10 traffic-characterization ( Highway Engineering Dr. Sherif El-Badawy )

1

Mansoura University - Faculty of Engineering – Public Works Engineering Dept.

1

Traffic Characterization


2

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


Traffic Loads Characterization

Pavement Thickness Design Are Developed

To Account For The Entire

Spectrum Of Traffic Loads

Cars Pickups Buses Trucks Trailers


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)

2


6

Traffic Composition


7

Axle Configuration

Single Axle Single Tires

Single Axle Dual Tires

Tandem Axle Dual Tires

Tridem Axle Dual Tires


8

Axle Configuration

3


Axle Configuration Parameters

Axle Width

Axle Spacing

Tire Pressure

Dual Tire Spacing

Wheel Base


FHWA Vehicle Classes

1 2 3 4

5 6 7 8

9 10 11

12 13


Normalized Truck Distribution by Truck Class

El-Badawy, et al. TRB, 2012


Sources of Traffic Data

• Weigh-in-motion (WIM)

• Automatic vehicle classification

(AVC)

• Traffic counts

4


Highway Graduation Project 2011


15

Design Approaches

•Fixed Traffic •Fixed Vehicle •Variable Traffic and Vehicle


16

Fixed Traffic

• Thickness design governed by single wheel load • Multiple wheels converted to equivalent single

wheel load (ESWL)

• Number of traffic repetitions is not a variable

• Historical usage: • Airport pavements

• Highway pavements w/ heavy wheel loads but light traffic volume

• Method rarely used today

Mansoura University - Faculty of Engineering – Public Works Engineering Dept. 18

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

5


19

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


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)


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


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.

6


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


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

WLEF

18

t

x

t

22x

t18

tx

β

G

β

G)4.33log(L)L4.79log(L1)4.79log(18

W

Wlog

1.54.2

p4.2logG t

t 3.23

2

5.19

3.23

2xx

L1SN

)L0.08(L0.40β

Where:

ESALs

12,000 lbs

LEF = 0.18928,000 lbs

LEF = 0.49528,000 lbs

LEF = 0.495


(Huang, 2004)


أوزان المحاور والحموالت المسموح بها طبقا للكود المصري للطرق

7


أوزان المحاور والحموالت المسموح بها طبقا للكود المصري للطرق



Approximate Relations to Compute LEF


REGULAR MIXED TRAFFIC

Equivalent Number of 18k Single Axle

Loads

8


31

Truck Factor = S (Number of axles * LEF)/number of vehicles

Truck Factor = S LEF = 0.189 + 0.495*2 = 1.368

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


TF = 1.5 to 2.5

According to Egyptian Code

32

Truck

Factor


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


34

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


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


9


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


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


36

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


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


Mansoura University - Faculty of Engineering – Public Works Engineering Dept. Mansoura University - Faculty of Engineering – Public Works Engineering Dept.

38

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

10


Percentage of truck traffic traveling

in one direction

47%

53%

Directional Distribution


ADT 20,000

25% trucks

75% trucks

ADT 60,000

8% trucks

39% trucks

53% trucks

Design for

worst case!!

Lane Distribution


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


42

Direction and Lane Factors (dfXfd)

Number of Traffic

Lanes

(Two Directions)

% Trucks in

design lane

2 50

4 45

6 or more 40

11


43

Growth Factor (G)

(Huang, 1993)

Based on standard compound growth equations.

r

rG

n11

r = growth rate

n = Design period, years

For annual growth rate (r=0), G = n

r = 2 -7 % Egyptian Code


Example of Single Axle Growth

0

100000

200000

300000

400000

2000 2010 2020 2030 2040

Year

Pro

jecte

d n

um

ber o

f

sin

gle

ax

les


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


12


Axle Load Spectra

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

0 10000 20000 30000 40000 50000 60000 70000 80000 90000

Pe

rce

nt

Axl

es

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