A researcher in an orange visibility vest adjusts a camera that is protected inside a wooden birdhouse.

Evaluating “Hot Shots”: Winter Roads Treatment of Hardest Icy Spots

A new six-part video series, “Hot Shots for Cold Climates” explores treating Minnesota’s iciest roadways, with safety and environmental impact in mind.

Chlorides in many lakes, rivers and even groundwater is a growing problem in Minnesota, leading transportation agencies to modify their winter road maintenance operations. Agencies continue striving to keep winter highways clear and safe for motorists while they also monitor and reduce salt use in deicing operations. Recent optimization of deicing applications has reduced costs and improved environmental impacts.

While MnDOT has been able to reduce the salt applied to typical winter highways, several types of winter road conditions require additional treatment. “Hot shot” treatments, which use extra deicing materials, are often needed to mitigate conditions on icy pavement surfaces to prevent roads from becoming treacherous and dangerous to motorists. 

“This project revealed that current treatment practices for difficult icy spots appear to be appropriate. In fact, few situations required additional salt use,” said Thomas Peters, research and training engineer, MnDOT Office of Maintenance.

MnDOT sought to evaluate deicing treatments for winter roadways with frequent difficult driving conditions caused by drifting snow, blow ice formation, ice fog (which produces black ice) and roadway shading, and on exposed bridge decks. The agency wanted to learn about the materials, labor and equipment costs of treating these hardest icy spots and the environmental impact, including whether treatments undercut environmental protection efforts elsewhere. 

What Was Our Goal?

This project’s primary objective was to investigate MnDOT’s current treatments of especially difficult pavement conditions on winter roads and determine the benefits and costs of different approaches, including environmental effects. One important aim was to distinguish between normal and difficult roadway conditions that could lead to specific targeting of these current winter maintenance efforts. 

What Did We Do?

Researchers and MnDOT winter maintenance personnel selected 19 road sites: 10 low-volume traffic sites that frequently developed difficult icy conditions and nine control sites that had similar traffic volume and design, but were less likely to develop severe pavement conditions. The 19 sites were monitored during the winter of 2019-2020. 

Several data collection methods were used. At each site, researchers installed instruments to collect highway level air temperature, dew point and light intensity. In addition, time-lapse cameras protected in wooden birdhouses captured site images to verify road conditions. Researchers obtained the amounts of deicing materials dispersed at each site from agency maintenance decision support system (MDSS) information and automatic vehicle locator (AVL) data collected from the plow trucks.

Cross-road snow drifting at a rural control site on a clear day.
Blowing and drifting snow can produce icy road conditions.

Regional weather stations provided measurements of precipitation amounts and types, and wind speed and direction. 

All on-site instrumentation was checked at least biweekly. Researchers had to adjust the heights of instrumentation posts throughout the winter, eventually raising some posts 5 feet as snowdrifts accumulated along the roadway sites. 

Throughout the monitoring period, 909 passes by plow trucks applying deicing materials were counted at the 19 study sites. 

What Was the Result? 

Only 255 plow truck passes (28%) applied hot shot treatments that used more than a typical amount of deicing material. Three types of difficult road conditions—drifting snow, blow ice formation and ice fog/black ice conditions—received normal winter roadway applications and were treated with typical amounts of deicing materials. Vehicle traffic enhanced the effectiveness of the normal treatments as vehicle tires worked the deicing salts into icy layers. Thus, more traffic actually improved the effectiveness of deicing treatments. 

“This work defined the balance between environmental protection and safety that winter highway maintenance crews struggle to achieve, particularly on low-volume highways in difficult geographies,” said Stephen Druschel, director, Minnesota State University Center for Transportation Research and Implementation.

More deicing salts were applied to exposed bridge decks and were needed to treat roadway shading. Two examples illustrate the need: At one exposed bridge deck study site, the deck temperature was 12 degrees colder than the approaching pavement. At a site where the road passed under a bridge, the shaded portion of the road was 28 degrees colder within 4 feet of the unshaded portion. Such sites with extreme temperature differences show the need for occasional additional deicing treatment.  

Researchers concluded that hot shot treatments were far less frequent than had been expected, and minimal extra salt was used. Operator judgment was the most important variable that researchers found. Recommendations included providing enhanced training for operators and encouraging operators to share their experiences. 

What’s Next?

The research team produced six short videos about the project and its findings. Because of restrictions on in-person reports due to the COVID-19 pandemic, researchers recommended the creation of a webinar to get the word out. 

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