Downtown is back, baby! Since 9/11, Lower Manhattan has recovered remarkably. But as ever-growing crowds congest neighborhood sidewalks, it may also be a victim of its own success. One World Trade Center, Santiago Calatrava’s Occulus and the National September 11 Memorial and Museum attract millions of visitors a year. And massive new residential towers have families pouring in. Now, residents say the once sleepy area (after work hours anyway) is becoming a nightmare to navigate.