Traffic along the fast-growing Rte. 5 corridor between the Capital Beltway and Southern Maryland has become so congested that Prince George's County officials are making road improvements a condition of selling water to Charles County, which needs a new source to further development.

Prince George's County Executive Parris Glendening said that until state officials place the Rte. 5 project on Maryland's schedule of capital road projects planned through 1990, "the water option for Charles is definitely on hold. . . . We realize that could take years."

"We want to be good neighbors, but Charles County wants to buy water that would bring more development to the same saturated, overcrowded strip of road where we are turning developers away," Glendening said.

Charles, which depends on groundwater, has proposed to tap into the water supply of the Washington Suburban Sanitary Commission, which supplies treated river water to suburban Maryland. The water request has been pending since last year.

One project that has been put on hold in Prince George's in the meantime is the Mattawoman New Town in the southern part of the county, turned down last year by the County Council because of "severe" transportation problems in the Rte. 5 corridor, Glendening said.

Officials of the four counties concerned about the road improvement -- Prince George's, Charles, Calvert and St. Mary's -- recently formed a regional transportation task force and plan to pressure the state to make the corridor a priority.

State officials say a major study of the corridor is to begin this summer, but no construction funds have been committed.

Marland Dean, chairman of the Charles County Commissioners, said that while water was critical to his county's growth, it was not the only reason for joining the regional effort to lobby for better roads.

"We can drill more wells and we will, if necessary. But our immediate concern is improving Rte. 5 because all four counties are being held captive by the lack of capacity on that road," Dean said.

Dean said he remained confident that a water compromise could be worked out with Prince George's.

A meeting on the proposal is scheduled for mid-March.

The idea to form a regional transportation committee originated with Glendening, who said the roads in question are "vital to the economic health of the whole Southern Maryland area. . . .If we are going to be effective, we have to speak with one voice."

Of specific concern to the counties is Branch Avenue -- the Rte. 5 and Rte. 301 corridor -- from the Capital Beltway south to Waldorf, Md., in Charles.

Commuter and tourist traffic on the fast-growing strip is expected to double at least over the next 20 years, from the current 45,000 cars a day to 90,000, said State Highway Administration district engineer Edward H. Meehan.

"Rte. 5 and the whole Branch Avenue area is definitely perceived as a major problem area. . . .The existing roads will not carry the load we are predicting for the year 2000," Meehan said.

A $142,000 study of ways to improve Rte. 5, which links Washington with Southern Maryland, will begin in July, said state highway engineer Mike Snyder.

"No construction money of any kind is committed at this point, but we are looking at bringing the whole road up to interstate standards by adding lanes and putting in a bypass around the Waldorf and St. Charles areas," Snyder said.

It will be two years before results of the study are known, he said