I live in New Hampshire, and we have a farm that we make half of our living from, and we have to complete the USDA requirements, and have to occasionally deal with people who are not farm friendly, but we are lucky for the most part that we do live in an AG friendly state where the right to farm is protected by law. Having that said, I was a police officer for years, and I try to stay within the boundaries of the laws where ever we have lived. The local, and state police are always bound to the letter of what they laws on the books say. That means that they can exercise judgement, and not enforce every law, but if they do decide to, they cannot enforce laws outside of what are on the books, and current in the state. Navigating these laws can be tricky, because most states repeal laws, or amend them each year, however almost every state publishes their motor vehicle laws online, and the only place I would really trust in regards to the laws, is a website maintained by a .gov site.
I have gotten misleading information from the registry in a few states, and there is not a police officer on the planet that knows every law in the book. Even when I thought I knew laws, they would change and be amended. Laws are always tough, and ignorance to the laws, or taking someone's word never holds up in court.
So, where I am going with all of this, is not to go against anyone on here, because there are very good people on here with great advice, but to protect yourself, not only from a citation, but the liability of an accident, research your local laws, and after you check you state laws, make sure that there are not additional local town or city ordinances that are stricter.
The general rule of thumb, is a county, province, city, or town cannot lessen state laws, by making things legal that are not at the state level, but the generally can strengthen those laws and make additional rules that make the existing laws stricter. An example would be if a State says that it is illegal to own a gun without a permit, and a city who says regardless of whether you have a permit you cannot own a gun in city limits, (that was just a reference, and not a point for a new discussion

),
So, even though I do not like sounding like a lawyer or police officer, we make sure on our farm that we have followed the written laws so there are no issues if we ever encounter an issue. The are some very scary drivers on the road, and it is scary enough for us having a small farm next to the road with our animals, and that whole liability issue, and our children, and occasionally driving my tractor, or working on fence line, mowing.
This is a very comprehensive article, and study done by the USDA relating to the use of agricultural equipment on public roads nationwide. The study really focuses on safety, talks about accident statistics.
http://www.csrees.usda.gov/about/white_papers/pdfs/ag_equipment.pdf
This article is for our neighbors to the north in Canada:
How the Farm Implements Act Protects You - Your Rights and Responsibilities Relating to Farm Machinery
While I legally have to have a plate to operate on the road,
This is an example of the law where I live in New Hampshire:
NH RSA 261:83 Farm Tractor Plates
I. A tractor shall qualify for farm tractor registration rates and shall be driven with such registration only when the tractor is used exclusively for agricultural purposes or to draw another vehicle in such a way that part of the load is carried on such towing vehicle.
II. Notwithstanding any provision of law to the contrary, a farm tractor that is fitted with attachments such as cultivators, loaders, or other equipment, specific to the activities of the farm, shall qualify for farm tractor registration rates and number plates, provided that it is being used exclusively for agriculture and farming purposes, and not being used for hire.
The laws even outline the inspection exemption for farm tractors, meaning someone probably enforced it at some point.
Saf-C 3224.04 Tread Depth of Farm Vehicles. Farm vehicles registered under RSA 261, self-propelled combines, self-propelled corn and hay-harvesting machines or
tractors used exclusively for agriculture purposes shall be exempt from the tread depth requirements of these rules.
This is what my tractor plate looks like here: