That's the paradox! The difference between a traffic route and a bandwidth-pure pipe (apologies for idiosyncratic terminology, i'm not a trained traffic planner) is that cars will choose their routes selfishly. The way I think of this difference is that even on a straight road that doubles its number of lanes, and we've all seen this, cars will change lanes with the idea that they'll get through traffic. Unfortunately, this lane changing, especially in gridlock situations, slows other cars down more than their overall speed is increased. Thus the overall bandwidth of the road, such as it is, is reduced. Add brake ripples and slow reaction time when moving in bumper-to-bumper, and the reduced overall speed should be obvious.