As part of a 35 percent service reduction planned for September, the Port Authority expects to close 18 park-n-ride lots with more than 2,900 spaces.

The lots slated for closing are served by routes that will be eliminated Sept. 2 if the state does not provide additional funding to the transit agency, which faces a projected $64 million deficit in its 2012-13 budget.

The closures would eliminate more than one-fifth of the agency's 13,000 park-n-ride spaces. In addition to displacing the riders who use those lots, the closures likely will affect users of other park-n-ride lots, many of which already fill to capacity by 7 a.m.

"Obviously this is going to put more stress on the [remaining] lots," authority spokeswoman Heather Pharo said.

"There are a million and one reasons we want to prevent these cuts," she said. "This is another of those reasons."

The biggest to close would be the 600-space lot on University Boulevard in Moon. It is served by the G3 Moon Flyer, one of 46 routes to be abolished. The lot typically fills to about two-thirds of its capacity, Ms. Pharo said.

The 245-space Alpine Village lot in Monroeville and the 100-space lot at Holiday Park Volunteer Fire Department in Plum, both of which reach their full capacity daily, will be closed as the P12 Holiday Park Flyer is axed.

Other lots scheduled for closure are Glenfield (served by 14 Ohio Valley, slated for elimination); North Park Pool (O5 Thompson Run Flyer); Springdale Volunteer Fire Department (P10 Allegheny Valley Flyer); Spring Garden (7 Spring Garden); East Allegheny High School (P76 Lincoln Highway Flyer); Forest Hills (69 Trafford, P69 Trafford Flyer and P76); Monroeville Mall (P67 Monroeville Flyer); North Versailles and Olympia Shopping Center (P76); Ambridge (14); Covenant Community Church in Scott (38 Green Tree); Neville (21 Coraopolis); North Fayette Volunteer Fire Department (29 Robinson); and Thorn Run in Coraopolis (21).