No roadway surface is indestructible and, by law, road agencies can enact seasonal weight restrictions on any road with the exception of designated “all season roads”. Restriction guidelines provided by the US Department of Transportation Federal Highway Administration (FHWA) advise that applying restrictions is dependent upon pavement structure thickness, base course thickness, local experience, and the Thaw Index (the cumulative number of degree-days above freezing for given time period).
This is a challenging balance for road agencies needing to protect the roadways and to keep routes used by heavy trucks open and accessible to traffic.
Accumulating the necessary data over time and making the calculations are cost and time prohibitive. Data sources and methods for calculating vary widely from state to state and even from county to county. A lack of consistency leads to the frustration of constituents affected by a road’s restriction status.
Thawcaster aids road agencies in the consistent assessment of roads during the more critical times of the year when some roads become quite fragile by providing agencies with a single tool to access weather data combined with thaw-weakening calculations and analyses. Thawcaster computes the thaw index for thaw periods using the reference temperature of 29 degrees Fahrenheit for the FHWA model and a floating reference temperature from February 1st to May 31st for the Minnesota Method Thaw Index calculations.
The Center for Technology & Training (CTT) develops and maintains Thawcaster based on research, experience, and the input of several Michigan road agencies.
This graph uses historical weather data to indicate the Thaw Index (TI) and Minnesota Method Thaw Index (MTI) for the entire season. The solid horizontal line displays the necessary TI or MTI prediction (based on selection) for investigating the removal of seasonal load restrictions (SLRs). The dotted yellow line depicts when the FHWA guidelines suggest agencies "should" consider imposing spring load restrictions, and the dashed red line is when the guidelines indicate the application of spring load restrictions "must" occur. The light blue diamonds on the graph denote documentation of SLR application.
Seasonal weather data collection begins with three consecutive days of positive Freeze Index after November 1st for TI, and after January 1st for MTI. This table is displaying seasonal data representing the application of seasonal load restriction based on MTI data and the "should" level (10 Thaw Index) for "thin" pavements.