Snowy / Icy Road Conditions and Crashes: What IS the Relationship?

1. Snowy / Icy Road Conditionsand Crashes:What IS the Relationship? Marc Briese, P.E., PTOE Office of Traffic, Security, and Operations (OTSO)

2. Brief History of Topic Historically, popular belief dictated the following Hypothesis: “There are more crashes in the winter, and specifically during snow events, but fewer Fatal and Life Changing crashes during snow events.”

3. Brief History of Topic, cont. • 2003-2004 Winter, Mn/DOT changed to new innovative plowing policy  cost savings. • Shoulders and Medians cleared ‘day after’ • Target for achieving bare pavement NOT changed (OT ok if needed) • As a result of change, Media and others ask: “What will be the effect of this change on traffic safety?” • Unfortunately, actual before/after analysis not possible lack of data

4. Brief History of Topic, cont. • Instead, analysis concentrates on: WHAT IS EFFECT OF SNOWY/ICY ROADS ON TRAFFIC SAFETY? - Results may shed light on potential impacts of any change in snow and ice removal efforts.

5. Inherent Limitations • Exposure Tough to Measure (be conservative) • Vehicle Miles Traveled, fewer discretionary trips • Actual amount of time where roads are snow or ice covered • Could have snowed 5” at MSP Airport, and 0” in 2/3 of metro area, or vice versa • Winter to Winter Comparisons are Tough • Snowfall varies greatly from winter to winter (1990’s varied from 34” to 89”) • Bridge/road icing happens under very specific, but not easily identifiable, conditions • Vehicle fleet continuously changing

6. Introduction • Current thinking from earlier: - “There are more crashes in the winter, and specifically during snow events, but fewer Fatal and Life Changing crashes during snow events, due to reduced speed.” • Intent of Analysis is to prove or disprove this hypothesis. • Analyses for Trunk Highways ONLY (ISTH, USTH, MNTH).

7. Introduction cont. • Part I: District Comparison - winter vs summer months • Part II: Statewide Comparison - winter vs summer and road conditions • Part III: Metro Division Days with Snow - ~150 days (1996-1999) with snowfall >= 0.1” • Part IV: Statewide Comparison - road conditions + weather conditions

8. Introduction Cont. Crash Severity Defined* * Note: crash severity is determined by most severe injury

9. Safety Analysis, Part I2000-2002 • There ARE more crashes, of all severity, in winter months .

10. Safety Analysis, Part I cont.2000-2002 • There ARE fewer Fatal and ‘A’ Severity crashes in a typical winter month.

11. Safety Analysis, Part II2000-2002 • Blanket Assumption: 20 full days of snow/ice covered conditions (13% of winter months) • Overall, Snow/Ice Road condition F and ‘A’ crashes represent a small % of all F and ‘A’ crashes, 13%. • But in winter months, they make up ~30%.

12. Safety Analysis, Part II cont.2000-2002 • Outstate Graphics

13. Safety Analysis, Part II cont.2000-2002 • Metro Graphics

14. Safety Analysis, Part II cont.2000-2002 SO WHAT DO THE PAST THREE SLIDES TELL US? • Crashes on Snowy/Icy Roads = small % of F and ‘A’ crashes overall, but large % during winter months. • From Earlier, roads are snow/ice covered full 20 days per winter = 13% of the time during winter months. • 30% of the F and ‘A’ Crashes occur during snowy/icy road conditions LARGE over-representation – 2x more than expected (conservatively).

15. Safety Analysis, Part III1996-1999 • Climatology data take from National Weather Service station at Minneapolis – St. Paul International Airport • Considered days with Snowfall >= 0.1” • BUT Again – inherent problems: • Could have snowed 5” at MSP Airport, and 0” in 2/3 of Metro area, or vice versa • Sample size of ‘snowy days’ is limited (148 with >= 0.1” over 4 years)

16. Safety Analysis, Part III cont.1996-1999 • As Snowfall Increases, Number of Fatal and A Severity Crashes increases (Metro Crashes)

17. Safety Analysis, Part III cont.1996-1999 • In comparison, Rainfall appears to affect crashes less than snow. (Metro Crashes)

18. Safety Analysis, Part IV1998-2002 • Road and Weather Conditions are Reported on the Minnesota Police Crash Report. • Road conditions either snowy or icy, but weather condition NOT listed as snow or freezing rain/sleet/hail. Primarily when: • After snow event when it’s no longer snowing, but road not restored to bare pavement yet. • Bridge decks icy at dawn or black ice. • Drifting or Blowing Snow

19. Safety Analysis, Part IV cont.1998-2002 • From earlier, 2x Over-representation of F & A crashes during poor road conditions • Opportunity to prevent 43 outstate, 19 Metro F & A crashes per year

20. Conclusions • More crashes (of all severity) in winter months. Fewer F and ‘A’ crashes in winter months.  Both consistent with popular belief. • With snowy/icy road conditions, there are higher number of F and ‘A’ crashes than expected – over TWICE the number expected. • By analyzing Snow Events in the Metro, it was found that there is a positive relationship between snowfall amount and the number of Fatal + A Severity crashes.

21. Conclusions cont. • There is potential to prevent F and ‘A’ Crashes during snowy/icy road conditions. Increased efforts statewide could prevent up to 62 F & A crashes per year. Further study necessary to determine appropriate strategies for each district. Possible approaches include: • Return THs to bare pavement more quickly • Be more aggressive in anti-icing • Improved Education to public regarding relative dangers of traveling during snowy/icy weather • Increased Enforcement Efforts • Increased awareness for EMS

22. Conclusions cont. As a Result of these analyses, old hypothesis appear to be incorrect. Analyses suggest: “Snowy/icy road conditions appear to be associated with a higher than expected frequency of ALL crash severities, including Fatal and Life Changing crashes.”