Despite a boost in Maryland's gasoline tax for highway improvements, the threat of a moratorium on new development still hangs over Montgomery County's flourishing I-270 corridor because roads there are inadequate, according to the county's planning board chairman.

Chairman Norman L. Christeller said the ordering of the development halt will depend on what road projects the state undertakes in the county with the extra tax money. County officials will be closely watching that decision making, and "we will be keeping our needs known," he said.

"A major project of concern to us is I-370, which is the connection between I-270 and the Shady Grove Metro station," Christeller said. Without the new highway, the area's potential development would be halved, he said. But he noted that the state has held public hearings on its plan for I-370, and "I trust they will be moving forward with it."

In the meantime, he said, the county will continue its efforts to avert a moratorium by spending millions of its own tax dollars to improve certain state roads and, in at least one case, by approving new developments on the condition the developers pay to expand a nearby artery.

County and state officials say the county acted because of inflation, budget cuts and occasional citizen opposition that over the past 10 years forced the state to delay road projects. The delays caused road improvements to lag development and led to the moratorium threat now facing Montgomery County. "We're on the edge of a moratorium all the time," Christeller noted.

The gas tax increase--two cents this year and 2 1/2 cents next year--is estimated to pump $110 million over the two years into the State Highway Trust Fund, which finances at least part of the construction and maintenance of state roads.

Christeller and other county officials had urged the state general assembly to raise the gas tax during its recent session because they feared traffic-clogged roads along the I-270 corridor would trigger a 1973 county ordinance that bans new development unless existing public facilities, including roads, are adequate to sustain the projects.

Christeller said he was disappointed that the legislature didn't go along with Gov. Harry Hughes' request for a tax increase that would have raised more revenue for road improvements.

With the higher gas tax, State Highway Administration officials said they have added the $60 million I370 and a $11 million interchange at I-270 and Falls Road (Rte. 189) in Rockville to their construction schedule. Both projects would begin in 1986 and would receive 10 percent funding by the state and 90 percent by the federal government, said Hal Kassoff, the highway agency's director of planning and preliminary engineering.

The tax increase also will help to insure that the state in 1985 will start building the $13 million interchange at I-270 and Clopper Road (Rte. 117) in Gaithersburg, Kassoff said. The project "will relieve the severe congestion" at I-270's Montgomery Village Avenue exit less than a mile away, he said.

The Clopper Road exit is a project cited by Christeller as a key factor in improving traffic flow along that interstate corridor. He also sees the need for better interchanges at Montgomery Village Avenue and Shady Grove Road. The latter is scheduled to be done with $500,000 in county funds and $3.5 million to $4.5 million in federal money.

Despite traffic snarls on certain roads in the corridor, county officials and developers said construction of already-approved development still is going full force, particularly along the stretch between the Shady Grove Road and Rte. 28 exits.

At Shady Grove, bricklayers are at work on a 110,000-square-foot office building that is the first phase of 800,000 square feet of office space planned by Spaulding & Slye Corp. of Rockville.

Ben C. Trotter, Spaulding & Slye vice president, says the company is proceeding with its 45-acre office park despite local traffic problems because "this area has so much going for it that we don't think the traffic will have a major effect on development."

Further out I-270, near the Germantown Road (Rte. 118) exit, Bellemead Development Corp. of New Jersey is planning several office buildings totaling more than one million square feet next to Fairchild Industries in Germantown. Groundbreaking for twin buildings of 106,000 square feet each is scheduled for this month.

County planners have approved 5.2 million square feet of commercial and industrial space in the Germantown and Gaithersburg areas and about 3.5 million square feet more along I-270 from North Bethesda to Shady Grove. About 500,000 to 600,000 square feet of office space is built annually in the corridor, planners said. In addition, developers have been given the go-ahead to build 4,800 houses, 12,000 town homes and 4,500 garden apartments between North Bethesda and Germantown.

While there is plenty of development already approved, county officials and some developers fear that I-270 corridor traffic congestion will scare away some new projects.

"A developer is not going to develop a speculative office building if the road situation is so bad that people can't get to work because of the traffic," Christeller said. "People have a much lower tolerance for traffic congestion out there in the suburbs than they do in downtown Washington. I guess they psychologically expect traffic downtown."

Jeff Zyontz, a county planner, noted: "There comes a point, and Shady Grove may be where we've reached this point, where government controls on growth are not as stringent as market controls. It may be no longer a desirable place to work, when it takes an hour and a half to get out of the parking lot."

Thus far, traffic problems alone have not changed corporate minds in locating along the corridor, although some firms have cited it as one of many reasons for building elsewhere in the Washington area, said Duc H. Duong, business development manager of the county's office of economic development.