Hi cunninghamair,
I don't think they've ever closed a road to car traffic on any organized century I've ridden in. The ride organizers stress the fact that even though we're on an organized ride, we MUST obey all local traffic laws and ride safely.
On a lot of the organized rides I've been on, the ride organizers also put up big signs on the road notifying drivers that there are bicycles on the road. There's also big signs reminding riders to ride singlefile!
Of course, your century may be different -- they may close the road.
There! How's that for the most waffling answer to a simple question? Maybe I should consider a career change to politics. 
- Melissa '08!
PS How in the world do you pronounce your screen name?!
I'll get back on the bike soon, I promise!