The Highway Traffic Act authorizes the province and municipalities to restrict truck weights during spring thaw to protect roads and highways:

1. Spring weight restrictions may be applied to schedule 1 King’s Highways at any time during March and April, to schedule 2 King’s Highways during March through May, and to schedule 3 King’s Highways during March through June depending on local conditions and weather.  A list of highways that may be affected is included in this [MTO memo].  Up-to-date information about actual weight restrictions can be viewed at: https://511on.ca/list/seasonalloads and drivers /carriers who use the listed highways are encouraged to carefully monitor this website. The 511 interactive map also provides up-to-date visual information – choose map menu item “Truck and Bus info”, then check “Seasonal Loads”.
2. Municipalities may also pass by-laws and set dates when spring weight restrictions apply to specified roads under their jurisdiction.   Watch for on-road signage.
3. During Reduced Load Periods, weights are generally limited to 5,000 kg per axle, although certain exemptions apply as set out in Highway Traffic Act, Section 122:  https://www.ontario.ca/laws/statute/90h08#BK217
4. Unless otherwise specified on the permit, ‘annual’ and ‘project’ overweight permits are not valid during March and April in southern Ontario, and during March, April and May in northern Ontario.