1. Examining the Role Weather Conditions Play in the Patterns and Outcomes of Motor Vehicle Crashes in New York State,
Motao Zhu, Michael Bauer, & Susan Hardman
New York State Dept. of Health
Bureau of Injury Prevention

2. Background
Motor vehicle crashes are a leading cause of death and injury in New York State
Weather plays an important role in motor vehicle crashes
Traffic related injuries have been and continue to be the leading cause of unintentional injury deaths among NYS residents between 5 and 65 years old, and over eleven thousand residents are hospitalized each year for a motor vehicle-related injury.
Weather directly influences driver’s visibility and road surface condition, which are important factors in motor vehicle crashes.

3. Study Objective
To evaluate the crash pattern and severity by different weather conditions
To assess the medical injury outcome, length of hospital stay, and hospital charges from motor vehicle crashes under different weather conditions

4. Methods
CODES 2000 was used to apply probabilistic technique to link the following databases:
Police Crash Report
Hospital Discharge Data
New York State Data
Since Police Crash Report (PCR) and Hospital Discharge Data (HDD) don’t have a common ID such as SSN for linkage, an MS-Access-based software named CODES 2000 was applied using probabilistic technique.
Match fields include event date, hour, location, occupant age, sex, birthday, first name, last name, and so on. The more match fields, the higher probability there is a true match of a PCR record with a HDD record.
The linked data provide detailed medical and financial outcomes of motor vehicle crashes.

5. Results
Motor Vehicle Crash Frequency and Rate by Weather, New York State,
Most crashes occurred in clear or cloudy weather.
The crash rate was higher in rain, snow or sleet.
In order to calculate weather days, all crashes were summarized by crash date. Suppose that 800 crashes occurred on January 1st, 1998, then each crash was assigned a 1/800 weather day. For example, if a crash occurred in clear condition, it was counted as a 1/800 clear day. Then all these weather days were summarized to get the total weather days for clear, cloudy, rain, snow, sleet, and fog conditions.

6. No. of Vehicles Involved by Weather
Under clear, cloudy or rainy conditions, two-car crashes accounted for more than 60% of crashes.
In contrast, in snow, sleet or fog, one-car only crashes accounted for nearly 50% of crashes.
Rain was associated with the highest percentage of multi-car crashes (10.5%).

7. Multi-car Crash Frequency and Rate by Weather, New York State, 1995-2000
It is defined as a multi-car crash if the crash involved 3 or more cars.
Rain was associated with the highest multi-car crash rate (89 per day).
Snow or sleet had a lower multi-car crash rate, compared with clear condition.

8. Distribution of Crash Class by Weather, New York State, 1995-2000
Snow was related to the highest percentage of property damage only crashes.
Fog was associated with the highest percentage of fatal crashes.

9. Top 5 First Collision Objects by Weather
Clear
Other motor vehicles
(73.0%)
Pedestrian
(8.0%)
Bicyclist
(4.0%)
Animal
(3.6%)
Earth Embankment
(1.5%)
Cloudy
(70.0%)
(6.5%)
(5.0%)
(2.7%)
(2.4%)
Rain
(74.9%)
(6.0%)
Guide Rail
(2.6%)
(2.2%)
Light Support
Snow
(54.6%)
(9.0%)
Tree
Sleet
(46.8%)
(12.2%)
(9.3%)
(6.3%)
(5.8%)
Fog
(48.3%)
(12.9%)
(7.8%)
(5.7%)
(4.6%)
Under snow, sleet or fog conditions, there were less collisions with other motor vehicles but more collisions with a guide rail, earth embankment, tree, or light support, compared with collisions under clear, cloudy or rainy conditions.

10. Top 5 Contributing Factors by Weather
No. 1
No. 2
No. 3
No. 4
No. 5
Clear
Driver inattention (16.7%)
Failure to yield right-of-way (14.6%)
Other human
(14.2%)
Follow too closely
(11.3%)
Unsafe speed
(4.4%)
Cloudy
Failure to yield right-of way
(15.6%)
Driver inattention
(10.8%)
(10.4%)
Animal’s action
(7.4%)
Rain
Pavement slippery
(16.6%)
Failure to yield right-of-way (12.7%)
(11.9%)
Driver inattention (11.8%)
(11.1%)
Snow
(35.3%)
(23.7%)
Failure to yield right-of-way
(6.4%)
(5.2%)
(4.8%)
Sleet
(46.4%)
(23.1%)
(5.0%)
(4.1%)
(4.0%)
Fog
(15.4%)
(10.1%)
(8.6%)
(7.8%)
Just state the No. 1 contributing factor and its percentage.

11. Motor Vehicle Crash Day of Week by Weather
Crashes in clear, cloudy, or rainy weather had a similar distribution. The lowest percentage of crashes were on Sundays and the highest percentage were on Fridays.
In snow, less crashes occurred on Sundays and Mondays, compared to other days.
Wednesday was associated with the least percentage of crashes in sleet.
Sunday was related to the highest percentage of crashes in fog.

12. Motor Vehicle Crash Time of Day by Weather
Crashes in fog were different from those in clear, cloudy, rain, snow, or sleet conditions. Its single peak was from 6-8 am.
Crashes under other weather conditions had two peaks. One was at 7-9 am and the other was at 4-6 pm.

13. Injury and Fatality Rates for Motor Vehicle Crashes by Weather, New York State, 1995-2000
Snow was associated with the lowest injury rate (32.9 per 100 involved).
Fog was related to the highest injury rate (42.9 per 100 involved) and the highest fatality rate (0.66 per 100 involved).

14. Traumatic Brain Injury Rate for Motor Vehicle Crashes by Weather, New York State, 1995-2000
Number
Rate per 1,000 involved
Clear
8,430
3.3
Cloudy
3,554
3.5
Rain
1,727
2.8
Snow
619
3.6
Sleet
129
3.9
Fog
123
7.9
Total
14,582
Fog was associated with the highest traumatic brain injury rate (7.9 per 1,000 involved).
* Based on matched pairs of 49,168 with a cut-off probability of 0.9.

15. Hospital Charges and Length of Stay for Inpatients by Weather, New York State, 1995-2000
Snow was associated with the lowest median hospital charges ($5,876).
Fog was related to the highest median hospital charges ($7,480) and the longest median hospital stay (4 days).
* Based on matched pairs of 49,168 with a cut-off probability of 0.9.

16. Conclusion
Most crashes occurred in clear or cloudy weather, however, the crash rate was higher in rain, snow or sleet conditions.
Under clear, cloudy or rainy conditions, two-car crashes accounted for more than 60% of crashes. In contrast, in snow, sleet, or fog, one-car only crashes accounted for nearly 50% of crashes.
Rain increased the risk for multi-car crashes. Snow, sleet or fog did not increase the risk for multi-car crashes.

17. Conclusion (continued)
Crashes in fog were related to a higher personal injury rate, fatality rate, and traumatic brain injury rate. Crashes in snow had a lower personal injury rate.
Crashes in snow had the lowest hospital charges, while crashes in fog had the highest hospital charges and the longest hospital stay.

18. Acknowledgement
NHTSA Crash Outcome Data Evaluation System (CODES) team: Ms. Barbara Rhea, Ms. Sandy Johnson
New York State Department of Motor Vehicles, Bureau of Program Analysis & Data Services
The New York State Governor’s Traffic Safety Committee (GTSC)
Dr. Mike McGlincy
Dr. Julie Eisele