dagger - it's not TWC that has the bandwidth restrictions, it's WRAL.
Each local station is alotted something like 22 Mbps of bandwidth (someone, I'm sure can provide the actual number ... or maybe I'll do some research and update the thread). They allocate the bulk (18Mbps or so) to the high def channel, and the other 3+ to the 24-hour weather/news station.
During the NCAA tournament, for example, when they have all four digital channels activated, they alot like 5-6 Mbps for each SD channel.
You'll also notice that PBS does the same thing OTA - from 8:00 pm on, they turn on their HD channel, give it lots of bandwidth, and turn off two of their SD channels to compensate. TWC actually gets the feed a different way, so they leave all 4 or 5 channels on all the time.