In Massachusetts cities and towns need permits to post speed limits. See General Laws Chapter 90 Section 18 and Chapter 85 Section 2. This permit is a “special speed regulation” signed by officials from the city or town, MassDOT, and the RMV. Often the speed limit will be written into city ordinance, but a separate speed regulation is required to validate it.
Both city and state should have copies and a request can be directed to either. MassDOT usually keeps more accurate records, and will often email copies of regulations for free. (They are allowed to charge a fee.) For a state-owned highway, only MassDOT is likely to have records.
You may get a response saying no regulation was found. It is approved by lawyers who do not want you to have a “get out of jail free” card. It is up to you to explain the letter’s significance to the judge.
There should also be an engineering study justifying the speed limit. This will normally be in MassDOT files only. If you want it, you have to specifically ask for it. Historically MassDOT has charged a fee in the range of $20-$50 for copies of engineering studies. If you know how studies are supposed to be done you may find the investment worthwhile because many are deficient.
In rare cases, MassDOT will not find any study. Without a study to back it up the speed regulation should be disregarded. Again, you will not get a “get out of jail free” card. You will have to explain the letter to the judge.
MassDOT’s explanation and email address for public records requests are on these pages:
https://www.massdot.state.ma.us/highway/Departments/TrafficandSafetyEngineering/PoliciesandDesignGuidelines/SpeedLimitRegulations.aspx
https://www.massdot.state.ma.us/ContactUs/PublicRecordsRequests.aspx
An additional source of information regarding public records requests is the NMA’s guide to writing effective public records requests.
MassDOT does not issue permits for school zone speed limits, and a few cities have special legislative permission to post speed limits without MassDOT approval. Only the city will have paperwork in those cases, and it will not be in the form of a special speed regulation. The city is responsible for keeping any supporting documentation like an engineering study.
A request directed to a city can cover both cases with language like
I am requesting copies of the ordinance setting a speed limit on (street),
and a copy of the special speed regulation from MassDOT if there is one.
Again, you will need to ask specifically for the engineering study if you want one.
The Turnpike, including harbor tunnels and I-93 downtown, is now under MassDOT control. Speed limits are supposed to follow the same procedure as for other state highways, but the Turnpike has resisted merging legal and engineering operations. Contact information is as above. You will probably get back a reference to Turnpike regulations which say the speed limit is 65 (45, 55, etc.). If you specifically want an engineering study or an acknowledgment that none could be found, you may need to write back and clarify your request.
Some roads in and around parks, as well as some major roads around Boston, are under control of the Department of Conservation and Recreation. The DCR is not required to obtain permits from MassDOT. The DCR does not keep records related to how speed limits were set. (Whether or not they should, they don’t.) If you need a written response either providing the records or saying they could not be found, request a copy of relevant information as explained here:
http://www.mass.gov/eea/agencies/dcr/inside-our-agency/public-records-requests/