Cabot, Cabot & Forbes’ Deco complex across from the MBTA’s Quincy Adams station is part of a multifamily building boom in Quincy.

A new report from the Metropolitan Area Planning Council suggests that the supply of parking in a multifamily building may affect who chooses to live there, and that Boston-area developers are building too much parking overall.

“Providing abundant parking does not necessarily result in a substantially larger percent of unused spaces, because the availability of easy parking may induce residents to buy or keep their car,” the report found. “Just like a gas expands to fill the space provided, it seems that parking demand expands along with supply, tending to fill about 70 percent of the spaces that are provided.”

The report found in the nearly 200 buildings surveyed in communities inside Route 128, most built since 2000, the relationship held true no mater how many parking spaces were provided per unit – even in developments with less than 0.75 spaces per unit, only 74 percent of the spaces were filled on average, while those with more than 1.5 spaces per unit saw only 68 percent of spaces used, on average. Many suburban communities’ zoning requires between one and two spaces per unit in multifamily buildings.

“Even acknowledging that there is a need for some unused parking to account for guests and unusual demand, it’s clear that the developments we surveyed provide more parking spaces than are needed by their residents: only 14 percent of the developments were more than 90 percent utilized,” the report stated.

The 5,910 unused parking spaces the MAPC’s researchers counted in the buildings it surveyed represented about $94.5 million in extra development costs, based on an estimate of $16,000 per parking space, or around $5,000 per housing unit. In majority-affordable developments, the empty spaces represented about $17.6 million, a sum researchers said could have been better put towards subsidizing more affordable units.

To understand current trends in parking supply and demand at multifamily developments, MAPC collected off-street parking data at developments in Arlington, Boston, Cambridge, Chelsea, Everett, Malden, Medford, Melrose, Newton, Quincy, Revere, Somerville, Waltham and Watertown. First, MAPC staff surveyed property managers to determine the amount and type of on-site parking provided for residents. MAPC staff then conducted in-person overnight parking counts to measure parking utilization during peak parking demand hours, when most people are asleep and the number of parked cars was expected to be highest.

Overall, the researchers found three factors explained the parking demand per unit: Parking supply per unit, access to jobs within a 30 minute trip and the share of affordable units in the project. Parking supply per unit was associated with the number of cars per unit parked in a project’s garage. As job accessibility increased, parking demand per unit went down. And as the share of affordable units went up, the parking demand epr unit dropped. Researchers found neither total unit count, the presence of indoor or outdoor parking and the presence of bicycle parking were correlated with parking demand per unit.

“Cities and towns shape the region’s future through their local land use regulations, and ought to implement parking requirements that align with actual use and demand,” MAPC Executive Director Marc Draisen said in a statement. “During this research, we witnessed oversupply of parking in every surveyed community. Reducing excess parking can encourage more housing units, at a lower rent or sales price. Aligning parking supply and demand at buildings near transit can also promote transit use over driving, which in turn alleviates congestion.”

The MAPC report suggested developers “unbundle” parking from the main rental package – requiring tenants to rent a parking space separately from an apartment – and offer fewer parking spaces when near transit or significant numbers of jobs.