Riding public transportation saves individuals, on average, $9,499 annually and $792 per month based on the Oct. 14, 2010 average national gas price ($2.83 per gallon-reported by AAA) and the national unreserved monthly parking rate, according to the American Public Transportation Association's (APTA) monthly "Transit Savings Report."

The national average for a monthly unreserved parking space in a downtown business district is $161.56, according to the 2010 Colliers International Parking Rate Study. Over the course of a year, parking costs for a vehicle can amount to an average of $1,939.

APTA calculates the average cost of taking public transit by determining the average monthly transit pass of local public transit agencies across the country. This information is based on the annual APTA fare collection survey and is weighted based on ridership (unlinked passenger trips).

The assumption is that a person making a switch to public transportation would likely purchase an unlimited pass on the local transit agency, typically available on a monthly basis.

APTA then compares the average monthly transit fare to the average cost of driving. The cost of driving is calculated using the 2010 AAA average cost of driving formula. AAA cost of driving formula is based on variable costs and fixed costs. The variable costs include the cost of gas, maintenance and tires. The fixed costs include insurance, license registration, depreciation and finance charges. The comparison also uses the average mileage of a mid-size auto at 23.4 miles per gallon and the price for self-serve regular unleaded gasoline.

The analysis also assumes that a person will drive an average of 15,000 miles per year. The savings assume a person in two-person household lives with one less car.