According to the International Energy Agency greenhouse gas emissions (GHG) from the transport sector have continued to rise in both the industrialized and industrializing countries, and are projected to continue to do so well into the 2030s and 2040s. One of the explanations for this is the strong network effects (path dependencies) associated with the system of car-based travel, where capacity expansions of the highway network induce further demand for higher automotive mobility.
Recent research from Aalborg University in Denmark suggests that, at least in the case of the Copenhagen and Portland, Oregon metropolitan areas, the existing system of dependence on high-carbon modes of transport may be weakening. Changes in demography, fuel prices, behavior, residential location preference, and environmental concerns are all contributing to a reduction in vehicle miles traveled (VMT) across North America and Europe, thereby undermining the business case for new large-scale transportation infrastructure projects such as the proposed Ring 5 in Copenhagen and the Columbia River Crossing project in Portland.
The path dependencies embedded within large-scale transportation infrastructure projects have dramatically different carbon trajectories, depending on the mode. If cities and regions are to meet their aggressive carbon reduction goals, then they will have to, sooner rather than later, begin to shift their strategic investment focus away from high carbon, motorized forms of travel toward low- or no-carbon modes of travel. Such decisions may become increasingly easier to implement in light of the decaying political support and economics of massive expansions of the road network.