Highway Pavement Design

Transportation · FE Reference Handbook section

Transportation

5 formulas

10 exam-style examples

~55 min

All Transportation lectures

Learning objectives

What you must be able to do before leaving this section.

This chapter section covers Highway Pavement Design within Transportation. Read it the way you would read a textbook chapter: the theory first so the relations mean something, then every equation with its use and its trap, then 10 fully worked examples with the arithmetic shown line by line, and finally a self-check you should be able to answer without notes.

Explain, in your own words, what highway pavement design describes physically and when it applies.
State every one of the 5 relations the handbook lists here and name each symbol with its unit.
Select the correct relation from the wording of an exam stem within 20 seconds.
Carry a complete solution from givens to a "most nearly" answer with the correct unit.
Recognise the distractors generated by the unit trap: grades as decimals in curve formulas, percent in the stem.

Lecture

Why this section exists. Highway Pavement Design is the part of Transportation that lets you connect a vertical or horizontal alignment, or a traffic stream to a number you can defend. Before any equation is useful you must be able to picture the physical situation it describes; the schematic below is that picture.

How the theory is built. The handbook prints results, not derivations. Each relation in this section comes from one governing principle applied to the idealised system: state the principle, impose the stated assumptions, and the printed equation follows. Knowing which assumption each relation rests on is what lets you reject a wrong answer choice in seconds.

How it is examined. Items from this page are written as a curve geometry element or a capacity/flow relationship. Roughly two thirds are direct substitution, one third require one intermediate quantity from a neighbouring relation, and a small number are conceptual — testing whether you know the assumption, not the arithmetic.

The habit that earns the points. Unit discipline. grades as decimals in curve formulas, percent in the stem. Every relation below is dimensionally consistent only when that rule is honoured, and the distractor set is deliberately built from candidates who ignored it. Write the unit next to every number you substitute, every time.

How to study this page. Read the theory, then cover the formula cards and try to reproduce each relation from its description. Then work the examples with the solution hidden, revealing one line at a time. Finish with the self-check questions; if you cannot answer one, return to the matching formula card.

Dense peak-hour traffic queued on an urban arterial at dusk. — Photo 1. Where this shows up in practice: highway pavement design.
Wikimedia Commons, CC BY 2.0

Transportation — Highway Pavement Design: reference schematic for orienting the symbols used in this section.

Theory, developed

Read this before the equations — it is what makes them memorable.

The physical situation

Every item from this section describes a vertical or horizontal alignment, or a traffic stream. Sketch it before you compute — a labelled sketch with the givens on it converts a wordy stem into a solvable problem and exposes the quantity the examiner left out on purpose.

The governing principle

The 5 relations on this page are consequences of one principle applied to that idealised system. Identify which quantity is conserved, balanced, or defined, and the correct equation follows without memorisation.

Assumptions and limits of validity

Each printed relation carries silent assumptions — linearity, steady state, uniformity, small deformation, or standard conditions, depending on the subject. Conceptual exam items are written by violating exactly one of these, so read the sentence above the equation as carefully as the equation itself.

Solution procedure you should automate

1) Read the last sentence of the stem to identify the requested quantity. 2) Locate the relation on this page whose left-hand side is that quantity. 3) Tabulate the givens with units and mark the missing symbol. 4) If a symbol is missing, find the one relation that produces it. 5) Rearrange symbolically, substitute once, evaluate, and round only at the end.

Notation used in this section

SN	Quantity produced by "SN = a1D1 +a2D2m2+ a3D3m3+...+anDnmn" — read its definition and unit from the handbook line directly above the equation.
ai	Quantity produced by "ai = layer coefficient" — read its definition and unit from the handbook line directly above the equation.
Di	Quantity produced by "Di = thickness of layer (inches)" — read its definition and unit from the handbook line directly above the equation.
mi	Quantity produced by "mi = drainage coefficient (assume m equals 1.0 unless otherwise given)" — read its definition and unit from the handbook line directly above the equation.

Handbook notes for this section

Definitions and conditions exactly as the handbook states them.

AASHTO Structural Number Equation
where
Load Equivalency Load Equivalency
Gross Axle Load Gross Axle Load
Factors Factors
Single Tandem Single Tandem
kN lb kN lb
Axles Axles Axles Axles
4.45 1,000 0.00002 187.0 42,000 25.64 2.51
8.9 2,000 0.00018 195.7 44,000 31.00 3.00
17.8 4,000 0.00209 200.0 45,000 34.00 3.27
22.25 5,000 0.00500 204.5 46,000 37.24 3.55
26.7 6,000 0.01043 213.5 48,000 44.50 4.17
35.6 8,000 0.0343 222.4 50,000 52.88 4.86
44.5 10,000 0.0877 0.00688 231.3 52,000 5.63
53.4 12,000 0.189 0.0144 240.2 54,000 6.47
62.3 14,000 0.360 0.0270 244.6 55,000 6.93
66.7 15,000 0.478 0.0360 249.0 56,000 7.41
71.2 16,000 0.623 0.0472 258.0 58,000 8.45
80.0 18,000 1.000 0.0773 267.0 60,000 9.59
89.0 20,000 1.51 0.1206 275.8 62,000 10.84
97.8 22,000 2.18 0.180 284.5 64,000 12.22
106.8 24,000 3.03 0.260 289.0 65,000 12.96
111.2 25,000 3.53 0.308 293.5 66,000 13.73

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1

Flexible pavement structural number — Highway Pavement Design

A flexible pavement has 6.0 in. of asphalt (a₁ = 0.44), 10 in. of base (a₂ = 0.14) and 12 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 6.0 in., a₁ = 0.44
D₂ = 10 in., a₂ = 0.14
D₃ = 12 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (6.0) = 2.640$
Base
$0.14 (10) (1.0) = 1.400$
Subbase
$0.11 (12) (1.0) = 1.320$
Total
$S N = 5.36$

Answer: SN ≈ 5.36

Why the other options are there

28.00 (thicknesses added without coefficients)
4.879 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 2

Flexible pavement structural number — Highway Pavement Design (2)

A flexible pavement has 5.5 in. of asphalt (a₁ = 0.44), 11 in. of base (a₂ = 0.14) and 7 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 5.5 in., a₁ = 0.44
D₂ = 11 in., a₂ = 0.14
D₃ = 7 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (5.5) = 2.420$
Base
$0.14 (11) (1.0) = 1.540$
Subbase
$0.11 (7) (1.0) = 0.770$
Total
$S N = 4.73$

Answer: SN ≈ 4.73

Why the other options are there

23.50 (thicknesses added without coefficients)
2.870 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 3

Flexible pavement structural number — Highway Pavement Design (3)

A flexible pavement has 5.0 in. of asphalt (a₁ = 0.44), 10 in. of base (a₂ = 0.14) and 13 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 5.0 in., a₁ = 0.44
D₂ = 10 in., a₂ = 0.14
D₃ = 13 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (5.0) = 2.200$
Base
$0.14 (10) (1.0) = 1.400$
Subbase
$0.11 (13) (1.0) = 1.430$
Total
$S N = 5.03$

Answer: SN ≈ 5.03

Why the other options are there

28.00 (thicknesses added without coefficients)
4.404 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 4

Flexible pavement structural number — Highway Pavement Design (4)

A flexible pavement has 6.0 in. of asphalt (a₁ = 0.44), 7 in. of base (a₂ = 0.14) and 11 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 6.0 in., a₁ = 0.44
D₂ = 7 in., a₂ = 0.14
D₃ = 11 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (6.0) = 2.640$
Base
$0.14 (7) (1.0) = 0.980$
Subbase
$0.11 (11) (1.0) = 1.210$
Total
$S N = 4.83$

Answer: SN ≈ 4.83

Why the other options are there

24.00 (thicknesses added without coefficients)
3.131 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 5

Flexible pavement structural number — Highway Pavement Design (5)

A flexible pavement has 5.5 in. of asphalt (a₁ = 0.44), 11 in. of base (a₂ = 0.14) and 9 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 5.5 in., a₁ = 0.44
D₂ = 11 in., a₂ = 0.14
D₃ = 9 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (5.5) = 2.420$
Base
$0.14 (11) (1.0) = 1.540$
Subbase
$0.11 (9) (1.0) = 0.990$
Total
$S N = 4.95$

Answer: SN ≈ 4.95

Why the other options are there

25.50 (thicknesses added without coefficients)
3.690 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 6

Flexible pavement structural number — Highway Pavement Design (6)

A flexible pavement has 5.0 in. of asphalt (a₁ = 0.44), 8 in. of base (a₂ = 0.14) and 13 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 5.0 in., a₁ = 0.44
D₂ = 8 in., a₂ = 0.14
D₃ = 13 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (5.0) = 2.200$
Base
$0.14 (8) (1.0) = 1.120$
Subbase
$0.11 (13) (1.0) = 1.430$
Total
$S N = 4.75$

Answer: SN ≈ 4.75

Why the other options are there

26.00 (thicknesses added without coefficients)
3.524 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 7

Flexible pavement structural number — Highway Pavement Design (7)

A flexible pavement has 4.5 in. of asphalt (a₁ = 0.44), 12 in. of base (a₂ = 0.14) and 11 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 4.5 in., a₁ = 0.44
D₂ = 12 in., a₂ = 0.14
D₃ = 11 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (4.5) = 1.980$
Base
$0.14 (12) (1.0) = 1.680$
Subbase
$0.11 (11) (1.0) = 1.210$
Total
$S N = 4.87$

Answer: SN ≈ 4.87

Why the other options are there

27.50 (thicknesses added without coefficients)
4.025 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 8

Flexible pavement structural number — Highway Pavement Design (8)

A flexible pavement has 5.5 in. of asphalt (a₁ = 0.44), 6 in. of base (a₂ = 0.14) and 6 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 5.5 in., a₁ = 0.44
D₂ = 6 in., a₂ = 0.14
D₃ = 6 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (5.5) = 2.420$
Base
$0.14 (6) (1.0) = 0.840$
Subbase
$0.11 (6) (1.0) = 0.660$
Total
$S N = 3.92$

Answer: SN ≈ 3.92

Why the other options are there

17.50 (thicknesses added without coefficients)
1.342 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 9

Flexible pavement structural number — Highway Pavement Design (9)

A flexible pavement has 6.5 in. of asphalt (a₁ = 0.44), 10 in. of base (a₂ = 0.14) and 7 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 6.5 in., a₁ = 0.44
D₂ = 10 in., a₂ = 0.14
D₃ = 7 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (6.5) = 2.860$
Base
$0.14 (10) (1.0) = 1.400$
Subbase
$0.11 (7) (1.0) = 0.770$
Total
$S N = 5.03$

Answer: SN ≈ 5.03

Why the other options are there

23.50 (thicknesses added without coefficients)
3.083 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Example 10

Flexible pavement structural number — Highway Pavement Design (10)

A flexible pavement has 4.0 in. of asphalt (a₁ = 0.44), 10 in. of base (a₂ = 0.14) and 7 in. of subbase (a₃ = 0.11) with drainage coefficients of 1.0. Find the structural number.

Given

D₁ = 4.0 in., a₁ = 0.44
D₂ = 10 in., a₂ = 0.14
D₃ = 7 in., a₃ = 0.11
m = 1.0

Find

Structural number SN

Start with the thinking

SN sums layer coefficient × thickness × drainage coefficient.
Thicknesses are in inches — never convert to feet here.

Step-by-step solution

AASHTO
$S N = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}$
Asphalt
$0.44 (4.0) = 1.760$
Base
$0.14 (10) (1.0) = 1.400$
Subbase
$0.11 (7) (1.0) = 0.770$
Total
$S N = 3.93$

Answer: SN ≈ 3.93

Why the other options are there

21.00 (thicknesses added without coefficients)
1.897 (terms multiplied)

Reference: FE Reference Handbook — Transportation → Highway Pavement Design

Self-check

Answer these without notes before moving on.

Without looking, state the relation on this page whose left-hand side is the quantity most often requested, and name every symbol in it.
Which assumption, if violated, makes the main relation of this section invalid?
Given a vertical or horizontal alignment, or a traffic stream, what is the first quantity you would compute, and why that one first?
Which unit conversion in this subject most often produces a wrong answer choice, and what is its numerical factor?
Rework Example 1 above from the givens alone, without reading the solution lines.

Chapter summary

Highway Pavement Design contains 5 relations; you must be able to find this page in under 15 seconds.
Exam style: a curve geometry element or a capacity/flow relationship.
Unit rule: grades as decimals in curve formulas, percent in the stem.
Work the 10 examples until the solution path, not the answer, is automatic.

Common traps in this section

grades as decimals in curve formulas, percent in the stem
Answering the intermediate quantity instead of the quantity requested.
Rounding intermediate values before the final step.
Using a relation from an adjacent handbook section that shares a symbol.
Skipping the sketch — most lost points on this page start with a misread geometry.