In what are seen as victories for suburban development interests, Transportation Department officials have changed two important Carter administration decisions on freeways designed to by-pass urban areas.

The department's Federal Highway Administration last week reversed the Carter decision to block an interestate highway bypass of Dayton, Ohio. A month earlier, the administration deleted some pro-downtown requirements from an uncompleted section of the Richmond beltway.

In both Dayton and Richmond, DOT came down on the side of the state highway departments of two Republican governors, James A. Rhodes and John Dalton. The Carter administration decisions had favored the positions of two Democratic mayors, James McGree and Henry Marsh.

Leonard B. Simons, assistant executive director of the U.S. Conference of Mayors, said the conference "is extremely disappointed" with DOT's Dayton decision. "We would think that any full review of the environmental impacts would include the very negative impact that building that road will have on the city of Dayton."

Pro-city interests couldn't wait to be included on the interstate building program when it was first announced in 1956, but they reversed their field a decade later after it became obvious that urban freeways were contributing to a decline in inner-city population and encouraging suburban sprawl. There are still about 20 controversial and unapproved segments on the interstate highway system map and current law requires final decisions within the next two years.

At issue in Dayton is a proposed 16.5-mile stretch of Interstate Route 695, an eastside bypass of the city. Carter administration Transportation Secretary Neil Goldschmidt, who successfully halted a major freeway himself when he was mayor of Portland, had said only four miles of the route should be built to interstate freeway standards, and that those traveling the remaining 12.5 miles of the suburban corridor could well be served by a less grand road.

Proponents for all 16.5 miles came from the suburbs and the state highway administration, which appealed Goldschmidt's decision after the Reagan team was sworn in at DOT. What DOT approved last week was an environmental impact statement affirming the environmental acceptability of the entire road, not just four miles of it. That was the last major administrative hurdle that the road faced.

In Richmond, Goldschmidt had ruled that the beltway could be completed with the construction of a bypass to the east, but that there would be an 18-mile stretch with no interchanges and that the road would carry signs advising motorists of a lack of tourist services.

After the national election, the Virginia Department of Highways and Transportation sought and subsequently received the restoration of two interchanges in the 18-mile stretch, plus the elimination of the signs.

As in Dayton, downtown Richmond forces were concerned that the high-speed bypass would draw inner-city businesses (and the tax base they provide) out to suburban interchanges.

Linda Gosden, an assistant to the current transportation secretary, Drew Lewis, said yesterday the Dayton and Richmond decisions were in accord with the affected areas. Highway administration officials, she said, "want to make these decisions so they are made in the context of normal environmental processing instead of a process that gives a single locality" the right to veto or modify a project.

In both Dayton and Richmond, Gosden said, "the state had complied with all existing social and environmental requirements, and the support for the highways was bipartisan, if not unanimous."

The Carter administration followed urban transportation guidelines that placed heavy emphasis on preserving and protecting cities. Those guidelines, according to a Reagan administration official, "are totally inconsistent with the Reagan philosophy of eliminating hurdles . . ."

McGree, the mayor of Dayton, said yesterday he had urged Lewis to uphold Goldschmidt's decision after he had learned it was being reviewed, but that he had "received no correspondence" concerning the basis for the new decisions.

"This administration will do whatever they want to do anyway," McGee said. ". . . The programs they are working with kill the cities. I think they believe the suburbs are going to carry this country."