
Turnout Exit Sight Distance
  • Place turnouts so motorists approaching from upstream can see low-speed vehicles exiting a turnout.
  • Use information on sight distance for right-skewed intersections to provide drivers exiting a turnout enough sight distance.
Exit Behavior
  • Avoid the use of long turnouts as drivers may try to use them as a passing lane.

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CONTACT:

Chris VanDyke

Research Scientist | Program Manager

chrisvandyke@uky.edu