The South Carolina Department of Transportation (SCDOT) has 336 live streaming video cameras covering over 1,000 miles of highway, mostly interstates, across the state. The camera system is part of the 511 Traffic Information System used by many US states.
The videos are mostly of good quality, except in heavy rain and fog, and easily accessible online.
The department has free Android and iPhone apps which allow you to view streaming video from these cameras, and more travel information, on your smartphone. In addition, South Carolina provides four Twitter accounts, that broadcast traffic incident information for each area of the state. (Listed below)
SCDOT has one of the more advanced live streaming video systems in use today, while apparently North Carolina DOT still uses the single frame pictures system. (I could find no link to live video). Other states have varying degrees of services, which you can research on their DOT websites.
A valuable and free service for travelers.