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Long‐term atmospheric NO_x/CO enhancement ratios in megacities provide evaluations of emission inventories. A fuel‐based emission inventory approach that diverges from conventional bottom‐up inventory methods explains 1970–2015 trends in NO_x/CO enhancement ratios in Los Angeles. Combining this comparison with similar measurements in other U.S. cities demonstrates that motor vehicle emissions controls were largely responsible for U.S. urban NO_x/CO trends in the past half century. Differing NO_x/CO enhancement ratio trends in U.S. and European cities over the past 25 years highlights alternative strategies for mitigating transportation emissions, reflecting Europe's increased use of light‐duty diesel vehicles and correspondingly slower decreases in NO_x emissions compared to the U.S. A global inventory widely used by global chemistry models fails to capture these long‐term trends and regional …