Complex sentence: Curitiba is the Brazilian capital with the highest automobiles/inhabitants ratio, with 1.2 million vehicles for a population of 1.8 million inhabitants, which explains the frequent traffic jams in the city.
Split into simpler sentences: Curitiba is the Brazilian capital with the highest proportion automobiles/inhabitants. The city has 1.2 million vehicles for a population of 1.8 million inhabitants, which explains the frequent traffic jams in the city.