Massport has filed plans with state environmental regulators to stack additional layers on top of the existing economy garage and build new levels on top of the Terminal E parking lot - moves that would require a waiver from the current state ban on new airport parking spaces.

In the filing, Massport explains what might seem counter-intuitive: How increasing the current number of 18,640 spaces by 27% would be environmentally friendly because of the unique nature of airport parking, the fact that Logan garages and lots are now often at capacity and the fact that people continue to drive to the airport despite such efforts as Logan Express:

Despite Massport's industry-leading efforts promoting and providing transit, shared-ride, and other HOV mode use, the number of private passenger vehicle trips to the Airport continues to increase as Logan Airport air traveler numbers increase. In particular, the constrained parking supply at Logan Airport, which results from the Logan Airport Parking Freeze (the Parking Freeze), has the unintended consequence of causing an increase in environmentally harmful drop-off/pick-up vehicle trips. Survey data collected from the 1970s to the present at Logan Airport have consistently shown that when demand for parking starts to exceed supply, approximately 75 percent of would-be parkers shift to drop-off/pick-up travel modes. These modes generate higher vehicle miles traveled (VMT) and associated vehicle emissions and exacerbate vehicle congestion on the Airport's terminal area roadway and at the terminal curbs during peak travel periods.

Massport further explains why it needs a waiver from the ban on new parking spaces - itself put in place to reduce emissions:

Airport commercial parking differs from other urban parking facilities in two important respects, which cause the drop-off/pick-up phenomenon. First, airport parking spaces turn over much less frequently; thus, more parking capacity is required at an airport to support the same number of vehicles than in an urban/workplace setting. Second, commuters traveling daily to an urban work location will not turn to drop-off/pick-up modes as an alternative means of travel if parking is constrained. Hence, in an urban core such as Boston, parking constraints tend to force commuters to travel by less environmentally harmful HOV transportation alternatives. Unlike urban commuters, air travelers do not go to the airport on a daily basis, so drop-off/ pick-up modes are more practical options. Accordingly, constrained parking at Logan Airport can have the unintended adverse environmental consequence of increased VMT and air emissions.

Massport claims that by building the new spaces, Logan would reduce miles driving by drivers around Logan - and the emissions their cars belch out - by 25% in 2022.

6,114 237 901

Logan parking environment filing (16.7M PDF)