Highway Engineering and Geometric Design - Concept Map
Geometric Design of Highways
Related concepts
Stopping Sight Distance (SSD)
Sub concepts
- Perception-reaction distance (0.278Vt)
- Braking distance (V²/[254(f±G)])
- Grade effect (uphill vs. downhill)
- Friction coefficient (f)
- Reaction time (t ≈ 2.5 s)
Relationship to central
Core safety requirement determining safe braking space
Horizontal Alignment & Superelevation
Sub concepts
- Minimum radius formula: Rmin = V²/[127(e+f)]
- Superelevation angle (e)
- Side friction (f)
- Design speed (V)
- Transition (spiral) curves
- Superelevation runoff
Relationship to central
Controls curve safety and centripetal force balance
Vertical Alignment
Sub concepts
- Parabolic curves (crest & sag)
- Grade limitations
- Crest curve sight distance
- Sag curve headlight throw
- Vehicle performance on grades
Relationship to central
Manages grades, drainage, and sight over crests/sags
Cross-Section Design
Sub concepts
- Lane widths (standard & design vehicle)
- Shoulder width and type
- Camber/crown for drainage
- Median design
- Clear zone width
- Traffic barriers and safety
Relationship to central
Defines lane widths, shoulders, drainage, and clear zones
Design Parameters & Standards
Sub concepts
- Design speed (km/h)
- Design vehicle (car, truck, bus)
- Traffic volume (AADT)
- Friction coefficients
- Grade limits
- Philippine standards (DPWH, NSCP)
Relationship to central
Foundational inputs for all geometric calculations
Safety & Operational Considerations
Sub concepts
- Passing sight distance
- Decision sight distance
- Night visibility
- Vehicle stability on curves
- Skid resistance
- Drainage slope
Relationship to central
Ensures geometry accommodates human and vehicle limits
Concept connections
- Stopping Sight Distance (SSD)Design SpeedSSD is calculated directly from design speed V; higher speed → longer SSD required
- Stopping Sight Distance (SSD)Vertical AlignmentSSD limits crest curve length; inadequate sight over crests violates SSD standards
- Stopping Sight Distance (SSD)Grade EffectGrade sign directly modifies braking distance denominator; downgrade lengthens SSD, upgrade shortens it
- Horizontal Alignment & SuperelevationDesign SpeedMinimum radius Rmin = V²/[127(e+f)] is quadratic in speed; faster speeds require much sharper curves or larger radii
- Horizontal Alignment & SuperelevationSuperelevation RunoffTransition spirals develop superelevation gradually over runoff distance; longer spirals reduce rate of change (comfort)
- Horizontal Alignment & SuperelevationFriction CoefficientSide friction (f) complements superelevation (e) in providing centripetal force; both contribute to Rmin formula
- Vertical AlignmentDesign SpeedDesign speed determines sight distance threshold; higher speeds require longer vertical curves
- Vertical AlignmentGrade LimitationsSteep grades (>6%) may trigger climbing lanes, runaway truck ramps, or design speed reduction
- Cross-Section DesignDesign VehicleDesign vehicle width and turning radius determine minimum lane width and shoulder width
- Cross-Section DesignDrainageCrown (camber) slope must match cross-fall percentage (2–3%) to shed water; affects lane edge profile
- Design Parameters & StandardsAll Geometric ElementsDesign speed, friction coefficient, and vehicle type are inputs to SSD, Rmin, grade limits, and cross-section
- Friction CoefficientStopping Sight Distance (SSD)Friction (f) in denominator of braking distance formula; higher f → shorter SSD
- Friction CoefficientMinimum RadiusSide friction (f) in Rmin formula adds to superelevation (e); higher f → smaller Rmin
- Grade EffectVehicle PerformanceDowngrade increases braking distance and brake fade risk; upgrade aids deceleration but limits engine braking
- Clear Zone WidthDesign SpeedHigher speeds increase clear zone requirement to safely recover from edge departures
- Superelevation AngleMinimum RadiusLarger superelevation (e) reduces Rmin; maximum e is typically 4–8% to prevent sliding on wet pavements
- Reaction TimeStopping Sight Distance (SSD)Reaction time (t ≈ 2.5 s) multiplies design speed in perception-reaction distance term (0.278Vt)
- Passing Sight DistanceVertical AlignmentOn undivided highways, crest curve length must accommodate passing sight distance (longer than stopping sight)
- Shoulder WidthClear Zone WidthPaved shoulder width contributes to clear zone; wider shoulders reduce clear zone barrier distance
- Lane WidthDesign VehicleDesign vehicle width plus lateral clearance (0.2 m per side) determines minimum lane width (typically 3.5–3.75 m)
Diagrams
Highway Geometric Design Concept Hierarchy
Comprehensive hierarchical overview of all geometric design concepts showing how stopping sight distance, horizontal alignment, vertical alignment, cross-section design, and parameters interconnect to form a complete highway design framework.
Stopping Sight Distance (SSD) Calculation Procedure
Step-by-step flowchart showing how to calculate stopping sight distance, including decision points for grade effects, component breakdown (reaction and braking), and comparison to design standards. Critical for exam problem-solving.
Minimum Radius Design Decision Tree
Decision flowchart for designing horizontal curves using the minimum radius formula (Rmin = V²/[127(e+f)]). Shows how to handle space constraints and iterate between design parameters—critical skill for board exams and field design.
SSD on Grades: Effect Analysis
Illustrates how grade sign (positive/negative) affects SSD calculations—uphill upgrades shorten SSD (better braking), downhill downgrades lengthen it (worse conditions). Essential for understanding why truck climbing lanes are needed on steep downgrades.
Cross-Section Design Integration Process
Systematic approach to designing road cross-sections, integrating lane widths, shoulders, drainage (crown/camber), and clear zones based on road classification and design vehicle. Shows how all cross-section components work together for safe operation.
Design Speed Selection & Justification
Logic for selecting design speed based on road classification, terrain, and adjacent land use—then cascading that speed through all geometric design calculations (SSD, Rmin, grades, cross-section).
Superelevation Runoff and Transition Curve Phases
State-transition diagram showing how a road section progresses from normal crown through spiral curves developing superelevation runoff, maintains full superelevation on the circular curve, then reverses the process on exit. Essential for understanding transition curve design.
Components of Total Stopping Sight Distance (SSD)
Typical distribution of the two SSD components at 80 km/h (level, f=0.35): reaction distance contributes ~44% and braking distance ~56%. Shows that on level roads, braking distance dominates, but reaction time is significant and must not be overlooked in calculations.
Vertical Curve Type Selection: Crest vs. Sag
Decision tree for choosing between crest and sag vertical curves based on grade profile, then determining minimum length using appropriate sight distance criteria (stopping sight for crests, headlight sight for sags).