The private owners of the Dulles Greenway said yesterday that they would increase weekday tolls by 25 cents for those who are not using Smart Tag, an electronic transponder mounted in car windows that automatically bills credit cards.

Greenway officials said they were not increasing the tolls to make more money but rather to ease traffic backups at the main toll plaza, which have become an increasingly common problem as more drivers use the nearly four-year-old road.

"The goal is to save as much time for everybody as possible," said Rick Froehlich, the Greenway's chief financial officer. "The way I can do that is to allow the traffic through the . . . toll plaza to flow as smooth as possible."

The new toll for drivers continuing onto the Dulles Toll Road will be $1.75 as of Sept. 13--up from $1.50. For vehicles with more than two axles, the toll will increase by 50 cents. Tolls at three of the road's exits will remain unchanged.

The Greenway had been plagued with financial problems since it opened in 1995, largely because drivers balked at paying the toll. But Greenway officials said that their financial outlook has improved dramatically and that daily trips on the roadway now average 43,000.

The Greenway has been marketing itself as a way to save time. The 14.5-mile road runs between Dulles International Airport and Leesburg and is an alternative to traffic-choked Route 7 and Route 50, which was not designed as a major east-west commuter route. But Froehlich said the time savings provided by the Greenway are being jeopardized by eastbound morning "slowdowns" at the toll plaza of up to two miles.

Currently, close to 60 percent of the drivers who use the Greenway use Smart Tag and can go through a specially designated lane where it's not necessary to stop at a toll booth. The Greenway has been using the Smart Tag system since May 1996. The device also can be used on the Dulles Toll Road.

Several people who use the Greenway said they did not want to hassle with Smart Tag and would use other roads instead.

"Almost $4 for a trip, that's just too much," said Diane Morano, as she pumped gas yesterday at the Shell gas station in Leesburg a few miles from the toll road exit. Morano, who is a stay-at-home mother with two children, said she uses the road a few times a week to go shopping in Fairfax County. "It's ridiculous to pay that much for a two-hour trip to the mall. That could be lunch instead of paying to drive on a road."

Tracy Caldwell, a benefits specialist who uses the toll road on her morning commute from Leesburg to Herndon, said she was relieved to already have Smart Tag.

"I normally don't have the cash on me, and I don't like being caught in all the traffic and stoplights and everything, so the toll road's easier in the mornings," Caldwell said.