According to the 2016 American Community Survey, 84.3% of working city of Montgomery residents commuted by driving alone, 8.8% carpooled, 0.4% used public transportation, and 0.6% walked. About 3.5% used all other forms of transportation, including taxicab, motorcycle, and bicycle. About 5.9% of working city of Montgomery residents worked at home. Despite the high level of commuting by automobile, 8.5% of city of Montgomery households were without a car in 2015, which increased to 11% in 2016. The national average was 8.7 percent in 2016. Montgomery averaged 1.62 cars per household in 2016, compared to a national average of 1.8 per household.. How many years did it take for an increase of 2.5% of residents to go without a car in Montgomery?