From Appropedia

Free car parking makes urban traffic congestion worse, increases pollution (air pollution and pollution in stormwater runoff from roads) and fuel consumption while reducing the quality of life for pedestrians and residents.

UCLA Professor Donald Shoup, in his 2005 book The High Cost of Free Parking, describes the true cost and the externalities of providing free parking.

"We have expensive housing for people and free parking for cars," he points out.