More Connected Urban Roads Decreases US GHG Emissions

More Connected Urban Roads Decreases US GHG Emissions

We quantify the importance of early action to tackle urban sprawl. We focus on the long-term nature of infrastructure decisions, specifically local roadways, which can lock in greenhouse gas emissions for decades to come. The location and interconnectedness of local roadways form a near permanent backbone for the future layout of land parcels, buildings, and transportation options. We provide new estimates of the environmental impact of low-connectivity roads, characterized by cul-de-sacs and T-intersections, which we dub street-network sprawl. We find an elasticity of vehicle ownership with respect to street connectivity of –0.15—larger than suggested by previous research. We then apply this estimate to quantify the long-term emissions implications of alternative scenarios for street-network sprawl. On current trends alone, we project vehicle travel and emissions to fall by ∼3.2% over the 2015–2050 period, compared to a scenario where sprawl plateaus at its 1994 peak. Concerted policy efforts to increase street connectivity could more than triple these reductions to ∼8.8% by 2050. Longer-term reductions over the 2050–2100 period are more speculative, but could be more than 50% greater than those achieved by 2050. The longer the timescale over which mitigation efforts are considered, the more important it becomes to address the physical form of the built environment.

Key findings

We find that reducing street-network sprawl can make a large contribution to greenhouse gas mitigation, particularly in the medium-to-long term as opposed to only bettering the types of fuels we use and becoming more fuel-efficient.

A doubling of housing density could reduce travel and therefore road congestion.

The effects of cul-de-sacs have the impact of lasting decades longer than a car on the streets.

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