What are the requirements for permanently posting a road for weight?
For towns, the only section of law that allows for permanently excluding trucks over a certain weight is Vehicle and Traffic Law 1660 (Traffic regulation in all towns) subdivision 17 which reads:
A town may -
17. Exclude trucks, commercial vehicles, tractors, tractor-trailer combinations, tractor-semitrailer combinations, or tractor-trailer-semitrailer combinations from highways specified by such town board. Such exclusion shall not be construed to prevent the delivery or pickup of merchandise or other property along the highways from which such vehicles and combinations are otherwise excluded.
There are similar laws for counties (Vehicle and Traffic Law §1650 (a)(4-a)) and villages (Vehicle and Traffic Law §1640 (a)(5)). However, that does not allow municipalities to just post the road without being reasonable. An opinion of the NYS Comptroller states:
Towns may by ordinance or local law, exclude trucks, tractor-trailer combinations, and trucks in excess of any designated weight from any town highway, but such action must be reasonable.
In deciding to post a road, the board needs to investigate why and provide a reasonable explanation for the decision. A road which would be overly damaged by heavy trucks or a need to exclude trucks due to poor sight lines may be considered reasonable, but this should be documented as part of the process. The municipality should comply with SEQRA when adopting a local law to establish a permanent weight restriction.
Note that such a posting does not stop local deliveries as might be needed for a business, farm, or home.
NYS Comptroller Opinion 1967-0325