12.8 Driver Information Needs at Complex Interchanges

  • Design interchanges so they conform with motorist expectations. Violating expectations increases the amount of time a motorist requires to execute a task as well as the number of errors. Table 12.3 lists design guidelines for geometric elements, signs, and sight distance.
Table 12.3 Design Elements of Complex Interchanges
Geometric Elements
Route Continuity
  • Provide a route that does not require motorists to change lanes to continue on the through route.
  • Assign the greatest number of lanes to the through movement.
Lane Balance
  • The number of lanes that leave a diverge point equals the number lanes that approach it, plus one.
  • Use option and auxiliary lanes to minimize required lane shifts.
Ramp Spacing
  • Space ramps to facilitate clear, simple guide signing and to prevent heavy traffic from entering and existing.
Error Handling
  • Establish a forgiving roadside at critical features.
  • Avoid using compound geometric features.
Signing
Error Handling
  • Make sure signs are not unclear, confusing, misplaced, obscured, small, or illegible, and that all required signs are present
Sign Placement
  • Spread out competing information sources. Place signs with less critical information upstream or downstream.
  • Leverage advanced warnings to structure driver expectations.
  • Repeat critical information on multiple signs.
Sign Content
  • Ensure signs offer appropriate guidance and fulfill all of a motorist’s information needs.
Sight Distance
Visibility
  • Avoid blocking sightlines.
  • Ensure the visibility of features is proportional to their level of criticality.

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Chris VanDyke

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