Potential Implications of NAFTA Truck Traffic:
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Potential Implications of Potential Implications of NAFTA Truck Traffic:NAFTA Truck Traffic:
for better or for worse? for better or for worse?
Potential Implications of Potential Implications of NAFTA Truck Traffic:NAFTA Truck Traffic:
for better or for worse? for better or for worse?
Dr. Jorge A ProzziDr. Jorge A ProzziThe University of TexasThe University of Texas
Border Partnership Working GroupBorder Partnership Working Group7 September 2006, Austin, Texas7 September 2006, Austin, Texas
Potential Implications of Potential Implications of NAFTA Truck Traffic:NAFTA Truck Traffic:
Presentation OutlinePresentation Outline
Effect of Loads on Pavement DamageEffect of Loads on Pavement Damage• Historical (ESAL)Historical (ESAL)
• Recent Findings (TxDOT Research 0-4510)Recent Findings (TxDOT Research 0-4510) Data AnalysisData Analysis Preliminary ResultsPreliminary Results Preliminary FindingsPreliminary Findings
Historical: ESALHistorical: ESAL
Where,Where, LEFLEFi i : load equivalency factor for load i-th: load equivalency factor for load i-th
nni i : number of axles of load i-th: number of axles of load i-th
xxi i : axle load i-th: axle load i-th
LLS S : standard axle load: standard axle load m m : power, usually 4.0: power, usually 4.0
k
iii nLEFESAL
1
m
s
ii L
xLEF
Historical: ESALHistorical: ESAL
Power value, m: Power value, m: 4.0 according to analysis of the AASHO Road Test 4.0 according to analysis of the AASHO Road Test
(based on (based on riding qualityriding quality)) 3.0-5.0 for fatigue 3.0-5.0 for fatigue crackingcracking Around 1.0-2.0 for rutting or surface Around 1.0-2.0 for rutting or surface deformationdeformation Other research studies found values varying from Other research studies found values varying from
less than 1.0 to 6.0 or even largerless than 1.0 to 6.0 or even larger So, which exponent (power) should we use?So, which exponent (power) should we use? Is it safe to use a higher power value? Is it safe to use a higher power value?
Historical: ESALHistorical: ESAL
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 2 4 6 8 10
Power
LSF
+0
+1
+2
+4
Historical: ESALHistorical: ESAL
0
0.5
1
1.5
2
2.5
15 20 25 30 35 40 45
AXLE LOAD (kips)
ED
F o
r "w
eig
ht"
EDF1
EDF2
EDF3
EDF4
Historical: ESALHistorical: ESAL
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 10 20 30 40 50 60 70 80
Axle Weight (kip)
Nor
mal
ized
Fre
quen
cy
Type IIType II
Type IIIType III
Type IType I
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 10 20 30 40 50 60 70 80
Axle Weight (kip)
Nor
mal
ized
Fre
quen
cy
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 10 20 30 40 50 60 70 80
Axle Weight (kip)
Nor
mal
ized
Fre
quen
cy
Mixed Lognormal distributionMixed Lognormal distribution
WhereWhere• xx: axle load, kip: axle load, kip• kk: the : the kk-th peak-th peak• KK: number of peaks, (1-3): number of peaks, (1-3)• λλkk, , ζζkk: parameters: parameters• WWkk: weights, : weights, ∑∑WWkk = 1 = 1
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0 10 20 30 40 50
Axle Load (kip)
No
rma
lize
d F
req
ue
nc
ypk1
pk2
pk3
all
K
k k
k
k
kkkk
xLn
x
WWxf
1
2)(
2
1exp
2),,;(
Recent Findings: TxDOT 0-4510Recent Findings: TxDOT 0-4510
Recent Findings: TxDOT 0-4510Recent Findings: TxDOT 0-4510
WhereWhere MM : total number of axles: total number of axles f(x)f(x) : relative frequency of axle load i-th,: relative frequency of axle load i-th, LSFLSF : load spectra factor, which represents the : load spectra factor, which represents the
number of ESALs of a “representative” axle.number of ESALs of a “representative” axle.
k
i
ii M
nLEF
M
ESAL
1
mi
k
i
mim
s
k
ii
m
s
i LSFxfxL
xfL
x
)(1
)(11
Advantages of the log-normal random variable X:Advantages of the log-normal random variable X: The logarithm of X (axle load) is normal.The logarithm of X (axle load) is normal. X is non-negative, as axle loads are.X is non-negative, as axle loads are. The parameters have physical meaning.The parameters have physical meaning. It has a close-form expression for the LSF, which It has a close-form expression for the LSF, which
captures the damaging effect of loads on captures the damaging effect of loads on pavements:pavements:
22
222
21
211 expexp mmWmmWLSF
Recent Findings: TxDOT 0-4510Recent Findings: TxDOT 0-4510
Data AnalysisData Analysis
TxDOT operates approximately 15-20 TxDOT operates approximately 15-20 permanent WIM stations, mostly on rural permanent WIM stations, mostly on rural interstate and US highwaysinterstate and US highways
Selection criteria:Selection criteria:• enough time series data, andenough time series data, and• close to US-Mexico borderclose to US-Mexico border
D522, near McAllen (Hidalgo), on US 281D522, near McAllen (Hidalgo), on US 281 D516, near San Antonio, on IH 35 D516, near San Antonio, on IH 35
Data AnalysisData Analysis
Class 9 (3S2) and Class 5 were analyzed in Class 9 (3S2) and Class 5 were analyzed in detail; they account for +80% of the volumedetail; they account for +80% of the volume
Analysis was doneAnalysis was done• per direction (north- and southbound)per direction (north- and southbound)• per axle type (steering, single, and tandem)per axle type (steering, single, and tandem)• per year (to establish and document trends)per year (to establish and document trends)
We calculated pavement damage by applying We calculated pavement damage by applying the LSF concept explained earlierthe LSF concept explained earlier
Preliminary ResultsPreliminary Results
D522 – US 281, McAllenD522 – US 281, McAllen
Class 9, Class 9, steeringsteering axle load distribution for axle load distribution for northboundnorthbound and and southboundsouthbound lanes lanes
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5 10 15 20 25 30
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Year98
Year99
Year00
Year01
Year02
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5 10 15 20 25 30
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Year98
Year99
Year00
Year01
Year02
D522 – US 281, McAllenD522 – US 281, McAllen
Class 9, Class 9, tandemtandem axle load distribution for axle load distribution for northboundnorthbound and and southboundsouthbound lanes lanes
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 10 20 30 40 50
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Year98
Year99
Year00
Year01
Year02
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 10 20 30 40 50
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Year98
Year99
Year00
Year01
Year02
D522 – US 281, McAllenD522 – US 281, McAllen
Class 9, Class 9, tandemtandem axle load distribution for axle load distribution for northbound and southbound lanes in northbound and southbound lanes in 20022002
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 10 20 30 40 50 60
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Northbound
Southbound
U.S. Legal Limit (34kip)
Mexico Legal Limit (40 kip)
D522 – US 281, McAllenD522 – US 281, McAllen
Class 5, Class 5, steeringsteering and and singlesingle axle load axle load distribution for distribution for southboundsouthbound direction direction
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25 30
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Year98
Year99
Year00
Year01
Year02
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 5 10 15 20 25 30
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Year98
Year99
Year00
Year01
Year02
Preliminary Findings (D522)Preliminary Findings (D522)
Class 9Class 9• bi-modal for tandem, uni-modal for steeringbi-modal for tandem, uni-modal for steering• small but steady rightward shiftsmall but steady rightward shift• northbound traffic is heavier than southboundnorthbound traffic is heavier than southbound• northbound is Type III; southbound, Type IInorthbound is Type III; southbound, Type II
Class 5Class 5• steering and single show uni-modal spectrasteering and single show uni-modal spectra• not significant directional distribution, nor load not significant directional distribution, nor load
increase (rightward shift)increase (rightward shift)
Preliminary Findings (D522)Preliminary Findings (D522)
Class 9, LSF estimation for Class 9, LSF estimation for steeringsteering and and tandemtandem axles axles in northbound and southbound directions.in northbound and southbound directions.
Note that the “number of ESALs per vehicle” is Note that the “number of ESALs per vehicle” is increasing.increasing.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
1997 1998 1999 2000 2001 2002 2003
Year
LS
F Northbound
Southbound
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1997 1998 1999 2000 2001 2002 2003
Year
LS
F Northbound
Southbound
D516 – IH 35, San AntonioD516 – IH 35, San Antonio
Class 9, Class 9, steeringsteering axle load distribution for axle load distribution for northbound directionnorthbound direction
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 5 10 15 20 25 30
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Y94
Y95
Y96
Y97
Y98
Y99
Y00
Y01
Y02
D516 – IH 35, San AntonioD516 – IH 35, San Antonio
Class 9, Class 9, tandemtandem axle load distribution for axle load distribution for northbound directionnorthbound direction
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0 10 20 30 40 50
Axle Load (kip)
No
rmal
ized
Fre
qu
ency
Y94
Y95
Y96
Y97
Y98
Y99
Y00
Y01
Y02
Preliminary Findings (D516)Preliminary Findings (D516)
Class 9Class 9• bi-modal distribution for tandem axlesbi-modal distribution for tandem axles• uni-modal for steering axlesuni-modal for steering axles• consistent rightward shiftconsistent rightward shift• spectra is changing from Type II to Type IIIspectra is changing from Type II to Type III• no significant difference per directionno significant difference per direction
Class 5Class 5• similar findings as for D522similar findings as for D522
Preliminary ConclusionsPreliminary Conclusions
In addition the volume increase, the LSF, ESAL per In addition the volume increase, the LSF, ESAL per vehicle, should be closely monitored.vehicle, should be closely monitored.
The LSF captures the load-associated pavement The LSF captures the load-associated pavement damage damage “contained”“contained” in the load distribution. in the load distribution.
We captured the change in the mid-1990s due to We captured the change in the mid-1990s due to NAFTA, therefore, we should capture any future NAFTA, therefore, we should capture any future shift, even is it is smallshift, even is it is small
Small changes, however, may cause significant Small changes, however, may cause significant increase in pavement damage and, therefore, on increase in pavement damage and, therefore, on TxDOT maintenance and rehabilitation activities. TxDOT maintenance and rehabilitation activities.
What is next?What is next?
We will incorporate at least two more WIM We will incorporate at least two more WIM stations from the border region or I-10.stations from the border region or I-10.
We are trying to obtain similar data from We are trying to obtain similar data from Mexican bordering states to carry out a similar Mexican bordering states to carry out a similar analysis.analysis.
We want to evaluate the potential increase in We want to evaluate the potential increase in the proportion of six-axle trucks (3S3). the proportion of six-axle trucks (3S3).
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