1 Investigating Crash Interaction of Younger and Older Drivers Iowa State University Hossein Naraghi Masters Thesis Defense October 15, 2004

2 Overview  The problem  Demographics  Risk assessment  Statewide interaction analysis  Causal factors  Spatial analysis  Geometric analysis  Temporal analysis  Conclusions, limitations, & recommendations

3 The Problem

4 Younger drivers (Y)  Young Drivers  Inexperience  Poor judgment  Risk taking behavior

5 Older drivers (O)  Older Drivers  Reduction in physical abilities  Reduction in cognitive capabilities  “Too” careful?

6 Compound effect? What will occur as a result of interaction of these two critical age groups?

7 Licensed Drivers by Age Group Iowa has 9% more Young Iowa has 20% more Older Iowa has 50% more Young Drivers Iowa has 18% more Older Drivers

8 Iowa older driver trends   Total population is relatively flat   Older driver population increase caused by …   More older population   Higher % licensed

9 Iowa older driver trends   In 1970 only 60% of older population was licensed (240,000)   In % was licensed (350,000, or 17% of all drivers)   In 2025 if 80% are licensed (500,000, or 24% of all drivers)   In 2025 if 87% are licensed (550,000, or 26.5% of all drivers)

10   Decrease (%) from 1970’s to early 1990’s  increase in younger population (compared to  10.7% increase in younger population from (compared to 4.8 % increase in total population)   In 2000, 157,000 younger drivers were licensed (7.4% of all drivers) Iowa younger driver trends

11 Assuming no increase in percentage of younger drivers … in 2025, as much as 35% of Iowa drivers will be older & younger drivers (compared to ~25% in 2000) In some parts of the state, this is already occurring … Iowa Y-O driver trends

12 53 counties have 20% or higher Over 20 percent Older drivers (2000)

13 23 Counties have 10% or higher Over 10 percent younger drivers (2000)

14 20 Counties Have Older & Younger Drivers > 1/3 of Driving Population Over 1/3 younger and older drivers (2000)

15 Risk Assessment

16 How much do these cohorts drive?

17 Ref # 1: 2001 National Household Travel Survey Ref # 2: Iowa Crash Facts 2000 Ref # 3: Iowa DOT crash database Iowa Y & O crash overrepresentation Interpretation: young drivers are 3.9 times as likely to be involved in a 2-veh crash as compared to the “average” driver E = H =73963

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19 The unadjusted probability that a driver involved in a 2-vehicle crash will be from a particular age group is the fraction of VMT driven by that age group … Statewide Interaction of Younger and Older Drivers

20 )*()* ( Assuming independence (e.g., the fact that an older driver is involved in a crash does not affect the likelihood of the other driver coming from a particular age group), the expected number of 2-veh crashes between age groups is … e.g., E(Y-O) = * * 34,264 = 150 What is expected?

21 Crash Outcome Expected 2-Veh Crashes TotalObserved Younger & Younger (16-19) & (16-19) E 1 (Y-Y) ,485 Younger & Middle age (16-19) & (20-64) E 1 (Y-M) + E 1 (M-Y) 1, ,385 2,7708,234 Younger & Older (16-19) & (65+) E 1 (Y-O) + E 1 (O-Y) ,084 Middle age & Middle age (20-64) & (20-64) E 1 (M-M) 25,340 25,34017,722 Middle age & Older (20-64) & (65+) E 1 (M-O) + E 1 (O- M) 2, ,741 5,4825,207 Older & Older (65+) 7 (65+) E 1 (O-O) TOTAL 34,264 Expected 2-veh crashes, Unadjusted Assuming independence, the joint probabilities are …

22 Based only on VMT YO crashes are clearly overrepresented… however, how much of this is due to the fact that older and younger drivers as groups are each overrepresented??? Statewide Interaction of Younger and Older Drivers

23 The adjusted probability that a driver involved in a 2-vehicle crash will be from a particular age group is … Statewide Interaction of Younger and Older Drivers

24 e.g., E(Y-O) = (0.18)*(0.11)*(34,262) = 659 )*()* ( Again, assuming independence … The expected number of 2-veh crashes between age groups is now … What is expected?

25 Expected 2-Veh crashes, adjusted for age

26 Statewide, we observe that Y-O crashes are underrepresented

27 Chi-square is used to make sure differences observed are significant 41,566 is much larger than the critical value of about 16 for 3 degrees of freedom and 99.9% significance level Unadjusted Analysis

28 Adjusted Analysis 258 is also much larger than the critical value of about 16 for 3 degrees of freedom and 0.001probability of exceeding the critical value

29 Causal Factors

30 Major Cause of 2-vehicle crashes

31 Driver contributing factors

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33  One Passenger Younger16% More than Older 27% More than All 2-veh  2 or More Passengers Younger 225% More than Older 30% more than all 2-veh

34 Spatial Analysis

35 County by County Analysis

36 Statewide YO under-representation is 18% Y-O representation (adjusted) Sample size >= 5

37 Proximity to High Schools at 1.5 mile distance from high schools: About 72% of all Y-O crashes occur Y-O crashes have the highest percentage of all 2-veh crashes Y-O crashes are 13% higher than all 2-veh crashes

38 Urban/Rural Analysis

39 Highway Geometry

40 Y-O crashes most likely at intersections Y-O crashes are about: 20% higher than Y-Y crashes 8% higher than O-O crashes 17% higher than M-M crashes

41 Intersection functional class Comparing Y-O to all 2-veh crashes

42 Are divided expressways a problem? e.g. 6.83% of all Y-O Crashes occur at intersections of divided roads

43 Temporal Analysis

44 3:00-4:00 PM is critical

45 When Do Older and Younger Drivers Crash?

46 Conclusions  Y-O under-represented at statewide level, … why?  Driving in different places and at different times?  Y-O significantly over-represented in some counties  High risk characteristics of Y & O drivers may be most problematic at intersections  After school hours are the most risky time of driving for both age groups

47 Conclusions (cont.)  Presence of passenger (esp. 2 or more) increases crash risk of younger drivers, but may have a beneficial effect on older drivers  Older drivers experiencing more difficulty at divided expressway intersections than their younger counterparts - Come across younger drivers is the most difficult

48 Limitations  Lack of local exposure (VMT) data  By age group in Iowa (used US averages)  No Iowa, county, or spatial + time of day exposure data  No knowledge of proportion of drivers in each age group that carry passenger(s)

49 Recommendations  Limit passengers for younger drivers  Educate older drivers about the benefit of a passenger  Educate older drivers about the risk associated with driving after school hours (they already know to avoid driving in congestion)  Collect more data (or at least a sample) on exposure, time of day, more stratified data (esp. for Y & O)

50 Future Research  Investigating if particular types of crashes occur more often in Y-O crashes e.g. rear-end, head-on, left turn, and sideswipe e.g. rear-end, head-on, left turn, and sideswipe  Investigating the overrepresentation in Y-O crashes in the vicinity of high schools by time of day and distance from high schools  The over-involvement in Y-O crashes by gender  Investigating safety implications of Y-O crashes by roadway functional class e.g., expressways, county roads, and city streets e.g., expressways, county roads, and city streets  Comparing Y-O overrepresentation in rural and urban areas

51 Thanks to   Dr. Reginald Souleyrette   Dr. Edward Kannel   Dr. William Q. Meeker, Jr.   Mr. Zachary Hans   Mr. Robert Thompson   The staff and students of CTRE

52 Questions?