MGL c. 90 § 17C defines a thickly settled or business district as "the territory contiguous to any way which is built up with structures devoted to business, or the territory contiguous to any way where dwelling houses are situated at such distances as will average less than two hundred feet between them for a distance of a quarter of a mile or over."

In 2016, legislation was passed (Sections 193 and 194 of Chapter 218 of the Acts of 2016) to create two new sections to MGL c.90§17C.

• Section 193 allows a municipality to opt-in to Section 17C of Chapter 90 of the MGL, thereby reducing the statutory speed limit from 30 mph to 25 mph on any or all city- or town-owned roadways within a thickly settled or business district. The legislation also requires cities and towns to notify MassDOT of these changes.
• Section 194 creates Section 18B of Chapter 90 of the MGL, allowing municipalities to establish regulatory 20 mph safety zones. Since this creates a regulatory speed limit, the MUTCD requires an engineering study prior to the establishment of the safety zone, and it should conform to the guidance found in the MassDOT Procedures for Speed Zoning.

If a municipality opts-in, it will not supersede any existing posted speed limit.  The legislation only affects streets that are currently governed by a statutory speed limit.  If an existing special speed regulation is in place, it will continue to govern.

MassDOT recommends that if a municipality opts-in to MGL c. 90 § 17C, that it does so on a city- or town-wide basis to avoid potential confusion for drivers. However, cities and towns do have the option to opt-in on a street-by-street basis. Once a municipality has opted-in to MGL c. 90 § 17C, it is required to notify MassDOT.