A Simple Tool to Estimate Transport GHGs Mitigated from Compact Urban Form

Compact urban form can reduce road transportation GHG emissions and mitigate resource supply bottlenecks associated with mass EV adoption. Global databases from Climate TRACE and the Global Human Settlement Layer are utilized to develop the Compact Urban Form Estimation Tool or CUFET for calculating the reduction in VKT and road transportation GHGs from shifting toward CUF. The CUFET does not explicitly account for mechanistic changes in driving (e.g., modal shift) but rather uses settlement density as a coarse proxy for walking and transit urban fabrics. VKT was modeled using weighted least squares regression from the independent variables settlement population, settlement population density, and country fixed effects. Population size banding was introduced to the model to improve explanatory power. The model was developed using 10,495 settlements in the 2021 Climate TRACE dataset. The CUFET VKT model was able to explain 78% (p < 0.001) of the variation in the VKT of test settlements and improved with the addition of a country fixed effect. The CUFET on average gave estimates of VKT within 24% of Climate TRACE-calculated VKT for countries with a GDP per capita between $20,000 and $45,000 and average estimates within 20% for countries with a GDP per capita above $45,000. Increased settlement density was associated with more substantial reductions in VKT in small (50,000 to 88,335) and medium (88,335 to 329,480) sized settlements relative to large (>329,480) settlements. Higher variability was observed in VKT estimates of small settlements. The CUFET VKT was validated by backcasting historical VKT data from 1960 to 2000. The backcasting exercise used historical administrative boundaries and only included high economic output nations (GDP per capita above $20,000 in 2021 USD). Despite these limitations, backcasting achieved a % difference of ~20% for settlements after 1990, suggesting the model can make useful estimates within 30 years of the model calibration year for high economic output nations. The VKT model was used to calculate emissions using a settlement-specific emissions factor. Settlements with annual road transportation emissions per capita greater than 2 t CO2eq have the lowest population densities relative to their populations and are mostly located in the United States, Japan, Canada, and Australia. The nations with the highest transportation emissions are also nations where the CUFET provides the most accurate VKT estimates. The CUFET aims to bridge the gap between academic consensus and local decision-making practice by reducing the barriers to estimate VKT and transportation GHG reduction from shifting to compact urban form.