Striking the right balance between preserving roadside trees and maintaining the roads in the winter is the focus of an ongoing study by U of M researchers. The stretch of State Highway (SH) 34 between the towns of Detroit Lakes and Osage is beautifully and quintessentially Minnesota: a towering mixture of deciduous and pine trees lines the roadway as it passes near lakes, a state forest, and a national wildlife refuge. However, the same trees that add to the road’s beauty make winter maintenance challenging for Minnesota Department of Transportation (MnDOT) crews—and make winter driving potentially dangerous for motorists.
MnDOT maintenance staff recently weighed in on an upcoming SH 34 paving project, reporting that they spent a lot of extra time and effort on that stretch of road, especially in the shaded areas, said MnDOT District 4 assistant district engineer Tom Lundberg in a recent CTS webinar. “Staff said, ‘We’re putting more [deicing] product on the road, we have slower recovery times to bare pavement, we have trees falling on the road, and we’re making special trips to touch this section up,’” Lundberg noted.
To address this challenge, MnDOT planned to cut the shade trees along the corridor to allow sunlight to reach the roadway. The agency reasoned that the sun would help melt snow and ice, thereby reducing staff and equipment time and saving money. This action would also reduce the amount of sand and salt applied to the road, which is better for surrounding water quality. Members of the public, however, wanted to limit tree cutting on the corridor as much as possible to preserve the trees and scenery.
In response to public input, MnDOT reduced the initial planned tree cutting and partnered with a U of M research team to determine the optimal balance of sun exposure and tree removal and explore how that will affect winter maintenance activities on SH 34 between Detroit Lakes and Osage. The U research team is being led by professor and CTS scholar Mihai Marasteanu of the Department of Civil, Environmental, and Geo- Engineering (CEGE).
The research team’s first task was to obtain the pavement temperature data needed for the analysis. The data included a heatmap generated by the infrared camera that’s part of MnDOT’s Road Doctor Survey Van; data from five new sensors embedded in select locations along SH 34, with various degrees of tree shade; and data from MnDOT’s Maintenance Decision Support System. The researchers proposed a new parameter—the “shade factor”—that they could incorporate into their heat transfer model, generating new insights into the effect of tree shade on pavement temperature.
“We found that the shade factor is related to the type of trees,” said CEGE associate professor Xue Feng, the study’s co-investigator. “Pine groves have the most shade, while deciduous trees, which shed their leaves, have a lower shade factor. In addition, shading is more pronounced in the winter months due to the lower sun angle.”
To determine shading’s effect on winter maintenance operations and costs, the research team analyzed shade and pavement temperature relative to the amount of deicing material used.
“The full-shade pavement required nearly double the amount of salt, sand and salt mix, and brine per lane mile compared with the full-sun pavement,” Marasteanu said. “While this is still preliminary data, it shows that the significant temperature differences between full-sun and full-shade areas of State Highway 34 do impact winter operations.”
Once the results of the research project are final, MnDOT plans to use them to help guide their decision making on SH 34.
“We’re looking for further recommendations to determine if we should clear-cut, limb, selectively cut, or do nothing,” Lundberg said. “At the end of the day, we’d love to limit material and equipment usage, save money and resources, and limit the chlorides in the watershed.”
Going forward, the research team hopes to investigate shade’s impact throughout the year. For example, on hot summer days, the shade factor may affect the durability of asphalt pavements. In addition, the researchers are interested in connecting the shade factor with more detailed tree configuration information such as height, canopy, and distance from the pavement.
—Megan Tsai, contributing writer
