The maths used for traffic projections (from the traffic engineers I've met in real life) is very basic - you may loose your faith in professional traffic engineers if you actually saw it and thought about it.
I love this one example from the U.S. of traffic projection by the Washington state Department of Transportation:
In many cases, traffic projections become a
self-fulfilling prophecy, because they can convince governments to heavily invest in wider roads and freeways, which in turn makes driving easier and leads to
induced demand - and that encourages more driving.
But population isn't directly linked to traffic. There are various trends at work, such as rising fuel costs, the younger urban professionals that are attracted to inner-city life typically are more environmentally conscious and are into more active transportation such as walking and cycling, also the CBD and inner suburbs are much denser (in walkability, public transport service, population, etc) that the percentage of commuters that live in those areas that travel via single-occupancy vehicles tend to be lower than someone living in a middle or outer ring suburb.
Also, our CBD is only about a 15 minute walk side-to-side. Even if public transportation in the CBD reached capacity, CBD residents that work and play in the CBD still have attractive alternatives such as walking or cycling that they don't have in the suburbs.
Sure, traffic is likely to grow a little, but I highly doubt it will grow linearly with population.
If the CBD becomes unnavigable by car and yet it continues to grow - then people are going to move to the next most convenient option (walking, cycling, public transport). By then, we may get a CBD tram loop out of it.
