Modern roundabouts are designed for trucks, large vehicles, and trailer towing vehicles by including the center flat area around the circle.  It’s not a sidewalk, it’s called a truck apron, and it’s for trucks to begin a sharp right or end a left or U-turn on.  

Roundabout Trucks Videos:
FHWA: http://www.youtube.com/watch?v=0nVzsC2fOQw  
Washington County, WI: http://tinyurl.com/trucksRABwi
U-turn: http://tinyurl.com/rabtruckuturn  
Windsor-Essex Parkway, Canada: http://tinyurl.com/windsoressexRAB