MDOT research on timing spring weight restrictions receives national recognition

LANSING, Mich. - The Michigan Department of Transportation (MDOT) was recently honored by the American Association of State Highway and Transportation Officials (AASHTO) with a national 2021 Sweet Sixteen award for research work aimed at improving predictions of when to impose and lift spring weight restrictions, work that is critical to reducing damage to Michigan's  roadways.

The Sweet Sixteen award is designed to highlight valuable research from four regions across the country, with four awards possible per region. MDOT received the award for research on developing and implementing a tool to help make load restriction decisions based on improved freeze-thaw models.

Each spring, roadbeds that have been frozen all winter begin to thaw from the surface downward. Melting snow and ice saturate the roadbed, as the frozen ground below prevents the water from draining. The roadbed, softened by trapped moisture beneath the pavement, is more susceptible to damage during spring than at any other time of year. It's also the time of year when potholes begin forming due to freeze-thaw cycles.

"Research shows that our pavements are most vulnerable during the spring thaw," said Richard Endres, head of MDOT's geotechnical services section. "The accuracy and timing of our seasonal load restrictions is critical. Even if it is a few days off, it could lead to significant damage."

During this crucial period, MDOT takes precautions to protect state roads, implementing spring load restrictions (SLR), or "frost laws." While there is no reduction in legal axle weights on state highways designated "all season" during SLR, no overweight loads are permitted. On seasonal state highways, truck weights are reduced by 25 percent for rigid (concrete) pavements, and 35 percent for flexible (asphalt) pavements. To protect roadway shoulders, which are not designed to carry regular traffic, permits for vehicles or loads exceeding 14 feet in width are not issued during seasonal restrictions.

These restrictions protect the roads, but they also disrupt local trucking routes - and the economy. It's important that the load restrictions begin before thawing reduces pavement strength but are lifted as soon as possible after the freeze-thaw cycles end and moisture drains from under the pavement. Then, trucking businesses can resume their usual routes.

Putting load restrictions in place early costs the trucking industry time and money. Putting them in late costs taxpayers through increased damage to the roads. The more accurate the restrictions, the more money can be saved. One major problem: the conditions under the pavement are hard to predict.

Traditional methods of determining load restriction dates have varied considerably throughout the state. Often, agencies have calculated dates manually using general data rather than site-specific information.

MDOT measures soil freezing and thawing manually using frost tubes installed throughout the state. Frost tubes are embedded in the ground and filled with a solution that changes color when it freezes. The tubes are checked periodically to determine how far down freezing temperatures have penetrated the soil. Over the years, MDOT has also invested in Road Weather Information System (RWIS) stations around the state that measure ground temperatures automatically.

In partnership with Michigan Technological University, MDOT undertook research to develop a web-based tool that draws from the statewide weather sensor network and weather forecasts to provide easily accessible site-specific guidance. This app data can assist road engineers with load restriction decision-making.

The new research helps MDOT leverage its 105 RWIS sites around the state. By filtering through vast amounts of data, researchers developed engineering models to better predict pavement and subsoil conditions and changes. Backed up by data from frost tubes and other indicators, these models assist in determining the optimal dates for placing and lifting SLR anywhere in Michigan, accessed through a user-friendly web application.

The web-based tool, MDOTSLR, provides MDOT, local agencies and road users with accurate and timely data on freeze-thaw index values in soil types based on ZIP code locations. By automating the required calculations, the tool saves road engineers time and effort. They can quickly validate data and easily determine and monitor dates for setting and lifting load restrictions.

Road engineers are already experiencing the benefits of the tool.

"We've used it as a comparative tool to assist in the decision-making discussion to enact or remove the spring weight restrictions," said Scott Greene, manager of MDOT's utilities and permits section. "In addition to the predictive models, we use frost tube data to get actual roadway conditions, along with physical observation of the roadside drainage, to determine when it is appropriate to implement and remove restrictions."

The new tool leverages technology that MDOT has already invested in, Greene explained. It also uses models developed specifically for Michigan climactic conditions instead of a generalized formula from the 1980s.

"With real-time pavement and underground temperature data from our RWIS system to determine when the frost is starting to thaw, along with the frost tube data from around the state and predictive formulas based on weather forecasts, we get a more complete picture of how the ground is thawing," Greene said. "That gives us a better basis for our decisions on when to enact and remove spring weight restrictions." 

To help spread the word and encourage adoption, MDOT conducted a training and outreach session for engineers, but the department cautions that the online tool is still a prototype. While it can give road users an idea of some of the information the department uses to make load restriction decisions, it doesn't show the field observations used to determine how well the pavement is draining, information that often factors heavily in SLR timing.

Another MDOT research project, examining the effectiveness of green strobe lights on snowplows and other winter maintenance vehicles, also received recognition and accolades from AASHTO in the supplemental category for safety.