MIDDLE EAST TECHNICAL UNIVERSITY 1 Shashi Nambisan Institute for Transportation Iowa State University Murat Ozen Middle East Technical University Department

MIDDLE EAST TECHNICAL UNIVERSITY

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Shashi NambisanInstitute for Transportation

Iowa State University

Murat OzenMiddle East Technical University

Department of Civil Engineering

Mid-Continent Transportation Research Symposium

Ames, IA

August 15, 2013

An Exploratory Analysis of Large Truck Crashes in the US


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Introduction• Truck operations: Key for trade & commerce

– Disproportionate share of fatal crashes

• Safety analysis: Macro-level; Fatal crash rates– Outcomes: Number and type of crashes– Exposure measures: #Trucks, Travel, Type & Extent of Roads

• Analyses of truck safety records: 2000 to 2010• Definitions of trucks before and after 2007 (US DOT)• Fatal truck crash analysis

– Spatial considerations– Temporal considerations


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Methodology• Large truck fatal crash

rates per 100 million VKT • Annual rates

– At national level– At regional level for the US DOT’s

10 federal regions

• Descriptive analyses • Statistical tests


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Data Sources• US DOT

– Federal Highway Administration (FHWA)– Federal Motor Carrier Safety Administration (FMCSA)

• Large trucks = Gross vehicle weight rating or gross combination weight rating of more than 4.5 tons

• National level fatal crash and truck Vehicle Kilometer of Travel (VKT) data available for the study period

• State level truck VKT data available only 2009, 2010– Average % of national VKT for each state in 2009, 2010– Apply to national VKT for the years 2000 to 2008

National Level Trends• % registered trucks in

national vehicle fleet increased from 3.5% in 2006 to 4.2% in 2007

• Truck VKT increased 36.7% in 2007 as compared to 2006

• Truck fatal crash rate per 100 million vehicle kilometers traveled by large trucks decreased from 1.21 to 0.86

Year FatalCrashes

Million TruckVKT

Fatal Crash Rate per 100 Million VKT

2000 4,573 330,752 1.39

2001 4,451 336,236 1.32

2002 4,224 345,369 1.22

2003 4,335 350,637 1.24

2004 4,478 355,360 1.26

2005 4,551 358,115 1.27

2006 4,350 358,099 1.21

2007 4,204 489,526 0.86

2008 3,754 499,990 0.75

2009 2,987 463,498 0.65

2010 3,261 461,213 0.71

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Check Data for Normality• Shapiro-Wilk normality test for normality assumption

– Ho: The data are normally distributed– H1: The data are not normally distributed

Period Data Test Statistic df Significance Reject Ho (95%)?

Before 2007

National .877 7 .214 YESUrban .941 7 .649 YESRural .983 7 .974 YES

After 2007

National .981 4 .908 YESUrban .982 4 .911 YESRural .963 4 .796 YES

Shapiro-Wilk Normality Tests Results For National Level Fatal Truck Crash Data

• Need to use non-parametric statistical methods


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Non-Parametric Tests• Wilcoxon Rank-Sum test and Mann-Whitney U test

– to test the null hypothesis that two populations are the same

• Kruskall-Wallis H test– to test the null hypothesis that more than two populations

are the same


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Urban vs. Rural• Are the Urban & Rural fatal truck crash rates the same before and after 2007?

o Before 2007 (i.e. Averagerural = 1.63 > Averageurban = 0.84)o After 2007 (i.e. Averagerural = 0.94 > Averageurban = 0.52)

• Ho: Urban and rural fatal truck crash rates are the same• H1: Urban and rural fatal truck crash rates are NOT the same

Hypothesis Test Type Test Statistics (ts)/ Z-Value

CriticalValue

Reject Ho (95%)

BEFORE 2007W R-S (ts) 28 L=36 YES

M-W U (Z) -3.137 -1.96 YES

AFTER 2007W R-S (ts) 10 L=11 YES

M-W U (Z) -2.309 -1.96 YES

Reject WR-S test Ho if ts < L or ts > UReject M-W U test Ho if absolute value of z > zC

• Rural fatal truck crash rates are higher than urban fatal truck crash rates


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Roadway Types• Compare fatal truck crash

rates for interstate highways, other arterials and other highways before and after 2007

• Interstate highways have lower fatal crash rates than other types of roadways.

• Are the differences in fatal crash rates on interstates and other arterials and other arterials and other highways statistically significant?


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Roadway Type Comparisons• Ho: Fatal truck crash rates are the same for interstate highways,

other arterials and other highways • H1: Fatal truck crash rates are NOT the same for interstate

highways, other arterials and other highways

Hypothesis Test Type Test Statistics (ts)

CriticalValue

Reject Ho (95%)

BEFORE 2007 K-W H (ts) 17.83 5.99 YES

AFTER 2007 K-W H (ts) 8.35 5.99 YES

Reject K-W H test Ho if H > Hcritical


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Regional Trends: Before 2007

• Fatal crash rate in Region 4 is the highest

• Fatal crash rate in Region 10 is the is lowest

• Fatal crash rates in Region 3, Region 8 and Region 10 seem to be lower than the others

Regions(b)

Regions(a)

Fata

l Tru

ck C

rash

Ra

te

Fata

l Tru

ck C

rash

Ra

te


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Regional Trends: After 2007

• Fatal crash rate in Region 4 is the highest

• Fatal crash rate in Region 10 is the is lowest

• Variations in the fatal truck crash rates across the regions are not pronounced

Regions(b)

Regions(a)

Fata

l Tru

ck C

rash

Ra

te

Fata

l Tru

ck C

rash

Ra

te


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Regional Comparisons: Pairwise• Fatal truck crash rates are not the same for different regions before and after

2007 based on pairwise comparisons.• Ho: Fatal truck crash rates are the same for each region • H1: Fatal truck crash rates are NOT the same for each region

Hypothesis Test Type Test Statistics (ts) Critical

ValueReject Ho

(95%)

BEFORE 2007 K-W H (ts) 55.93 16.92 YES

AFTER 2007 K-W H (ts) 24.73 16.92 YES



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Regional Ranks: Before 2007


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Regional Ranks: After 2007


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State Level Analyses• Comparison of state level fatal truck crash rates across the

states before and after 2007• Ho: Fatal truck crash rates are the same across the states• H1: Fatal truck crash rates are NOT the same across the states

Hypothesis Test Type Test Statistics (ts) Critical

ValueReject Ho

(95%)

BEFORE 2007 K-W H (ts)

240.84 66.34 YES

AFTER 2007 K-W H (ts)

126.36 66.34 YES



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State Level Ranks: Before 2007


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State Level Ranks: After 2007


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Summary• Illustrated methods to analyze truck safety

– Non-parametric techniques

• Variations in fatal truck crash rates – Before and after 2007– Urban vs rural– Across roadway types– Across federal regions– Across states– Northwest US vs Southeast US

• Limitations– Focused only on fatal truck crash rates– Need to evaluate other factors / metrics


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Implications• Spatial and temporal variations in truck safety records warrant

further study• Changes in truck definitions affected estimated crash rates

– Use caution when comparing Pre-2007 and Post -2007 data

• Method can be adapted for safety analyses in other contexts• Support policy making

– Planning– Design– Operations– Law enforcement strategies– Resource allocation– Others


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Contact Information

Shashi Nambisan, Ph.D., P.E.

Professor of Civil Engineering

Institute for Transportation

Iowa State University

Ames, IA 50011 [email protected]

Murat Ozen, Ph.D. Candidate

Middle East Technical University

Department of Civil Engineering 06800, Ankara - TURKEY

[email protected]

mailto:[email protected]

mailto:[email protected]

Documents

MIDDLE EAST TECHNICAL UNIVERSITY 1 Shashi Nambisan Institute for Transportation Iowa State University Murat Ozen Middle East Technical University Department