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.

The real problem is that the roads don't seem to be engineered with a "bandwidth" mentality. Instead I see interchanges where 2-3 lanes from a new road are being piped into an existing 2-3 lane road. Did the existing road get expanded to 4-6 lanes? Very rarely, because you would see a lot more 10-20 lane roads running through downtown.

Instead, it seems a lot of "traffic" studies are just minor tweaks around the edge, add a lane or remove a lane from a 4-6 lane main corridor and the traffic patterns basically don't change that much because its massively oversubscribed already.

actually they manipulate the flow of cars to the smaller roads by making u wait on the offramp longer when its busy iirc, after watching a few docu's on the engineering i think theyre engineers are really good and bright