I'm not saying that idiot drivers don't help... man do they.. I just remember working in a City and having to deal with their planning group. There was an issue of capacity of the roads and they needed to add lanes to handle the amount of cars going down the roads. It appears, at least to me and I admit I know almost nothing on the subject so take what i say with a grain of salt, but it appears that the problem is two fold. The amount of cars on the roads and that's compounded by idiot drivers. I can see the second part fixed with driverless cars, but if they become more popular the amount will still cause issues.
yeah, I think there certainly is a density part to the math--think of the intertubes and the amount of data, the roads being the max bandwidth. But that is also a good analogy for how much data (cars) can be crammed down such little space, with very little to no issues,
if that data is crammed through efficiently.
The reason your contractors are claiming x lanes need to be added to accommodate x cars/people, is because that math is corrected for idiot human drivers. There is enough data now where your typical city planner is going to understand that a certain population, in heavy commute zones will need x amount of lanes...but human brains are the most important part of their equation--the brains controlling those death machines. Density is a part of it, but a very small part of it. You could probably chart two lines for density and population, what is required for commuter traffic, and those lines even out quite early on in an automated system. The fact that humans are the primary component of the current paradigm is very well hidden within the math, and it would surprise most people how few lanes are truly needed to sustain major metro commuter corridors in an automated system.
This is a really cool article I read years ago about traffic and the conditions that cause traffic. The primary focus is Moscow (notoriously one of the worst traffic places in the world), but talks about the disasters in other 3rd world and developing countries like Pakistan, India, China (also--what happens when traffic laws don't exist. basically: lol)
http://www.newyorker.com/magazine/2010/08/02/stuck-3
There's also a lot of information coming out more recently as the driverless vehicle paradigm approaches. It's going to be a real game-changer and it can't come soon enough, imo.