Your speculation is poor - since there are 193+ countries and the imports from foreign countries is roughly 6 that are road legal by NHTSA.
Germany - Japan - Italy - Sweden - South Korea - England.
Foreign cars that are not produced under approved brands from these countries would be classed as grey market and are subject to import laws, that would likely make your proposed mandate impractical.
Also the number of foreign diplomats in DC's ~690,000 population is unlikely to skew the numbers.
Foreign cars that are not produced under approved brands from these countries would be classed as grey market and are subject to import laws, that would likely make your proposed mandate impractical.
Also the number of foreign diplomats in DC's ~690,000 population is unlikely to skew the numbers.