The supply is the built infrastructure (e.g., streets, overpasses, highways, and parking lots) to accommodate an increasing number of person-trips or person-miles traveled (demand).
To meet the growing demands of commuters, most cities first try to expand the infrastructure, but many are now:
Running out of space
Suffering from health problems related to air pollution due to auto exhaust
Finding their city centers and neighborhoods disrupted by cars and roads and no longer walkable or livable
Increasing housing costs due to off-street parking requirements
Fostering car trips due to free on-street parking
Paying exorbitant costs for transport-related issues. In some cases, more than 10 percent of a country’s GDP is lost due to wasted time, traffic fatalities, and other things…(Dalkman, H. & Sakamoto, K., 2012)
And yet they are still unable to provide high-quality transportation.