Frost laws are seasonal restrictions on traffic weight limits and speeds on roadways subject to thaw weakening.
In climates that experience below-freezing temperatures, damage to roads from thaw-weakening have led to many US states, Canadian provinces and other jurisdictions to enact laws that restrict vehicle loads during spring months, when road structures are thawing from above in a manner that limits water from escaping the soil structure, thereby weakening the pavement underpinnings.   The US state of Michigan, for example, during the months of March, April and May reduce legal axle weights of vehicles by up to 35%. Some areas also require heavy vehicles to travel a maximum of 35 miles per hour (56 km/h), regardless of the posted limit. Some states allow load increases on roads during the freezing season.
Since its inception in 1949, the Frost Laws or (Seasonal Weight Restriction) had been posted roadside with signage that informed truck drivers and other commercial traffic of the effective frost law.
Over the years, technological improvements have provided tools to detect when frost is present in the ground. The result has been a lifting of the effective dates of the frost laws when frost is no longer a threat to roadways. This created confusion as there was no uniform method of communicating these changes to the public and enforcement agencies.
In 2007 this issue was introduced to the state legislature. In 2008, in response to this issue, the state legislature passed a law requiring local (enforcing) municipalities to post their frost law information on the homepage of their website or the website of a statewide road association of which it is a member.
In 2009, Wayne County Mi discontinued the practice of posting signs. This resulted in thousands of tickets being issued to commercial drivers and truck drivers who would happen onto these roadways without posted notice. Many drivers were issued citations well above $3,000 while a few received citations in excess of $8,000.
As of August 2015, this issue is being challenged in the Taylor Michigan court. At issue is the legal requirement of the responsible party, to post the signs if they intend to enforce and whether police are required to ensure legal compliance prior to enforcement.
- Canadian Strategic Highway Research Program (C-SHRP) (September 2000), Seasonal Restrictions in Canada and around the World (PDF), C-SHRP Technical Brief #21, Ottawa, Ontario, p. 8, ISBN 1-55187-059-2
- Wisconsin Department of Transportation (2007), Springtime posted roads (state numbered highways only), retrieved 2010-03-28
- 2009 Minnesota Statutes (2009), 169.87 Seasonal Load Restriction; Route Designation., retrieved 2010-03-29
- Transports Quebec (2010), Québec Pavement Story, retrieved 2010-03-21
- Van Buren County Community Center (2010), Seasonal Weight Restrictions, retrieved 2010-03-28
- 2009 Minnesota Statutes (2009), 169.826 Gross Weight Seasonal Increases., retrieved 2010-03-29