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3.2. Geometric Design Engineering (GDE)

This knowledge area covers the geometric design of highways, streets, and intersections: design controls and criteria, horizontal and vertical alignment, cross-section and roadside design, at-grade intersections and interchanges, and context-sensitive, multimodal design practice.

Knowledge Area (KA)	Core Tier1	Core Tier2
3.2.1 Design Controls and Criteria	Elective
3.2.2 Horizontal Alignment Design	Elective
3.2.3 Vertical Alignment Design	Elective
3.2.4 Cross-Section Elements and Roadside Design	Elective
3.2.5 At-Grade Intersections and Interchanges	Elective
3.2.6 Context-Sensitive and Multimodal Design	Elective

3.2.1. GDE/Design Controls and Criteria ↑ Back to top

Design vehicle and driver performance characteristics, functional classification and design domain, design speed selection, and the design exception process that together govern every subsequent geometric design decision.
Topics:
Core

• Design vehicle types and turning template characteristics
• Driver performance characteristics: perception-reaction time and eye/object height
• Functional classification of highways and streets and its relation to design criteria
• Design speed selection and its relationship to operating speed
• Design domain concept and flexible design criteria ranges
• Design exception process for substandard criteria

Learning Outcomes:
Core:

1. Select an appropriate design vehicle and apply its turning template to a layout [Familiarity]
2. Apply driver performance parameters (perception-reaction time, eye/object height) in design computations [Usage]
3. Determine the functional classification of a facility and the design criteria it implies [Assessment]
4. Select a design speed consistent with context, function, and operating speed expectations [Assessment]
5. Explain the design domain concept and its use in flexible design [Familiarity]
6. Document a design exception for a criterion that cannot be met [Usage]

3.2.2. GDE/Horizontal Alignment Design ↑ Back to top

Design of horizontal curves and their transitions, superelevation, and the sight-distance and comfort criteria that govern safe horizontal alignment.
Topics:
Core

• Simple circular curve geometry and layout
• Spiral transition curves and their application
• Superelevation design, rates, and runoff/transition lengths
• Side friction factors and minimum radius determination
• Horizontal sight distance and sight obstructions on curves
• Compound and reverse curve design considerations

Learning Outcomes:
Core:

1. Design simple circular horizontal curves for a given design speed [Usage]
2. Apply spiral transition curves where required by design speed and curvature [Assessment]
3. Design superelevation rates and transition/runoff lengths for a horizontal curve [Assessment]
4. Determine minimum curve radius from side friction and superelevation limits [Usage]
5. Verify horizontal sight distance and identify sight obstructions along a curve [Assessment]
6. Evaluate compound and reverse curve layouts against design guidance [Familiarity]

3.2.3. GDE/Vertical Alignment Design ↑ Back to top

Design of grades and vertical curves, and the stopping, passing, and vertical-clearance criteria that determine safe and comfortable vertical alignment.
Topics:
Core

• Maximum and minimum grade control and critical length of grade
• Crest vertical curve design for stopping sight distance
• Sag vertical curve design for headlight and comfort criteria
• Stopping sight distance computation and application
• Passing sight distance on two-lane highways
• Vertical clearance requirements at structures and underpasses

Learning Outcomes:
Core:

1. Apply grade control criteria including critical length of grade for trucks [Assessment]
2. Design crest vertical curves satisfying stopping sight distance [Assessment]
3. Design sag vertical curves satisfying headlight and comfort criteria [Assessment]
4. Compute stopping sight distance for a given design speed and grade [Usage]
5. Determine passing sight distance requirements on two-lane highways [Usage]
6. Verify vertical clearance at structures against design standards [Familiarity]

3.2.4. GDE/Cross-Section Elements and Roadside Design ↑ Back to top

Design of travel lanes, shoulders, medians, and other cross-section elements, and the roadside design, clear-zone, and barrier criteria that protect errant vehicles.
Topics:
Core

• Lane and shoulder width selection by functional class and context
• Median types, widths, and median barrier warrants
• Cross slope, superelevation transition, and drainage considerations
• Clear zone width determination and roadside hazard rating
• Roadside barrier selection, placement, and end treatments
• Sidewalk, bike lane, and buffer cross-section design

Learning Outcomes:
Core:

1. Select lane and shoulder widths appropriate to functional class and context [Usage]
2. Determine median type, width, and barrier warrant for a given facility [Assessment]
3. Explain cross slope and drainage requirements in cross-section design [Familiarity]
4. Determine clear zone width from roadside hazard rating and design speed [Assessment]
5. Select and place roadside barriers and end treatments [Usage]
6. Design sidewalk, bike lane, and buffer cross-sections for a complete street [Assessment]

3.2.5. GDE/At-Grade Intersections and Interchanges ↑ Back to top

Geometric design of at-grade intersection channelization and turning lanes, and the selection and ramp/weaving design of grade-separated interchanges.
Topics:
Core

• At-grade intersection types and alignment/skew considerations
• Channelization and turning lane design (left/right turn lanes)
• Intersection sight distance and approach sight triangles
• Interchange types and selection criteria (diamond, cloverleaf, directional)
• Ramp geometric design and gore area treatment
• Weaving section geometry and auxiliary lane design

Learning Outcomes:
Core:

1. Select an at-grade intersection type and alignment appropriate to approach conditions [Assessment]
2. Design channelization and turning lanes for an at-grade intersection [Usage]
3. Verify intersection sight distance against approach sight triangle criteria [Usage]
4. Select an interchange type appropriate to traffic, right-of-way, and terrain constraints [Assessment]
5. Design ramp geometry and gore area treatments [Usage]
6. Analyze weaving section geometry and auxiliary lane requirements [Assessment]

3.2.6. GDE/Context-Sensitive and Multimodal Design ↑ Back to top

Context-sensitive design practice and complete-streets principles for balancing multimodal users, together with the three-dimensional coordination of horizontal and vertical alignment.
Topics:
Core

• Context-sensitive design process and stakeholder-based criteria flexibility
• Complete streets principles and multimodal accommodation
• Traffic calming elements for urban and residential streets
• Three-dimensional coordination of horizontal and vertical alignment
• Aesthetic and environmental considerations in geometric design
• Access management and driveway spacing criteria

Learning Outcomes:
Core:

1. Explain the context-sensitive design process and when design flexibility is appropriate [Familiarity]
2. Apply complete streets principles to accommodate multiple travel modes [Usage]
3. Select traffic calming elements appropriate to an urban or residential context [Assessment]
4. Coordinate horizontal and vertical alignment for a three-dimensionally consistent design [Assessment]
5. Consider aesthetic and environmental factors in geometric design decisions [Familiarity]
6. Apply access management criteria to driveway and intersection spacing [Usage]