AFAICT Somerville requires $1/car for "one-time event passes" (see the first bullet point (http://www.somervillema.gov/section.cfm?org=traffic&page=916)), which is far less generous than Cambridge's just ask us first (http://www.cambridgema.gov/traffic/Consideration.cfm) policy (you can even ask Cambridge online (http://www.cambridgema.gov/CityOfCambridge_Content/forms/tpat_consideration2.cfm)).