Prince William County will establish an express bus service to employment centers in Northern Virginia and the District late this fall, providing another option for 45,000 residents who commute by car.

"We're looking for cost-effective ways to help residents get to and from work," County Transportation Planner Craig T. Leiner said.

Using a $1.4 million grant from the Virginia Department of Highways and Transportation, the county will buy about 20 renovated buses and lease them to private operators. The county, in one of a handful of such public-private sector arrangements in the nation, will retain title to the buses and will monitor their use and maintenance, Leiner said.

Board of County Supervisors chairwoman Kathleen K. Seefeldt has called transportation a major problem for the county that must be solved within this decade.

The State Aid for Experimental Mass Transportation program that provided this grant also enabled the county in 1980 to establish its ride-sharing program, which now has a full-time director, County Transportation Planner Raymond B. Palfrey said.

"On a per capita basis," Palfrey said, "it's the largest in the country, having matched abut 3,400 residents over the three years with a car pool or van pool. And currently an estimated 7,650 commuters ride-share."

Nine hundred Prince William commuters daily use Trailways, Greyhound and Colonial Transit, Palfrey added.

Other transportation options for Prince William commuters are under consideration by the Metropolitan Washington Council of Governments. A Virginia highway department commuter rail study, examining the feasibility of service on existing railroad lines, is expected to be completed next spring.

The county's timetable for beginning the express bus service calls for several steps. The first--to select a manufacturing company to locate, buy and renovate the buses--should be completed after mid-July.

Companies receiving invitations to bid include such firms as the Blitz Corp. of Chicago, NIMCO Bus Division of New Jersey and Hausman Bus Sales and Parts of New Jersey, Leiner said.

ATE Management Services, the county's consultant firm that helped to develop vehicle specifications for the bidding, will provide two inspectors at the renovation facility to monitor the process.

Renovated buses can be expected to last eight to 10 years and cost $50,000 to $80,000 each, or about 50 percent less than the price of new buses, Leiner said.

The county is beginning to develop a competitive process for the next step in the program: selecting one or more operators later this summer.

"One of the most important factors in the selection is the company's ability to safely operate and properly maintain the buses," Leiner said.

The county will look to its consultants for assistance in developing maintenance requirements and a monitoring system, he said.

While the operator selected will establish the routes and the fares, Leiner said, riders will be picked up in the county and taken to such employment centers as the District, Rosslyn, the Pentagon and possibly Tysons Corner.

The county recently completed a mass transit study and a telephone survey to determine commuting patterns. Most trips average 35 miles one way, Palfrey said, and end in the District or Arlington.

"There will be opportunities for this bus service to complement the county's ride-sharing program," Leiner said. "For instance, a car-pool rider who wants to work late may be able to catch one of the express buses."

The county has not decided how to spend the money collected from leasing the buses, Leiner said, but it probably will be used for administrative costs or go into a fund to buy replacement buses.