Plunge into the narrow streets of the St-Severin quarter to the east (rue St-Severin, rue de la Harpe, and rue Galande).
Cars cannot fit down the narrow streets of St-Severin.  
mnli hypothesis: Cars cannot fit down the narrow streets of St-Severin.   premise: Plunge into the narrow streets of the St-Severin quarter to the east (rue St-Severin, rue de la Harpe, and rue Galande).