(b) after thirty years from the imposition of the restriction, unless (1) the restriction is imposed as part of a common scheme applicable to four or more parcels contiguous except for any intervening streets or ways, and provision is made in the instrument or instruments imposing it for extension for further periods of not more than twenty years at a time by owners of record, at the time of recording of the extension, of fifty per cent or more of the restricted area in which the subject parc...
