A SMALL and, to our mind, insignificant roadblock threatens to halt the work of the Pennsylvania Avenue Development Corporation. It takes the form of a rider to a bill, now awaiting President Carter's signature, which would extend the life of the corporation and continue the flow of federal financial support. The troublesome, rider would establish a historic district in San Antonio, Texas. There is some sentiment in the White House that is a waste of money, and we can sympathize with the administration's urge to economize - and to be consistent about it. But we think that putting a hold on the Pennyslvania Avenue plan for the sake of killing the San Antonio project might very likely turn out be a case of false economy.

The development of Pennsylvania Avenue, after all, is a matter of establish policy; once begun, it makes no sense not to finish it. Since its creation in 1972, the PADC has worked to redevelop and revitalize the Avenue from the White House to the Capitol. It has come up with a careful plan for a mixture of commercial, residential and cultural activity. It has solicited significant amounts of private investment. Those investments, in turn, stimulate the local economy by providing jobs as well as new sales-and income-tax revenue.

But to continue its work, the corporation needs an uninterrupted flow of federal support. It goes almost without saying that a slowdown and eventual halt to this work for any protracted period, at a time of steadily rising constrution costs, would only make the whole thing more expensive in the end.

There has been some legitimate concern in Congress about the continuing federal expense of maintaining the San Antonio park project, once it has been completed.But that problem has been eased by provisions for the National park Service and the Catholic diocese to share in the maintenance. The Justice Department has been assigned to protect the federal government's interests in this arrangement.

So we think, on balance, that Mr. Carter can safely sign this legislation without violating his commitment to across-the-board economy. We would further urge him to waste no time filling the lame-duck appointments at the PADC; they include the chairman, the vice-chairman and some corporation members. That would clear the way for the corporation, when it convenes in the next few weeks, to get to work quickly on the business at hand.