By most accounts, the one downside to Wynn’s Everett casino plan is how to handle the heavy traffic it’s expected to generate. That’s because there’s pretty much only two major access routes to the Everett casino site from the south, Route 99 through Charlestown’s Sullivan Square and Route 28 through Somerville. And as local residents know, both those routes are jammed with traffic already, and that's one of many concerns Somerville Mayor Joseph Curtatone has about the project.