Metro's subway trains will begin running on the new "blue line" July 1 and for the first time commuters will be able to cross the Potomac River between Washington and Virginia without having to battle traffic, bridges and weather.

After seven years, six months and 22 days of construction, after expenditures totaling $1.2 billion, the subway system will expand from five to 17 miles and will give area residents their first real opportunity to find out what rapid rail can mean to their travel habits.

Twelve new miles of rail and 17 new stations will open, linking by rapid transit all of the major federal employment centers of the area's urbanized core, its downtown business district, its primary airport and its stadium.

Then, in phases over the next few months, two-thirds of the 775 bus routes, which look like a tangle of spaghetti on a regional map, will be changed or eliminated to make them feeders to the rail system, with its improved speed and reliability. However, no route changes are planned for the same day the subway opens. The first shifts will come on July 17.

The opening of the new subway line will be accompanied by four major changes in the way area citizens use their public transit system:

D.C. residents will have to pay 50 cents to board Metrobuses during rush hour, a 10-cent per ride increase. The fare remains 40 cents the rest of the time. There are no changes in suburban bus fares.

An electronic fare-collecting system becomes the only way to buy a subway ride. The system, called Farecard, requires subway riders to feed a prepurchased ticket into a machine at both ends of their train trip.

Transfer procedures between train and bus change drastically. In brief, riders will no longer be able to transfer from bus to train, but must pay two full fares. However, they will be able to transfer from train to bus. Total round trip fares for most people will be about the same as they are for bus-only trips today.

Elderly and handicapped citizens who are properly certifi- with Metro will be able to ride both the bus and the subway at reduced rates all the time. Today, they must pay the full, regular fare during rush hours.

Auto commuters who do not live close to Metro stations will find it very difficult to park cheaply and walk to the train until more stations are opened in the coming months. Metro estimates that there are 860,000 commercial parking spaces close to Metro stations, but that most of them are taken and all of them are expensive, because the new Metro stations are mostly in downtown D.C. or high-density Arlington locations.

There is an exception. The Stadium-Armory Metro station is close to a 1,200-space parking lot located north of the Armory on East Capitol Street. That lot will charge only $1 for day-long parkers.

Traffic around the stadium station will be impaired for a few weeks yet, however, because not all of the Metro-related surface construction is complete.

Even though there aren't parking places, regional traffic planners expect that automobile movements around the Metro stations will increase, particularly in some critical locations such as Rosslyn.

"My biggest concern is that people will try and develop a 'kiss'n ride' stop at the Rosslyn station," said Henry Hulme, Arlington's transportation director. "Kiss 'n Ride" is the transit industry term for spouses who drop off spouses at the train station and keep the car for the day. The Rosslyn station is in a traffic-dense area and will be a major bus terminal as well. There simply isn't room for a lot of cars.

There are Kiss 'n Ride facilities in the operating Metro station at Rhode Island Avenue and a Kiss 'n Ride lot is under construction by the D.C. department of transportation near the new Stadium-Armory Metro station. It will not be completed by next Friday, however, and when it is completed it is not going to be as easy to use as it probably should be. Kiss 'n Ride drivers are certain to clog traffic immediately next to the station entrances and the bus bays on 19th Street because to do so will appear to be quicker than using the Kiss 'n Ride lot.

Regardless of what the planners think, however, they won't really know what they've got in terms of traffic patterns and changes in commuter habits until the trains have been operating for a while, the bus routes have been changed to feed them and people have learned to use the system.

For example, it will take riders a certain amount of time to learn how to buy Farecards and operate the automated subway fare collecting system. That is one of the reasons Metro is not changing its bus routes the same day it opens the new subway route. "Maybe we can spread the confusion out a little," one official said.

People who use both bus and subway will have to learn to juggle Farecards, exact change, tickets, tokens and transfers in different denominations than they use today.

That inconvenience arises because bus fares are established by each jurisdiction, or "balkanized" as Metro General Manager Theodore Lutz has said. That results from differing philosophies as to how much public transit should be subsidized.

In the District, about half the transit cost is picked up by the taxpayer. In the Virginia suburbs, it is policy to collect two-thirds of the cost from the fare box. In Maryland, the practice has been to subsidize at a rate somewhere in between the District and Virginia.

The result is a wide difference in bus fares and how they are charged. Those differences were compromised, however, when the subway fare structure was decided.

A subway ride will cost 40 cents for the first three miles, no matter when the rider boards. During rushhour, (6 to 9:30 a.m., 3 to 6:30 p.m., Monday through Fr*iday) it will cost the rider 7.5 cents for each additional mile will cost 3.75 cents. Farecard figures it all out instantly and charges accordingly, which is the reason the rider must report to the computer at both ends of his trip.

Washington area residents will be able to move around downtown with remarkable ease when the new line opens and gives the system a total of 17 miles and 24 stations. The Connecticut Avenue corridor is now connected with the traditional central business district on E, F and G Streets in a matter of minutes.

Major federal employment centers - Capitol Hill, Federal Triangle, New Southwest, Rosslyn, Pentagon, Crystal City, National Airport - are all easily reachable by rail. There is a heavy amount of mid-day traffic between the innumerable agencies and offices that have employees in these areas.

Will the government encourage such travel by subway instead of staff car or interagency shuttle?

When Metro opened its first five-mile segment downtown more than a year ago, officials thought that the rush hour would follow the traditional morning-evening pattern. Not so. Metro's rush hour is noon, as workers shop, take a ride or eat lunch in a different neighborhood.

Will Pentagon workers ride to eat lunch at Connecticut Avenue just as attorneys and employees at the District courthouses began to do when the first line opened? Nobody knows. But travel times are so impressive - the Pentagon to 18th and I Streets NW in six minutes, for example - that the temptation will certainly be there.

What does it mean for Washington's taxi business? If downtown travel during rush hour has been the problem, it's no longer a problem for those who don't mind walking a block or two to the nearest Metro station.

Subway users will find that the system has some disappointments. The National Airport station is just not as convenient to the airport terminals as it should be; the Stadium-Armory Station would be more appropriately named the Jail-Hospital station, because it is closer to both the D.C. jails and the D.C. General Hospital than it is to the Stadium; there is no station in Georgetown, nor will there ever be one; the closest station to the Kennedy Center is five blocks away.

But those who are willing to try the system will find that the heart of area's employment center is very well served and that there is, suddenly, a quick way to avoid the traffic, the noise and the hassle of driving or even taking the bus through downtown.

At a small private luncheon gathering of federal transportation specialists recently, a man who has major responsibility for cost-cutting decisions on Metro's future and who has a reputation for being very tough on such questions, was asked by another federal employee what he thought.

"Fifteen years from now," he said, "everybody will look back and say what remarkable foresight we had to build Metro - and all the cost questions will seem insignificant in relation to the benefit."