Opponents of a plan to convert downtown parking lots into affordable housing say they've collected enough signatures to put the issue before voters, potentially halting an effort to build 345 affordable units on lots that currently provide more than 500 parking spaces. Save Downtown Menlo Park, a coalition of local businesses and downtown patrons, submitted 3,440 signatures to City Hall last Friday well above the 10% threshold of registered voters needed to qualify the measure for the ballot.