It is unarguable that transit is vastly more effective at moving large numbers of people per unit area than personal motor vehicles. As such, it might be reasonably assumed that increasing the transit capacity of a city would decrease motor vehicle congestion as users of the latter shifted towards the former, thus freeing scarce road space. Yet, to understand why this is so rarely the case, we must dig deeper into the root causes of transportation demand.
By prioritizing spatially efficient modes like transit, walking, and biking, the Tilikum Crossing is able to increase transportation capacity, and therefore potential economic activity, far more than an equivalently sized bridge built for private vehicle traffic, and without the negative impacts on the surrounding environment as well.
Transportation demand is a classic example of what is known in economics as “derived demand”, or demand as a means to satisfy some other more primary demand. That is to say that transportation is rarely a demand unto itself, but a necessary component in achieving some other end. And while a leisurely stroll around the block or a drive along a scenic parkway may occasionally serve as its own end, the overwhelming majority of trips are rooted in objectives like transporting one’s self or one’s goods to a specific destination. Broadly speaking, we could take “economic activity” to represent the primary demand from which transportation demand is derived. I don’t mean to suggest that people travel only for economic purposes, however freight transport, commuting, shopping, and even traveling for leisure can be thought of as under the umbrella of “economic activity”, if only as a proxy.
Ultimately, traffic congestion is an indication that there is latent demand for economic activity that is being constrained by the capacity of a city’s transportation system. While adding additional capacity may have the first order effect of reducing, congestion, the excess capacity will soon be absorbed by the appetite of latent demand for economic activity, à la tragedy of the commons
*. That is, even if by adding one’s vehicle to the road the overall condition for everybody is deteriorating, it may still be in an individual’s self-interest to exploit the newfound time savings. Of course there are other forces that undergird the demand for economic activity, and it is unlikely to grow perpetually. But only once increased transportation capacity exceeds the transportation demand derived from economic activity will congestion abate.
On the flip side, transportation capacity can also not grow perpetually. Capacity inherently contains some spatial cost, and at a certain point the spatial cost of transporting people to a place will begin to erode the very place that it is intended to deliver people to. Considering both the benefits of tapping into latent economic demand via transportation capacity and the spatial constraints inherent to developing that transportation, it is apparent that cities with more spatially efficient transportation have proportionally more transportation capacity to serve that latent economic demand. After all, it is not a coincidence that New York, the city with by far the highest transit ridership in the U.S., is also the city with the largest economy in the U.S. The graph below illustrates the strong relationship between transit ridership and GDP, created using publicly available information for a selection of large U.S. cities.
This back-of-the-envelope graph illustrates the clear relationship between a city’s GDP and its transit mode share, based on  U.S. Census Bureau, American Community Survey, 2010 and  U.S. Department of Commerce, Bureau of Economic Analysis, 2015.
Conversely, consider the effect of replacing New York’s nearly six million average daily subway riders, over half of whom are destined for Manhattan, with an equivalent number of personal vehicles. Manhattan’s notorious traffic congestion is caused by a mere three quarters of a million trips into the borough below 60th Street. Imagine the sheer impossibility of fitting an additional three million vehicle trips, or 400% more than personal vehicles currently crossing into the central business district. To even entertain such a preposterous scheme would require leveling every economically productive destination in favor of highways and parking lots. Surely the economy of the city―and by extension, the region―would collapse under this extraordinarily counterproductive effort. But while this may sound comical in the New York context, it is exactly what has happened in so many U.S. cities. Ironically, using the justification of economic development, cities have hollowed out their urban centers to accommodate cars to the point of reducing the economic vitality of the areas they sought to serve. Clearly this approach is untenable.
Returning to the original point, our cities’ transportation systems ought to be built to serve the needs of the population, economic and otherwise, in contrast to the transportation development ethos of the past century. The problem with expecting mass transit to relieve vehicle congestion, aside from framing the issue in such a way as to prioritize the driving class above all others, is that it makes the mistake of treating transportation demand as some unchanging, immutable force, rather than as part of a feedback system between a population’s travel demand and a city’s economic activity. And so long as the underlying economic activity is growing, the travel demand will follow, quickly filling however many billions of dollars worth of infrastructure were just built. Now, many would consider increased economic activity as a good thing regardless of the transport mode supporting it, but we must consider the return on investment for different modes of transport as well as the positive and negative externalities of each. As discussed, the return of mass transit is orders of magnitude greater than that for personal vehicles in terms of how much additional transportation capacity can be added to fill any latent economic demand. But additionally, mass transit comes with vastly less public space used, pollution generate, and traffic violence committed than do personal vehicles. Frankly, the choice for cities is downright obvious.
So why aren’t cities making the obvious choice to invest their transportation resources into mass transit rather than personal vehicles? Why is it so controversial to replace a congested mixed traffic lane with a bus lane that will prioritize the many over the few, as per the tweet below? Why are mass transit projects so heavily scrutinized facing years of obstructions while greater sums of money may be unflinchingly dropped to shave mere seconds off a motor vehicles commute with a new highway interchange? Well, there is no concise answer but it certainly involves political and philosophical ideologies that bias many U.S. decision makers towards some romanticized, if false, sense of individualism at the expense of the public good, worsened by fragmented governance structures that make strategic regional transportation planning all the more difficult. Of course it is also not a coincidence that there are colossal corporate interests from the automotive industry to keep people reliant on expensive vehicles for all personal mobility.
This is all to say that arguments for increasing vehicle travel within cities premised on improving the economy are inherently disingenuous when failing to consider the obvious economic superiority of transit. We should not be resigned to accept transit improvements contingent only on the spurious claim that they will alleviate personal vehicle traffic congestion when the economic, social, and public health benefits stand on their own merit.
*For this reason, traffic congestion may increase even holding economic activity equal since those wealthy enough to be unbounded by the monetary cost of driving are now unencumbered by the time cost as well.