NCHRP 15-35, Geometric Design of Driveways

Many years since comprehensive national guidelines for the geometric design of driveways

• Has been a growing emphasis on …
  
  • access management
  • designing for pedestrians with disabilities.

Issues to Address

1. Plan-view connection transition design
   1. Document the operational attributes of the curve radius and the flare/taper designs, and evaluate the strengths and weaknesses of each in various environments
   2. Evaluate connection-transition dimensions (e.g., radius or flare size), width of the outer through lane, and driveway lane width to determine minimal acceptable dimensions
   3. Determine at what connection-transition dimension and lane width entry speeds began to increase
   4. Accommodating pedestrians and pedestrians with disabilities
2. Accommodating pedestrians and pedestrians with disabilities
   1. Determine how the placement of the sidewalk relative to the edge of the traveled way affects safety and operations
   2. Assess the degree to which driveway width affects pedestrian safety
   3. Determine if 2% is the best value for sidewalk cross slope
   4. Test effectiveness of treatments proposed to assist pedestrians with disabilities as they cross driveways
3. Truncated radius designs
   1. Validate or refute the assumptions upon which these designs are predicated
4. Driveway-connection length
   1. Rationale and dimensions for minimum throat lengths in various situations
   2. Guidelines for the minimum length of paved connections for gravel and dirt driveways
5. Triangular islands to prohibit left-turns
   1. Evaluate triangular “pork-chop” islands intended to restrict left-turn movements
6. Multiple exit lanes
   1. Do side-by-side exit lanes at Stop-controlled driveways create problems?
7. Driveway divider-median widths
   1. When does median become too wide?
8. Vertical alignment at driveway thresholds
   1. How much vertical lip is acceptable, and under what conditions?
   2. Evaluate increased gutter slopes.
9. Justification for multiple driveways
   1. What combination of factors?