This document discusses traffic characterization and loadings for pavement design. It covers topics like vehicle characteristics, axle configurations, traffic composition, sources of traffic data, load equivalency factors, truck factors, and how to calculate estimated 18-kip equivalent single-axle loads (ESALs) using traffic data. The goal is to account for the full spectrum of traffic loads that pavement will experience over its design life when determining appropriate pavement thickness.

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Traffic Characterization
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Traffic Loading
• Primary input to any design procedure
• Issues:
• Vehicle Characteristics
• Weight
• Wheel and axle configuration
• Frequency of occurrence
• Traffic Volume
• Current
• Future (traffic growth)
• Other
• Lateral wander
• Often not well-defined in practice

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Traffic Loads Characterization
Pavement Thickness Design Are Developed
To Account For The Entire
Spectrum Of Traffic Loads
Cars Pickups Buses Trucks Trailers
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Critical Response Values
et
ec
et
ec
et at surface + bottom of all bound layers (cracking)
ec at midthickness of all layers + top of subgrade (rutting)

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Traffic Composition

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Axle Configuration
Single Axle
Single Tires
Single Axle
Dual Tires
Tandem Axle
Dual Tires
Tridem Axle
Dual Tires
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Axle
Configuration

5. 5
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Axle Configuration Parameters
Axle Width
Axle
Spacing
Tire
Pressure
Dual Tire
Spacing
Wheel Base
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Vehicle
Classes
(NCHRP 1-37A, 2001)

6. 6
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FHWA Vehicle Classes
1 2 3 4
5 6 7 8
9 10 11
12 13
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Normalized Truck Distribution by Truck Class
El-Badawy, et al. TRB, 2012

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Sources of Traffic Data
• Weigh-in-motion (WIM)
• Automatic vehicle classification
(AVC)
• Traffic counts
Mansoura University - Faculty of Engineering – Public Works Engineering Dept.
• , Delta
Highway Graduation Project 2011,
Delta

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Design Approach
(Fixed Vehicle)
• Thickness design governed by number of
traffic repetitions
• Vehicle type/load is not a variable
• 18 kip (80 kN) single axle load (SAL)
• Standard aircraft
• Multiple axles converted to equivalent single axle
load (ESAL)
• Most common current design procedure
Mansoura University - Faculty of Engineering – Public Works Engineering Dept.
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Important Definitions
• Standard Axle Load (SAL): A single axle
with dual tires that has a weight of 18,000
lb (80 KN).
• ESAL: The number of repetitions of an
18,000 lb single axle load
18,000 lb
SAL

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Equivalent Single Axle Load
(ESAL)
• Most common standard load
• Developed at the AASHO Road Test
• Load equivalency factors (LEFs)
• The number of ESALs needed to cause the
same damage as the load in question
1 ESAL = damage caused by one 18,000 lb single axle load
Mansoura University - Faculty of Engineering – Public Works Engineering Dept.
Load Equivalency Factors (LEF)
LEFs Can Be Based On:
• Equivalent Stress or Strain at a Given Location
(Mechanistic)
• Equivalent Deflection at a Given Location
(Mechanistic)
• Equivalent Serviceability Loss
(AASHTO)

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LEF = EALF
• Ratio of damage caused by a certain axle to
that caused by the standard one.
• Based on Fatigue,
• LEF ≈ {Wi / Ws}4,
where W = weight of axle
• Based on AASHTO Equation:
• For Flexible Pavements
• For Rigid Pavements
Mansoura University - Faculty of Engineering – Public Works Engineering Dept.
AASHTO Load Equivalency
Factors
Load Equivalency Factor (LEF):
No. of repetitions of 18-k SAL Load causing given PSI
No. of repetitions of X- k Y-Axle Load for a same PSI
Change for each:
Pavement Type
Thickness
Terminal Serviceability.

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ESALF
• Ratio of damage caused by a certain axle to
that caused by the standard one.
• Based on Fatigue,
• ESALF ≈ {Wi / Ws}4,
where W = weight of axle
• Based on AASHTO Eqn,
• For Flexible Pavements, see Equation in Next
Slide
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AASHTO 1993 Equation for LEF for Flexible
Pavement
Wtx = number of applications of given axle
Wt18 = number of standard axle passes (18-kips single axle)
Lx = load of axle group being evaluated, kips
L2 = axle code (1 for single, 2 for tandem, and 3 for tridem, 4 for quad)
b18 = value of bx when Lx = 18 and L2 = 1
pt = terminal serviceability index
SN = structural number
LEF = load equivalency factor
tx
t18
W
W
LEF 
18
t
x
t
22x
t18
tx
β
G
β
G
)4.33log(L)L4.79log(L1)4.79log(18
W
W
log 














1.54.2
p4.2
logG t
t
  3.23
2
5.19
3.23
2x
x
L1SN
)L0.08(L
0.40β



Where:
ESALs
12,000 lbs
LEF = 0.189
28,000 lbs
LEF = 0.495
28,000 lbs
LEF = 0.495

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(Huang, 2004)
Mansoura University - Faculty of Engineering – Public Works Engineering Dept.
‫للطرق‬ ‫المصري‬ ‫للكود‬ ‫طبقا‬ ‫بها‬ ‫المسموح‬ ‫والحموالت‬ ‫المحاور‬ ‫أوزان‬

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Mansoura University - Faculty of Engineering – Public Works Engineering Dept.
‫للطرق‬ ‫المصري‬ ‫للكود‬ ‫طبقا‬ ‫بها‬ ‫المسموح‬ ‫والحموالت‬ ‫المحاور‬ ‫أوزان‬
Mansoura University - Faculty of Engineering – Public Works Engineering Dept.

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Approximate Relations to Compute LEF
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REGULAR MIXED TRAFFIC
Equivalent Number of 18k Single Axle
Loads

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Truck Factor = S (Number of axles * LEF)/number of vehicles
Truck Factor = S LEF = 0.189 + 0.495*2 = 1.179
ESALs
12,000 lbs
LEF = 0.189
28,000 lbs
LEF = 0.495
28,000 lbs
LEF = 0.495
Truck Factors
Truck Factor (TF) = sum of all LEFs for each truck
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TF = 1.5 to 2.5
According to Egyptian Code
32
Truck
Factor

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Truck Factors – Urban
Vehicle Type Interstate Freeways
Other
Principal
Minor
Arterials Collectors
Single-unit trucks
2-axle, 4-tire 0.002 0.015 0.002 0.006 -
2-axle, 6-tire 0.17 0.13 0.24 0.23 0.13
3-axle or more 0.61 0.74 1.02 0.76 0.72
All single units 0.05 0.06 0.09 0.04 0.16
Tractor semi-
trailers
4-axle or less 0.98 0.48 0.71 0.46 0.40
5-axle 1.07 1.17 0.97 0.77 0.63
6-axle or more 1.05 1.19 0.90 0.64 -
All multiple units 1.05 0.96 0.91 0.67 0.53
ALL TRUCKS 0.39 0.23 0.21 0.07 0.24
From MS-1, Table IV-5
Mansoura University - Faculty of Engineering – Public Works Engineering Dept.
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Truck Factors – Rural
Vehicle Type Interstate
Other
Principal
Minor
Arterials
Major
Collector
Minor
Collector
Single-unit trucks
2-axle, 4-tire 0.003 0.003 0.003 0.017 0.003
2-axle, 6-tire 0.21 0.25 0.28 0.41 0.19
3-axle or more 0.61 0.86 1.06 1.25 0.45
All single units 0.06 0.08 0.08 0.12 0.03
Tractor semi-trailers
4-axle or less 0.62 0.92 0.62 0.37 0.91
5-axle 1.09 1.25 1.05 1.67 1.11
6-axle or more 1.23 1.54 1.04 2.21 1.35
All multiple units 1.04 1.21 0.97 1.52 1.08
ALL TRUCKS 0.52 0.38 0.21 0.30 0.12
From MS-1, Table IV-5

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Truck Distribution - Rural
Vehicle Type Interstate
Other
Principal
Minor
Arterials
Major
Collector
Minor
Collector
Single-unit trucks
2-axle, 4-tire 43 60 71 73 80
2-axle, 6-tire 8 10 11 10 10
3-axle or more 2 3 4 4 2
All single units 53 73 86 87 92
Tractor semi-trailers
4-axle or less 5 3 3 2 2
5-axle 41 23 11 10 6
6-axle or more 1 1 <1 1 <1
All multiple units 47 27 14 13 8
ALL TRUCKS 100 100 100 100 100
From MS-1, Table IV-1
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Truck Distribution – Urban
Vehicle Type Interstate
Other
Principal
Minor
Arterials
Major
Collector
Minor
Collector
Single-unit trucks
2-axle, 4-tire 52 68 67 84 86
2-axle, 6-tire 12 12 15 9 11
3-axle or more 2 4 3 2 <1
All single units 66 82 85 95 97
Tractor semi-trailers
4-axle or less 5 5 3 2 1
5-axle 28 13 12 3 2
6-axle or more 1 <1 <1 <1 <1
All multiple units 34 18 15 5 3
ALL TRUCKS 100 100 100 100 100
From MS-1, Table IV-1

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Traffic Analysis
ESAL = Equivalent 18,000 lb single axle load
AADTi = First year annual average daily traffic for axle i in both directions
Gjt = growth factor for growth rate j and design period t
fd = lane distribution factor
df = directional distribution factor
Ni = Number of axles
FEi = Load equivalency factor for axle i
Ti = Truck Factor
ESAL = 365(fd)(df)(Gjt)S(AADTi)(Ni)(FEi) using LEF
ESAL = 365(fd)(df)(Gjt)S(AADTi)(Ti)  using Truck Factor

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Percentage of truck traffic traveling
in one direction
47%
53%
Directional Distribution
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ADT 20,000
25% trucks
75% trucks
ADT 60,000
8% trucks
39% trucks
53% trucks
Design for
worst case!!
Lane Distribution

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Lane Distribution Factor
N o. of Traffic Lanes
in each Direction
% Trucks in
Design Lane
1 100
2 80–100
3 60–80
4 or more 50–75
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Direction and Lane Factors (dfXfd)
Number of Traffic
Lanes
(Two Directions)
% Trucks in
design lane
2 50
4 45
6 or more 40

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Growth Factor (G)
(Huang, 1993)
Based on standard compound growth equations.
  
r
r
G
n
11 

r = growth rate
n = Design period, years
For annual growth rate (r=0), G = n
r = 2 -7 % Egyptian Code
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Example of Single Axle Growth
0
100000
200000
300000
400000
2000 2010 2020 2030 2040
Year
Projectednumberof
singleaxles

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Example 1 - Calculation of ESALS
Axle Load, kips # of Axles per
1000 trucks
Single Axles
2 753
4 299
6 105
8 34
10 42
12 30
14 41
16 93
18 110
20 80
22 50
24 11
Axle Load, kips # of Axles per
1000 trucks
Tandem Axles
14 1
16 5
18 15
20 20
22 36
24 42
26 84
28 92
30 50
32 12
34 8
36 4
38 2
40 1
42 1
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Axle Load Spectra
0%
2%
4%
6%
8%
10%
12%
14%
16%
18%
0 10000 20000 30000 40000 50000 60000 70000 80000 90000
PercentAxles
Axle Load, lb
000169-South,VC 9, Tandem Axles
January
February
March
April
May
June
July
August
September
October
November
December
El-Badawy, et al. TRB, 2012