Highway Alignment and Geometric Design

Introduction

Highway geometric design is the process of selecting the three-dimensional form of a road so that vehicles can move safely, comfortably, economically, and efficiently. It determines the position of the highway in plan, profile, and cross-section. Although pavement materials and structural thickness control the load-carrying capacity of a road, geometric design strongly influences crash risk, operating speed, construction cost, drainage, land acquisition, and future maintenance.

An alignment must respond to design speed, traffic composition, terrain, climate, soil and geology, drainage, environmental constraints, adjacent land use, and the characteristics of vehicles and drivers. Good design does not simply maximize speed. It provides a consistent relationship between speed, visibility, curvature, gradients, intersections, and roadside conditions.

This chapter explains the principles and practice of horizontal and vertical alignment, cross-section design, sight distance, superelevation, gradients, intersections, and interchanges. Numerical values used in practice must ultimately be selected from the governing national road-design standard, because legal requirements and design classifications vary among jurisdictions.

Learning Objectives

After studying this chapter, the learner should be able to:

1. Explain the relationship between design speed, terrain, vehicle characteristics, and geometric design.
2. Describe the elements of highway cross-sections and select appropriate dimensions from design criteria.
3. Calculate stopping sight distance, horizontal-curve radius, superelevation, and summit-curve length under stated assumptions.
4. Design and assess horizontal and vertical alignments.
5. Apply principles of intersection and interchange layout.
6. Evaluate geometric alternatives in terms of safety, cost, accessibility, environment, and constructability.

1. Design Controls and Alignment Principles

1.1 Design speed and operating speed

Design speed is the selected speed used to determine geometric features such as curve radius, sight distance, superelevation, and vertical-curve length. It is not necessarily the posted speed or the speed at which every driver travels. It should be credible for the road function, terrain, adjacent development, and expected traffic.

Operating speed is the speed drivers actually select under free-flow conditions. A design is safer when successive elements produce a consistent speed pattern. Sudden changes from a high-speed tangent to a sharp curve, or from a high-speed curve to a constrained intersection, can produce unstable operations and unexpected braking.

1.2 Functional classification

Roads may be classified as local roads, collectors, arterials, expressways, or motorways. Local roads emphasize access; arterials emphasize movement; collectors provide the transition between them. Higher functional classes generally require greater design speed, stronger access control, larger sight distances, and more substantial intersection treatment.

1.3 Terrain and environmental controls

Terrain is commonly described as level, rolling, or mountainous. In difficult terrain, forcing a high design speed may create excessive earthwork, retaining structures, environmental damage, and unsafe roadside exposure. Alignment should follow stable ground where possible, avoid flood-prone areas and landslide zones, and respect watercourses, wetlands, cultural resources, and settlements.

A practical alignment process is iterative:

1. establish the road function and design criteria;
2. identify environmental, geological, hydraulic, and property constraints;
3. develop corridor alternatives;
4. compare safety, earthwork, drainage, cost, access, and environmental effects;
5. refine plan, profile, and cross-section together; and
6. check constructability, consistency, and compliance with standards.

1.4 Three-dimensional coordination

Plan alignment, profile alignment, and cross-section cannot be designed independently. A horizontal curve combined with a crest vertical curve may restrict visibility. A steep profile combined with a sharp curve may cause heavy vehicles to slow and create conflicts. Superelevation must transition smoothly, and drainage must remain effective through transitions and low points.

2. Highway Cross-Sections

A typical cross-section may contain traffic lanes, shoulders, a median, drainage facilities, side slopes, a verge or clear zone, and pedestrian or cycle facilities. The arrangement depends on road class and context.

Lane width should accommodate the swept path of the design vehicle while allowing lateral clearance. Wider lanes may improve comfort on high-speed roads but increase construction cost and crossing distance for pedestrians. Narrower lanes can be appropriate in constrained urban environments if speed and context support them.

Shoulders provide emergency stopping space, improve pavement support, and may accommodate cyclists. On high-speed roads, a paved shoulder can significantly reduce disruption from disabled vehicles. The crossfall of lanes and shoulders must drain water without creating uncomfortable or unstable vehicle behavior.

The normal crossfall of a pavement is commonly directed from the crown toward the edges. A representative value might be about 2%, but the governing standard should determine the value according to surface type, rainfall, material, and climate. At a superelevated curve, the crossfall rotates so that the entire pavement slopes toward the inside of the curve, subject to drainage and transition requirements.

Side slopes should be stable, erosion-resistant, and traversable where feasible. Fixed objects such as trees, signs, utility poles, and culvert ends should be removed, relocated, shielded, or placed beyond an appropriate clear zone. Roadside safety is a design issue, not merely a maintenance issue.

3. Sight Distance

Sight distance is the length of roadway visible to a driver. Adequate visibility is required for stopping, passing, decision-making, intersection entry, and recognition of signs and hazards.

3.1 Stopping sight distance

Stopping sight distance (SSD) is the distance traveled during perception and reaction plus the distance required for braking. On a level roadway, using consistent units:

where:

• SSD = stopping sight distance in m;
• V = speed in km/h;
• t = perception-reaction time in s; and
• f = coefficient of longitudinal friction, dimensionless.

The factor 0.278 converts km/h to m/s. The factor 254 incorporates unit conversion and the braking relationship under the assumptions of constant friction and level grade. The formula does not include a grade effect.

For a grade, a commonly used form is:

where G is grade expressed as a decimal; use “+” for an upgrade in the direction of travel and “−” for a downgrade. A downgrade reduces the effective resisting force and therefore increases braking distance. Standards may specify different reaction times, friction values, or grade conventions.

Worked example. A vehicle travels at 80 km/h on a level road. Let reaction time be 2.5 s and friction coefficient be 0.35.

Reaction distance:

0.278 × 80 × 2.5 = 55.6 m

Braking distance:

80²/(254 × 0.35) = 72.0 m approximately.

Therefore:

SSD = 55.6 + 72.0 = 127.6 m, or approximately 128 m.

The result is sensitive to speed because braking distance varies with the square of speed. Wet pavement, worn tires, driver condition, and heavy vehicles may require more conservative values.

3.2 Decision and passing sight distance

Decision sight distance is longer than ordinary stopping sight distance and is used where drivers must detect, recognize, interpret, and respond to an unexpected or complex situation. It is especially relevant near interchanges, unusual intersections, work zones, and locations with confusing geometry.

Passing sight distance is the distance needed for a vehicle to pass another vehicle safely on a two-lane road. It depends on speeds, vehicle acceleration, opposing traffic, and clearance. Because assumptions differ considerably among standards, passing design should use the adopted national or agency procedure rather than an improvised equation.

3.3 Sight distance at curves and intersections

At a horizontal curve, the inside of the curve may be obstructed by a cut slope, vegetation, wall, building, or barrier. The obstruction must be set back sufficiently to provide the required line of sight. At intersections, sight triangles should be kept free from obstructions. Signs, landscaping, parked vehicles, and utility equipment must not compromise the sight triangle.

4. Horizontal Alignment

Horizontal alignment consists of tangents, circular curves, and transition curves. A good alignment avoids unnecessary bends, provides smooth curvature, and coordinates with the vertical alignment.

4.1 Circular curves

For a vehicle traveling at speed V on a curve of radius R, lateral acceleration is approximately:

where a_l is lateral acceleration in m/s², V is speed in m/s, and R is radius in m. Superelevation and tire-road friction resist this lateral acceleration. The basic equilibrium relationship is:

where e is superelevation expressed as a decimal, f is the assumed side-friction factor, and g is gravitational acceleration, approximately 9.81 m/s². If speed is in km/h, the relationship becomes:

The equation is a design model, not a justification for using the maximum possible friction. Standards normally limit both superelevation and side friction for comfort, safety, slow vehicles, snow or ice conditions, and construction practicality.

Solving for radius:

with V in km/h and R in m.

Worked example. For V = 80 km/h, e = 0.06, and f = 0.10:

R = 80²/[127(0.06 + 0.10)] = 314.96 m.

Thus the calculated minimum radius is approximately 315 m, subject to the agency’s minimum-radius, maximum-superelevation, and minimum-curve-length rules.

4.2 Transition curves

A transition curve gradually changes curvature from zero on a tangent to the curvature of the circular curve. It allows gradual development of lateral acceleration, superelevation, and steering input. A spiral is a common transition form. The transition must be long enough for comfort and for the required superelevation runoff, while avoiding excessive length that may confuse drivers or create drainage problems.

4.3 Compound and reverse curves

A compound curve contains two or more curves turning in the same direction with different radii. It should be used only when justified by terrain or access, with compatible radii and adequate transitions. Reverse curves turn in opposite directions. They require careful coordination of transition lengths and superelevation because the crossfall must change from one direction to the other.

4.4 Horizontal alignment principles

Long tangents can encourage excessive speed and may cause headlight glare at night. Very short tangents between curves can appear as a kink. Sharp curves should not be hidden immediately beyond a crest vertical curve. Curvature should be consistent with the surrounding road and should not surprise drivers.

5. Superelevation

Superelevation is the transverse slope provided by raising the outside edge of a pavement on a horizontal curve. It reduces the side-friction demand and improves comfort. The total resistance to lateral movement is represented by e + f, but neither component should be treated as unlimited.

Maximum superelevation depends on climate, vehicle mix, urban constraints, snow and ice, drainage, and agency standards. Excessive superelevation can cause slow vehicles to drift inward, create difficulties at intersections and driveways, and cause operational problems during icy conditions.

Superelevation is introduced through a runoff length. Part of the rotation may occur on a tangent, called tangent runoff, and part on the curve. The pavement rotation should be smooth, with a rate of change that satisfies the applicable comfort and drainage criteria. The axis of rotation may be the centerline, crown line, or another reference line specified by the standard.

At the beginning and end of runoff, drainage must be checked. Flat areas can retain water, while abrupt changes can produce uncomfortable steering or vehicle instability. Superelevation diagrams should show normal crown, removal of adverse crown, full superelevation, and return to normal crossfall.

6. Vertical Alignment

Vertical alignment consists of grades connected by vertical curves. It controls vehicle energy, truck speed, drainage, earthwork, visibility, and accessibility.

6.1 Grades

Grade is the change in elevation divided by horizontal distance:

where Δh is elevation change and L is horizontal distance. Grade is often expressed as a percentage:

A 4% upgrade rises 4 m in every 100 m horizontally. Maximum grades should reflect road class, terrain, climate, heavy-vehicle proportion, and desired operating speed. Long steep grades may require climbing lanes, truck escape ramps, or warning and speed-management measures.

6.2 Vertical curves

Parabolic vertical curves are commonly used because they provide a constant rate of change of grade. Let g₁ and g₂ be algebraic grades in decimal form. The algebraic grade difference is:

For a symmetric curve of length L, the grade at distance x from the beginning is:

The elevation relative to the beginning of the curve is:

The equation assumes a parabolic curve and consistent units. For a crest curve, A is negative; for a sag curve, A is positive.

The highest or lowest point occurs where g(x)=0. Therefore:

provided the value lies between 0 and L. The elevation at this point is important for drainage, sight distance, and vertical clearance.

Vertical-curve length is controlled by sight distance, comfort, drainage, headlight illumination, and appearance. Crest curves are often controlled by daytime sight distance. Sag curves may be controlled by headlight sight distance, comfort, drainage, and structural clearance.

For a symmetric curve, the rate of grade change is often expressed as:

where K has units of m per one-percent grade difference when A is expressed in percentage points. Standards provide minimum K values for specific design speeds and sight-distance requirements.

Worked example. A crest curve connects g₁ = +2% and g₂ = −3% over L = 200 m. Then A = −5%. The point of zero grade is:

x₀ = −(0.02)(200)/(−0.05) = 80 m.

Its elevation relative to the curve beginning is:

y = (0.02)(80) + [(-0.05)(80²)]/(2×200) = 0.8 m.

Thus the high point is 80 m from the beginning and 0.8 m above the beginning elevation.

7. Coordination of Plan and Profile

A safe road should appear continuous and understandable. Important coordination principles include:

• avoid placing a sharp horizontal curve at or immediately beyond a crest;
• avoid hiding an intersection beyond a crest or sharp curve;
• provide adequate drainage at sag points;
• coordinate superelevation runoff with vertical grades;
• avoid successive changes that cause misleading perspective;
• ensure bridge, culvert, railway, and utility clearances;
• check the alignment from the driver’s eye perspective, not only from drawings.

Computer-aided design and three-dimensional visualization are valuable for checking sight lines, earthwork, drainage, and driver expectancy. They do not replace engineering judgment or field verification.

8. Intersections

An intersection is a location where traffic streams cross, merge, diverge, or turn. Its design must manage conflict points, visibility, speed, pedestrian movement, drainage, and traffic control.

8.1 At-grade intersection types

Common forms include T-intersections, four-leg intersections, staggered intersections, roundabouts, and channelized intersections. Four-leg intersections concentrate conflicts and may require signal control or other management. Staggering two nearby T-intersections can reduce direct crossing conflicts, but the spacing must support safe operation.

8.2 Intersection sight distance

Intersection sight distance allows a driver on one approach to see enough of the major road to enter, cross, or turn safely. Required distance depends on the maneuver, vehicle acceleration, major-road speed, grade, and control type. The sight triangle must be protected from obstructions, and the design vehicle’s swept path must be checked.

8.3 Channelization and turning lanes

Channelization uses islands, medians, markings, and lanes to guide vehicles and separate conflicting movements. Right- or left-turn lanes may reduce rear-end collisions and through-traffic delay where turning demand is significant. A turn lane should include appropriate deceleration, storage, and taper lengths based on the governing standard.

Pedestrian crossings should be direct, visible, accessible, and aligned with desire lines. Refuge islands, raised crossings, signals, lighting, and reduced approach speeds may be appropriate. Designers must account for wheelchair users, people with visual impairments, cyclists, children, and older adults.

8.4 Roundabouts

Roundabouts reduce severe crossing conflicts by converting movements into entering, circulating, and exiting interactions. Their safety depends on appropriate entry deflection, speed control, visibility, yield control, truck accommodation, pedestrian crossing placement, and adequate capacity. A roundabout is not automatically appropriate where heavy pedestrian volumes, very high traffic demand, unusual geometry, or insufficient space exists.

9. Interchanges

Interchanges separate traffic streams by grade and connect roads through ramps. They are used where high traffic volumes, high speeds, or access-control objectives make frequent at-grade intersections unsuitable.

Common forms include diamond, trumpet, cloverleaf, partial cloverleaf, and directional interchanges. A diamond is compact and often suitable where a motorway crosses a lower-volume road. A trumpet is useful at a terminal connection. Cloverleafs reduce direct crossing but may have weaving problems and require substantial land. Directional ramps provide high-speed, high-capacity movements but are expensive.

Interchange design requires attention to ramp radius, superelevation, acceleration and deceleration lanes, merge and diverge sight distance, signing, lane balance, weaving, drainage, bridge clearance, and emergency access. Ramp alignment should create realistic operating speeds; a ramp with a severe speed reduction immediately after a high-speed exit can produce crashes unless the transition is clearly communicated.

10. Vehicles, Accessibility, and Roadside Safety

The design vehicle represents the physical and operating characteristics of a selected vehicle class. Passenger cars, buses, rigid trucks, articulated trucks, and emergency vehicles have different wheelbases, turning paths, acceleration, braking, and off-tracking. Swept-path analysis should be performed at intersections, roundabouts, terminals, and constrained sites.

Heavy vehicles are especially affected by steep grades, small-radius curves, and inadequate merge lengths. A road that is acceptable for passenger cars may create unacceptable delay or safety problems for trucks.

Universal accessibility requires continuous, obstruction-free pedestrian routes, manageable crossfalls, appropriate curb ramps, detectable warnings where required, and safe crossings. Accessibility is a fundamental design obligation, not an optional enhancement.

Roadside safety treatments include clear zones, breakaway supports, median barriers, bridge railings, crash cushions, and slope flattening. Barriers should not be installed without checking their need, end treatment, working width, drainage, and maintenance implications.

11. Technology and Contemporary Practice

Modern geometric design uses digital terrain models, geographic information systems, building information modeling, mobile mapping, LiDAR, drone surveys, and three-dimensional visualization. These tools improve quantity estimation, sight-distance checks, clash detection, and stakeholder communication.

Traffic simulation can test queues, merging, roundabout entry, and signal timing, but its outputs depend on assumptions about demand, driver behavior, and calibration. Artificial intelligence may assist with crash prediction, image-based pavement or roadside inspection, traffic forecasting, and alignment optimization. However, AI results must be explainable, independently validated, checked for biased data, and reviewed by a qualified engineer. Automated optimization should not be allowed to trade away safety, accessibility, or environmental obligations merely to reduce cost.

12. Illustrative Case Study: Mountain Corridor Improvement

Background

A two-lane rural road crosses rolling-to-mountainous terrain. It has frequent crashes on a sharp bend near a crest, poor drainage, and long queues behind heavy trucks. The existing road also passes close to a village school.

Problem

The original alignment has inadequate stopping sight distance, a short reverse curve without adequate superelevation transition, and an intersection hidden behind the crest. Widening the existing pavement alone would not solve the visibility and speed-consistency problems.

Analysis

A survey and digital terrain model were used to compare three corridors. The first followed the existing road and required extensive retaining walls. The second shifted uphill, reducing the curve severity but affecting a wetland. The third used a longer radius curve, a modest realignment, and a lower-speed approach near the school.

Checks included stopping sight distance, truck swept paths, grade length, drainage, earthwork, land acquisition, wetland impact, pedestrian access, and construction staging. The third alternative produced a more consistent speed profile and avoided the wetland, although it required a short retaining structure.

Approach and solution

The selected design used a larger horizontal radius, a properly developed transition, a crest vertical curve satisfying the adopted stopping sight-distance criterion, improved crossfall and side drainage, and a protected pedestrian crossing near the school. A climbing lane was introduced on the longest upgrade after confirming truck demand and available width. Warning signs and speed management complemented, but did not replace, the geometric improvements.

Outcome

The design reduced the number and severity of conflict locations, improved visibility, reduced water ponding, and provided safer pedestrian movement. Construction cost was higher than simple resurfacing but lower than the alternatives requiring extensive retaining works and wetland mitigation.

Learning points

1. Alignment decisions must consider plan, profile, cross-section, and roadside context together.
2. A low-cost pavement treatment cannot correct a fundamentally unsafe geometric layout.
3. Design speed must be credible and consistent with the surrounding environment.
4. Safety, accessibility, environmental protection, and life-cycle cost should be evaluated together.

13. Limitations, Governance, and Best Practice

Geometric formulas simplify real driver behavior. Friction varies with weather, tires, pavement condition, and speed. Design vehicles are representative rather than universal. Traffic forecasts may be uncertain, and construction constraints may require staged or nonstandard solutions.

Professional practice requires using the current legal and agency standards, documenting assumptions, obtaining independent checks, engaging affected communities, protecting vulnerable road users, and maintaining transparent records of alternatives. Designs should be subjected to road safety audit at appropriate stages. Ethical practice requires that convenience, political pressure, or short-term cost reduction never justify knowingly inadequate sight distance, unsafe roadside conditions, or exclusion of people with disabilities.

Chapter Summary

Highway geometric design creates the three-dimensional form of a road in response to function, speed, terrain, vehicles, traffic, environment, and human behavior. Cross-sections must provide appropriate lanes, shoulders, drainage, clear zones, and facilities for pedestrians and cyclists. Stopping sight distance combines perception-reaction distance with braking distance and increases strongly with speed.

Horizontal alignment uses tangents, circular curves, and transition curves. Curve radius, superelevation, and side friction are related through e+f=V²/(127R) when speed is in km/h. Vertical alignment uses grades and parabolic curves; curve length is controlled by sight distance, comfort, drainage, and clearance. Plan and profile must be coordinated to avoid hidden hazards and inconsistent speed changes.

Intersections and interchanges must manage conflict points, visibility, turning paths, merging, weaving, pedestrians, cyclists, and heavy vehicles. Digital tools and AI can support analysis, but qualified engineering judgment, standards, safety audits, accessibility, environmental responsibility, and ethical governance remain essential.

Key Terms

Design speed: Selected speed used to establish geometric design criteria.

Operating speed: Speed commonly chosen by drivers under free-flow conditions.

Horizontal alignment: Road location viewed in plan.

Vertical alignment: Road profile showing grades and elevation changes.

Cross-section: Transverse arrangement of lanes, shoulders, drainage, and roadside elements.

Stopping sight distance: Distance required for perception, reaction, and braking to a stop.

Decision sight distance: Visibility needed to detect and respond to an unexpected situation.

Superelevation: Transverse slope that raises the outside edge on a horizontal curve.

Side friction: Tire-road resistance contributing to lateral stability on a curve.

Transition curve: Curve that gradually changes curvature between a tangent and circular curve.

Circular curve: Horizontal curve with constant radius.

Grade: Rate of elevation change relative to horizontal distance.

Vertical curve: Smooth connection between two grades.

Crest curve: Convex vertical curve joining an upgrade to a downgrade.

Sag curve: Concave vertical curve joining a downgrade to an upgrade.

Clear zone: Recoverable roadside area relatively free of fixed hazards.

Channelization: Use of markings, islands, or lanes to guide traffic movements.

Interchange: Grade-separated facility connecting roads through ramps.

Design vehicle: Vehicle selected to represent controlling dimensions and movements.

Road safety audit: Formal examination of a road project for crash-risk reduction.

Review Questions

Short Answer Questions

1. Distinguish design speed from operating speed.
2. Identify four major elements of a highway cross-section.
3. What is the purpose of a transition curve?
4. Define superelevation and explain why it is provided.
5. State two reasons why plan and profile must be coordinated.

Long Answer Questions

1. Explain the factors that control highway alignment selection in mountainous terrain.
2. Derive and interpret the stopping sight-distance relationship, including the effect of grade.
3. Describe the design procedure for a horizontal curve, including radius, superelevation, friction, and transition.
4. Explain the components and controlling considerations of a parabolic vertical curve.
5. Compare at-grade intersections and interchanges with respect to safety, capacity, cost, and land requirements.

Analytical / Application Questions

1. A road is designed for 90 km/h with reaction time 2.5 s and friction coefficient 0.34. Calculate level stopping sight distance and explain how wet conditions could affect the result.
2. A horizontal curve has radius 400 m, speed 80 km/h, and superelevation 0.06. Calculate the side-friction demand using f=V²/(127R)−e, and assess whether it should be accepted without checking the applicable standard.
3. Propose and justify an alignment improvement for a two-lane road where a sharp curve, a crest, a school crossing, and a truck-climbing problem occur within 500 m. Discuss safety, construction, drainage, accessibility, and environmental trade-offs.

Multiple Choice Questions with Answers

Question 1

Which factor most directly determines the selected geometric design criteria for a highway?

A. Design speed

B. Paint color

C. Contract duration

D. Pavement brand

Correct Answer: A. Design speed

Question 2

Using SSD = 0.278Vt + V²/(254f), what is the approximate level stopping sight distance for V = 80 km/h, t = 2.5 s, and f = 0.35?

A. 72 m

B. 96 m

C. 128 m

D. 180 m

Correct Answer: C. 128 m

Question 3

What is the principal purpose of a transition curve?

A. To eliminate drainage

B. To change curvature and superelevation gradually

C. To increase the maximum grade

D. To replace all vertical curves

Correct Answer: B. To change curvature and superelevation gradually

Question 4

In the relationship e + f = V²/(127R), speed V is expressed in:

A. m/s

B. km/h

C. miles per minute

D. degrees per second

Correct Answer: B. km/h

Question 5

For a curve with V = 80 km/h, e = 0.06, and f = 0.10, the calculated radius is approximately:

A. 160 m

B. 250 m

C. 315 m

D. 640 m

Correct Answer: C. 315 m

Question 6

A downgrade generally has what effect on stopping distance?

A. It decreases braking distance

B. It has no possible effect

C. It increases the distance required to stop

D. It eliminates reaction distance

Correct Answer: C. It increases the distance required to stop

Question 7

For a parabolic vertical curve, the point of zero grade is found by setting which expression equal to zero?

A. The horizontal radius

B. g(x)

C. The lane width

D. The superelevation rate

Correct Answer: B. g(x)

Question 8

Which feature is especially important for reducing severe crossing conflicts at many intersections?

A. Uncontrolled access everywhere

B. Roundabout or grade separation where appropriate

C. Removing all pedestrian facilities

D. Using shorter sight triangles

Correct Answer: B. Roundabout or grade separation where appropriate

Question 9

Which vehicle characteristic is particularly important in checking a tight intersection or roundabout?

A. Vehicle paint reflectance

B. Swept path and off-tracking

C. Radio frequency

D. Engine color

Correct Answer: B. Swept path and off-tracking

Question 10

Why should AI-generated geometric design recommendations be independently reviewed?

A. AI always produces identical designs

B. Engineering standards are unnecessary

C. Results depend on data and assumptions and may contain unsafe bias

D. Digital terrain models cannot represent elevation

Correct Answer: C. Results depend on data and assumptions and may contain unsafe bias