This is what is done in tokyo, and if it can be done in tokyo it can be done in any city in the world.
A 10-15km circut is blocked off from the public for a day, sometimes a weekend, and you get to ride long crit style laps. The roads are nearly always in good condition, are wide, and there are races ranging from U15 to 60+ masters.
Usually it is held in the port districts, which are not in use on the weekends anyway. This is basically where people do their training too.
I like the idea of long stages better, but the logistics are a bit crap: getting back 100km or so, road closures/support cars, safety, unknown hazards and all that.
To quote the Jens "When you are pushing that hard that you get that little taste of blood in the back of your mouth, you know?" that is all I ask for in a road race.