design criteria

Chapter 4: Basic Design Criteria

4.1 Overview

• Designs for highway and bridge projects are based on established design controls and criteria.

• Elements include roadway width, side slopes, horizontal/vertical alignment, drainage considerations, and intersecting roads.

4.6 Terrain

• Terrain is classified for design purposes:

  • 4.6.1 Level Terrain: Relatively flat; sight distances are generally long.

  • 4.6.2 Rolling Terrain: Natural slopes rise and fall; presents design challenges.

  • 4.6.3 Mountainous Terrain: Sudden elevation changes; requires careful planning for road alignment.

4.7 Horizontal Alignment

• Establish proper relationship between design speed and curvature; includes:

  • 4.7.1 General Considerations: Use of flatter than minimum curvature to enhance aesthetics and driver perception.

  • 4.7.2 Curve Radius: Minimum radii based on design speed; important for safety.

  • 4.7.3 Superelevation Rate: Rate at which road is banked in a curve to counteract lateral acceleration.

  • 4.7.6 Superelevation Transition Length: Length required to comfortably transition from normal crown to superelevated section.

4.8 Vertical Alignment

• Vertical alignment impacts grades, visibility, and vehicle operation. Key points include:

  • 4.8.1 Grades: Limit steep grades to improve safety and performance;

  • 4.8.2 Vertical Curves: Used for smooth transitions between grades; length based on algebraic change in grades.

  • 4.8.4 Vertical Alignment at Railroad Crossings: Ensure level alignment for visibility and safety.

4.9 Combination of Horizontal and Vertical Alignment

• Ensure both alignments accommodate safe and efficient vehicle operations.

4.10 Cross Sectional Elements

• Includes lane configurations, shoulders, tapers, and safety features. Important elements include:

  • 4.10.1 Through Travel Lanes: Design width based on functional classification.

  • 4.10.2 Speed Change Lanes: Important for safe vehicle merging and turning; taper lengths defined.

  • 4.10.10 Right of Way Width: Ensure adequate space for safe transportation and environmental considerations.

4.11 Sight Distance

• Critical for safe driving; involves:

  • 4.11.1 Stopping Sight Distance (SSD): Distance needed for drivers to stop safely.

  • 4.11.2 Decision Sight Distance: Longer than SSD; accommodates complex maneuvers.

  • 4.11.3 Passing Sight Distance: Distance required for safe passing maneuvers on highways.

4.12 Drainage Facilities

• Design for effective stormwater management while ensuring safety and accessibility:

  • 4.12.1 Design Treatment of Cross Drainage Culvert Ends: Important for maintaining safety;

  • 4.12.7 Side Ditches: Must be safely traversable and properly sized for drainage.

4.13 Environment

• Ensure that highway development enhances rather than harms the environment; consider ecological impacts and local regulations.

4.14 Human Factors

• Consideration of older drivers and pedestrian needs in design:

  • Suggested improvements include better signage, clear zones for pedestrians, and improved intersection design.

4.10.15 Medians

• Use to enhance traffic management and safety between opposing lanes:

  • Types include depressed, raised, flush, and Two-Way Left Turn Lanes.

4.10.16 Parking

• Discusses the features of parking facilities, including dimensions and accessibility.