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Midpoint of April
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In seasonal frost areas, as freezing occurs the road becomes stiffer and stronger. Many state Departments of Transportation take advantage of the period of higher strength in mid-winter by applying winter weight premiums (WWPs), increasing the allowable weight that trucks can haul. During late winter and early spring when the top road layers begin thawing and can’t drain excess water, the road is highly susceptible to damage. To reduce roadway damage, many highway agencies apply spring load restrictions (SLR) during the critical time interval when the pavement is most vulnerable to damage. The Minnesota Department of Transportation (MnDOT, 2014) has recommended guidelines for applying and removing WWPs and SLRs are based upon freezing and thawing indices.
Cumulative Freezing Index for Establishing WWP Timing: MnDOT recommends that the WWP can be allowed when the 3-day weather forecast indicates that the cumulative freezing index (CFI) will exceed 280°F-days and extended forecasts predict continued freezing temperatures. The end date of the WWP period is determined when forecast air temperatures predict daily thawing, as indicated by the cumulative thawing index, and the impending placement of spring load restrictions.
Cumulative Thawing Index for Establishing SLR Timing Guidelines: MnDOT recommends that the SLR be scheduled when the 3-day weather forecast indicates that the cumulative thawing index (CTI) for a zone will exceed 25°F-days and longer-range forecasts predict continued warmth. MnDOT suggests removing SLRs using measured frost depths, forecast daily air temperatures, and other key indicators.
Minnesota Department of Transportation. (2014). Engineering Services Division Technical Memorandum No. 14-10-MAT-02. Process for Seasonal Load Limit Starting and Ending Dates.