RICHMOND, FEB. 11 -- The plan to build a privately owned 17-mile extension of the Dulles Toll Road hit a detour today when a member of the Senate Transportation Committee warned about "possible collusion" between the would-be owners of the highway and the developers who have offered to donate land for the right-of-way.

State officials have supported the bill, which they said would substantially speed the construction of the road in an area where traffic congestion is already severe. Extending from Dulles International Airport to Leesburg, the highway would be one of the first privately owned toll roads in the country.

However, Sen. Thomas J. Michie Jr. (D-Charlottesville) said today that because private construction would not require competitive bidding, the company that wants to build the road as a profit-making enterprise might inflate its costs, thereby justifying higher tolls and increasing profits.

To prevent such an occurrence, Michie proposed that anyone connected with the 15 property owners who would give the land be prohibited from owning shares in the road, which would be financed by bonds to be paid off by tolls.

Alexandria lobbyist William G. Thomas, who already nursed the bill through 11 drafts and dozens of "flyspecking" changes for the would-be road builder, Municipal Development Corp. of New York, said such a concern "doesn't make any sense. What would you achieve if you had to bid more and charge more?"

Apprehensions about the novel proposal, also voiced by several others on the 15-member committee, resulted in delayed action on the measure to allow Michie time over the weekend to prepare an amendment banning the joint ownership.

Postponing committee action until Monday leaves less than 24 hours for the measure to come before the full Senate before its Tuesday deadline for acting on Senate-generated bills.

While no one predicted that the legislation would die, one backer on the committee, Sen. Richard L. Saslaw (D-Springfield), said it "has a slight fever."

John D. Miller, president of Municipal Development Corp., said his company could begin work on the road next year and have it "in revenue service" before the end of 1991.

If the state were to build the road, Transportation Secretary Vivian E. Watts said, it could not begin construction until the fall of 1989 and it would take three years to complete. She said the private company could get the road built faster because it would not have to go through the time-consuming process of acquiring rights-of-way, nor would it have to seek bids on construction.

Watts said that even if the legislation is passed, the state could step in and take over the project if it determines the promoters are not carrying through on promises to "build it faster and at an appreciable saving."

The four-lane highway, with space in the median for rail service, would also be a toll road if the state built it, Watts said.

The private operators would be regulated like a public utility, with the tolls and profits set by the State Corporation Commission. The state would take control of the highway 10 years after the private owners paid off the bonds they plan to sell to finance the construction.

"The incentive for the property owners" to donate the land, Thomas said, is that quick completion of the road will allow them to develop their land sooner along the booming Dulles-Leesburg corridor.