WHATS THE FASTEST way to get to work? Or the cheapest? Or the easiest? While it won't end the argument that goes on perpetually in most shops and offices, this newspaper sent its staff out to test the various possibilities on seven common routes and it reported the results in aseries of articles last week. Usually the private car was the fastest way to travel - but the differences in time were not as large as we would have expected.

The combination of bus and subway turned out to be very competitive on most routes. The longer the route, the closer the transit system generally came to matching the speed of a private car in rush hour - particularly if the trip required crossing the Potomac. In one case, from Takoma Park to Rosslyn, the bus-subway combination was actually faster. Those results seem to us to reinforce powerfully all the other arguments for completing the full rail system as it is now planned, with its lines reaching well out into the suburbs. It is in the longer commutes that the value of the rail lines will be most fully demonstrated.

The great commuter races, as our staff ran them, were not what you would call a rigorously scientific experiment. A number of people have phoned and written to us to object that in some respects the comparison was biased in favor of cars. For one example, traffic was unusually light over the holidays. For another, some of the reporters were on routes unfamiliar to them and wasted time - as no trained veteran of the daily struggle would do - getting farecards on the way into stations, or looking for bus stops. That's quite true. But there is more involved here than time alone.

Driving a private car to work usually costs between two and five times as much as transit, depending on the route and parking rates.And a faster trip does not necessary mean time saved, if you are the kind of commuter who can usefully read a book - or, might we even suggest, a newspaper? - on Metro instead of staring grimly at the rear end of the car immediately in front of you. To those citizens who have not yet become regular travelers by subway, we will observe that a subway car is a much more comfortable place to read, and is better lighted, than a bus.

The commuter races also illustrated one peculiarly exasperating practice among Metro bus drivers. A reporter starting from Central Avenue in Prince George's County missed the bus because, as it happened, the bus left six minutes ahead of schedule. Running ahead of schedule is rather common - and a major nuisance to commuters, who are usually on tight schedules themselves. That dereliction invites more attention than Metro's management is currently giving it. Metro is already indispensable to the people who live in and around Washington. But they need service that is convenient and reliable as well.