Are laws of states and municipalities in the US not public domain automatically? While works of the states and municipalities might not be public domain, the text of law I thought was. 

The municode link there is to the Somerville Code of Ordinances. That said, I can provide attestation that the street is signed as Grand Union Boulevard, in alignment with local law.

I do not believe Somerville provides an online browsable street GIS solution (Vision primarily contains parcel data, and the street basemap is demonstrably out of line with surface conditions and ordinance), however public domain ESRI data is here: https://data.somervillema.gov/GIS-Data/Streets/7jtq-qmnf/about_data