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Snowy / Icy Road Conditionsand Crashes:What IS the Relationship?

Marc Briese, P.E., PTOEOffice of Traffic, Security, and Operations (OTSO)

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Brief History of Topic

“There are more crashes in the winter, and specifically during snow events, but fewer Fatal and

Life Changing crashes during snow events.”

Historically, popular belief dictated the following Hypothesis:

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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

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Instead, analysis concentrates on:

WHAT IS EFFECT OF SNOWY/ICY ROADS ON TRAFFIC SAFETY?

Brief History of Topic, cont.

- Results may shed light on potential impacts of any change in snow and ice removal efforts.

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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

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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).

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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

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Introduction Cont.Crash Severity Defined*

* Note: crash severity is determined by most severe injury

Fatal (F) – one or more people killed

‘A’ Severity – incapacitating injury

- E.g. severe laceration, broken/distorted limbs

‘B’ Severity – non-incapacitating injury

- E.g. minor laceration, bruises, bump on head

‘C’ Severity – possible injury - E.g. momentary unconsciousness, claimed injury

PD – property damage only

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Safety Analysis, Part I2000-2002

There ARE more crashes, of all severity, in winter months .

All Severity TH Crashes Per MonthBy District and Season

0

500

1000

1500

2000

Metro 1 2 3 4 6 7 8District

Summer (Apr-Oct)

Winter (Nov-Mar)

Year Round

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Safety Analysis, Part I cont.2000-2002

There ARE fewer Fatal and ‘A’ Severity crashes in a typical winter month.

Fatal + 'A' TH Crashes Per MonthBy District and Season

0

5

10

15

20

25

30

Metro 1 2 3 4 6 7 8District

Summer (Apr-Oct)

Winter (Nov-Mar)

Year Round

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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%.

Statewide Year RoundFatal and 'A' Crashes TH 2000-2002

NOT Snow /Ice Road

Conditions87%

Snow /Ice Road

Conditions13%

Statewide Summer (Apr-Oct)Fatal and 'A' TH Crashes 2000-2002

NOT Snow /Ice

Road Conditions

98%

Snow /Ice Road

Conditions2%

Statewide Winter (Nov-Mar)Fatal and 'A' TH Crashes 2000-2002

NOT Snow /Ice

Road Conditions

71%

Snow /Ice Road

Conditions29%

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Safety Analysis, Part II cont.2000-2002

Outstate Graphics

Outstate District Year RoundFatal and 'A' TH Crashes 2000-2002

NOT Snow /Ice Road

Conditions85%

Snow /Ice Road

Conditions15%

Outstate District Summer (Apr-Oct)Fatal and 'A' TH Crashes 2000-2002

Snow /Ice Road

Conditions3%

NOT Snow /Ice

Road Conditions

97%

Outstate District Winter (Nov-Mar)Fatal and 'A' TH Crashes 2000-2002

Snow /Ice Road

Conditions34%

NOT Snow /Ice

Road Conditions

66%

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Safety Analysis, Part II cont.2000-2002

Metro Graphics

Metro Year RoundFatal and 'A' TH Crashes 2000-2002

NOT Snow /Ice Road

Conditions92%

Snow /Ice Road

Conditions8%

Metro Summer (Apr-Oct)Fatal and 'A' TH Crashes 2000-2002

Snow /Ice Road

Conditions1%

NOT Snow /Ice

Road Conditions

99%

Metro Winter (Nov-Mar)Fatal and 'A' TH Crashes 2000-2002

Snow /Ice Road

Conditions18%

NOT Snow /Ice

Road Conditions

82%

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Safety Analysis, Part II cont.2000-2002SO 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).

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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)

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Safety Analysis, Part III cont.1996-1999

As Snowfall Increases, Number of Fatal and A Severity Crashes increases (Metro Crashes)

1996-1999Fatal + A Crashes by Snowfall

Metro

20

25

30

35

40

45

50

0-0.25" 0.25-0.5" 0.5-0.75" 0.75-1.0" 1-1.5" 1.5-2.0" 2.0-2.5" 2.5-5.0" >5.0"

Daily Snowfall Amount (inches)

F &

A C

rash

es

By SnowfallAnnual AverageSummer AverageWinter Average

33 12 17 10 26 17 9 17# Days-->

Note: Crash data Normalized for # per 30 Days)

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Logarithmic Fit

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Safety Analysis, Part III cont.1996-1999

In comparison, Rainfall appears to affect crashes less than snow. (Metro Crashes)

1996-1999Fatal + A Crashes by Summer Rain

Metro

20

25

30

35

40

45

50

0-0.1" 0.1-0.2" 0.2-0.3" 0.3-0.4" 0.4-0.5" 0.5-0.75" 0.75-1.0" >1.0"

Daily Rainfall Amount (inches)

F &

A C

ras

he

s

By Rainfall

Annual Average

Summer Average

Winter Average

130 46 28 25 17 23 14 20# Days-->

Note: Crash data Normalized for # per 30 Days)

Logarithmic Fit

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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

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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

F and 'A' TH Crashes in Snow and Ice Road ConditionsOUTSTATE 1998-2002

Icy Rd, Weather OK

40%

Snow y Rd, Weather OK

10%

Snow /Ice Rd, Weather Bad

50%

Sample of F&A Crashes = 430

Preventative Reactive

??

F and 'A' TH Crashes in Snow and Ice Road ConditionsMETRO 1998-2002

Icy Rd, Weather OK

25%

Snow y Rd, Weather OK

14%

Snow /Ice Rd, Weather Bad

61%

Sample of F&A Crashes = 138

Preventative

Reactive??

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Conclusions

I. More crashes (of all severity) in winter months.Fewer F and ‘A’ crashes in winter months. Both consistent with popular belief.

II. With snowy/icy road conditions, there are higher number of F and ‘A’ crashes than expected – over TWICE the number expected.

III. 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.

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IV. 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

Conclusions cont.

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“Snowy/icy road conditions appear to be associated with a higher than expected

frequency of ALL crash severities, including Fatal and Life Changing crashes.”

Conclusions cont.

As a Result of these analyses, old hypothesis appear to be incorrect. Analyses suggest: