Winter driving: risks, costs, and safety strategies
Driving in poor winter weather conditions can be compared to another kind of risky behavior: drinking and driving.
Winter snowstorms are a fact of life in the Upper Midwest, and Iowans tend to accept driving on snowy, icy roads as one of the inevitable consequences of living here. With winter approaching, now is a good time to remember the risks motorists face during inclement winter weather and redouble our efforts to keep motorists safe.
High-risk driving
From 1996 to 2000, almost 20 percent of all crashes in Iowa occurred during the five percent of the year (based on an average of 18, 24-hour storms) in which inclement winter weather affected road conditions. See Table 1.
Winter weather–related crash rates are even more startling.
In a CTRE study1, crash rates were calculated for sections of Iowa’s Interstate highways when snowfall exceeded 0.2 inches per hour. The rates were compared to crash rates for the same highway sections (same month, weekday, and time) when the weather was clear.
When road surfaces were icy or snowy, or weather conditions were snow or sleet, crash rates were 13 times higher than during clear weather.
To put this increase in perspective, compare driving in poor winter weather
conditions to another kind of risky behavior that increases a driver’s
crash risk: drinking and driving.
In a commonly quoted paper2, researchers reported that to increase
their chances of getting into a crash by 13 times, they would need a blood
alcohol content (BAC) of about 0.13.
This is much higher than the legal BAC for impairment in every state. Most states, including Iowa, have significant legal sanctions for drivers caught with a BAC at 0.13.
Trip deferrals
Both number of crashes and crash rates increase during inclement winter weather even though fewer travelers are on the road during snowstorms. As visibility declines to a quarter mile, as many as 80 percent of road trips are cancelled3.
The number of trips deferred depends on trip type. People who drive as part of their job have less discretion about canceling trips.
Commercial vehicle operators, for example, are less likely than other drivers to cancel trips due to bad weather. Therefore, as shown in Table 1, a disproportionately high percentage of winter weather–related crashes involve trucks.
Table 1. Comparison of Winter Weather-related Crashes to All Crashes: 1996 to 2000
| All Crashes | Winter Weather Crashes | ||||||
|---|---|---|---|---|---|---|---|
| No. of Crashes | Average Severity | Average Loss | No. of Crashes | Percent of Total Crashes | Average Severity | Average Loss | |
| Commercial vehicles | 22,048 | 13.6 | $39,103 | 4,962 | 23% | 11.5 | $33,860 |
| All vehicles | 342,732 | 9.4 | $19,374 | 64,410 | 19% | 7.7 | $15,866 |
Source: Iowa DOT crash data; Iowa Traffic Safety Data Service
Traffic speeds
Motorists do tend to slow down during inclement winter weather—but not all that much.
A study of traffic data from the Twin Cities Metropolitan Area freeway system shows average decrease in speeds of 8 to 10 percent for snowfalls of less than 0.1 inch per hour and more than 0.1 inch per hour, respectively. See Table 2.
Table 2. Summary of Capacity Analysis
| Capacity | Speed | |
|---|---|---|
| Clear weather | 2,400 vph | 65 mph |
| Snows less than 0.1 inch per hour | 2,000 vph (17% reduction) |
60.6 mph (8% reduction) |
| Snows greater than 0.1 inch per hour | 1,800 vph (25% reduction) |
58.7 mph (10% reduction) |
Source: Maze T.H., Integration of Road Weather Information with Traffic Data (preliminary results), Aurora pooled-fund project
As part of an evaluation of Iowa’s Maintenance Decision Support System, CTRE shot some local traffic video that indicates how much Iowa drivers reduce their speeds during snowstorms. Traffic on the U.S. Highway 30 bypass around Ames was videotaped from the State Street bridge4. This is a fully access-controlled, four-lane, divided highway.
Figure 1. Video of traffic on U.S. Highway 30 bypass from State Street
bridge (Ames)
during light snowstorm
Figure 2. Video of U.S. Highway 30 bypass from State Street bridge (Ames)
during heavier snowstorm
Traffic speed data were collected via video during two different storms. The first storm was light, and the Iowa DOT was doing a great job of keeping the roadway clear. See Figure 1. Average vehicle speed was 70.6 miles per hour.
The second storm was more severe, and the Iowa DOT was unable to keep the roadway clear. See Figure 2.
During data collection, when the roadway was mostly ice covered, the average speed was 62 miles per hour. One vehicle was recorded at 89 miles per hour.
The average speed reductions were rather meager compared to the stark differences in the pavement surface conditions shown in Figures 1 and 2.
Highway capacity
In the Twin Cities study, highway capacity dropped during inclement winter weather—from 17 to 25 percent for snowfalls of less than 0.1 inch per hour and more than 0.1 inch per hour, respectively. See Table 2.
The drop in capacity exceeded the drop in traffic speeds, indicating that drivers are increasing headway between vehicles.
Crash severity
Some people argue that, while winter crashes are more frequent, vehicles are traveling at lower speeds and, therefore, crashes are less severe.
Winter weather–related crashes in Iowa do appear to be less severe, but not significantly—only about 15 percent. See Table 1.
Bottom line
Snowstorms have a significant impact on roadway performance and impair motorists’ ability to drive safely. During snowy winter weather, drivers tend to increase their headways but moderate their speeds only slightly.
Enhancing winter roadway safety for motorists therefore requires a combined effort involving the three Es: education, enforcement, and engineering.
Driver education
Drivers—including commercial vehicle operators—need to
- be better educated about the increased risk they’re assuming for themselves, their passengers and loads, and other travelers when they drive during snowstorms.
- learn how to drive during snow and ice events: reduce speeds, increase headway, etc.
- know how to access weather-related traveler information systems so they can make more informed choices about where and when to drive.
Law enforcement
Officers need to ticket individuals for driving at unsafe speeds for winter roadway conditions. Judges need to understand the safety challenges of drivers traveling at speeds above those warranted by road conditions.
Roadway design
Designers need to understand and implement roadway design elements to mitigate the effects of winter storms. For example, designing and building the roadway to slope into the sun can reduce ice formation. Designing with an understanding of prevailing winds can reduce snow blowing across the roadway and snow removal.
For more information
For information about the study, contact Tom Maze, 515-294-9459, tmaze@iastate.edu.
Notes
1 Knapp K.K., Kroeger D., and Giese K., Mobility and Safety Impacts of Winter Storm Events in a Freeway Environment, Center for Transportation Research and Education, Iowa State University, 2002, www.ctre.iastate.edu/reports/winstorm.pdf
2 Compton R.P., Blomberg R.D., Moskowitz H., Burns M., Peck R.C., and Fiorentino D. Crash Risk of Alcohol Impaired Driving. In Proceedings of the 16th International Conference on Alcohol, Drugs, and Traffic Safety, Montreal, Canada, August 4–9, 2002, Montreal: International Council on Alcohol, Drugs, and Traffic Safety, 2002
3 Maze T.H., Burchett G.D., and Hawkins N.R., Using Data to Predict Relative Road Safety and Requirements for Road Closures During Severe Winter Weather Conditions, prepared for presentation at the Transportation Research Board, Washington, D.C., January 2005
4 Andrle S.J., Kroeger D.A., and Sinhaa R., Deploying the Maintenance Decision Support System (MDSS) in Iowa, Center for Transportation Research and Education, Iowa State University, November 2003, www.ctre.iastate.edu/reports/MDSS.pdf
