The safest part depends a lot on the storm, and each storm is different. In general, given a 'typical' supercell structure, the most prefered place for tornado viewing is 90 degrees to the right of the storm motion. For example, for a storm moving northeast, the preferred spot would be to the south of the storm. The "safest" is probably behind the storm, but viewing can often become obscured by wrap-around precipition (or the DRC I would think, should this turn out to be real)... Sometimes, however, the wrap-around precip is so dense that you can only really see the tornadic while in the 'Bear's Cage", or the area between the tornadic circulation and the front-flank precipitation. This can be a very dangerous locations however (anyone in this position on the Hallam storm? LOL I think not, or at least not for very long). Of course, as you noted, it does depend on the road network as well as other factors.