Shashi Nambisan Institute for Transportation Iowa State University Murat Ozen

1. An Exploratory Analysis of Large Truck Crashes in the US Shashi Nambisan Institute for Transportation Iowa State University Murat Ozen Middle East Technical University Department of Civil Engineering Mid-Continent Transportation Research Symposium Ames, IA August 15, 2013

2. 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

3. 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

4. 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

5. National Level Trends • % registered trucks in national vehicle fleet increasedfrom 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 MIDDLE EAST TECHNICAL UNIVERSITY

6. Check Data for Normality • Shapiro-Wilknormality test for normality assumption • Ho: The data are normally distributed • H1: The data are not normally distributed Shapiro-Wilk Normality Tests Results For National Level Fatal Truck Crash Data • Need to use non-parametric statistical methods

7. 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

8. Urban vs. Rural • Are the Urban & Rural fatal truck crash rates the same before and after 2007? • Before 2007 (i.e. Averagerural= 1.63 > Averageurban= 0.84) • 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 Reject WR-S test Ho if ts < L or ts > U Reject M-W U test Ho if absolute value of z > zC • Rural fatal truck crash rates are higher than urban fatal truck crash rates

9. 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 interstatesand other arterials and other arterials and other highwaysstatistically significant?

10. 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 Reject K-W H test Ho if H > Hcritical

11. 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

12. 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

13. 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 Reject K-W H test Ho if H > Hcritical

14. Regional Ranks: Before 2007

15. Regional Ranks: After 2007

16. State Level Analyses • Comparison of state level fatal truck crash rates across the statesbefore 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 Reject K-W H test Ho if H > Hcritical

17. State Level Ranks: Before 2007

18. State Level Ranks: After 2007

19. 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

20. 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

21. Contact Information Shashi Nambisan, Ph.D., P.E. Professor of Civil Engineering Institute for Transportation Iowa State University Ames, IA 50011 shashi@iastate.edu Murat Ozen, Ph.D. Candidate Middle East Technical University Department of Civil Engineering 06800, Ankara - TURKEY mozen@metu.edu.tr