A week or so ago, when a reader complained about the special bus lanes on Route 50 in Arlington, I took the opposite side of the issue.

I said it is futile to try to build enough roads to accomodate a nation that travels exclusively by private auto. We must put more emphasis on mass transit.

There was a lively response from readers. Few topics command as much interest as those that relate to the ease or difficulty people encounter in moving about, especially in going to and from work.

Getting there used to be half the fun, as the travel agents say. For a few minutes each day, even the most harassed among us could enjoy the peace and solitude of a private car - a powerful machine that could be made to go fast or slow at the driver's whim, and guided along shortcuts or over long, scenic routes, depending on the driver's mood. But now, alas, the driver's route and pace are determined by circumstances that are beyond his control - and getting there is no longer even half the half-the-fun it used to be.

Today there are too many people, too many cars, too much pavement, too much crowding, too many accident victims,too much pollution being created and too much energy being used. And each time we build a nw bridge or highway, I said, additional automobiles seem to come out of the wood-work to make the new facility as crowded as the older ones.

Jack C.Martin of the Highway Users Federation immediately challenged this view, and the remainder of the letters that arrived from readers divided about evenly between those who agreed with me and those who thought I was some kind of nut.

What unsettled me was that the best-written letters were from people who disagreed with me.

A note from Arlette Hennessey of Falls Church will illustrate the point. "Generally your answer may have been right," she wrote. "A bus lane is justified on corridors with a regular flow of buses. But setting aside 33 percent of the carrying capacity of a main artery for an occasional bus is indefensible.

"During rush hours, that additional lane could permit a steady flow of hundreds of cars that now plod along in a start-stop, start-stop energy-consuming snarl on two lanes.

"To shunt hundreds of cars aside for one or two buses is also an indefensible waste of taxpayers' dollars.

"I would be more than happy to take a bus to work to avoid driving, but it is absolutely impossible for me to catch one on Route 50 to get to work on time (7 a.m.) at Crystal City. I have verified this with Metro."

Good point. The unavailability of Metro service affects more people than those who work from 9 to 5 may realize. There are tens of thousands of policemen, firemen, nurses, cleanup crews, maintenance people, waiters, butchers, produce handlers, truck drivers, telephone operators, public utility troubleshooters, printers and people in similar callings who must arrive before Metro service begins, or leave work after Metro services ends, or simply live in areas that have little or no public transit service.

Several who wrote said that police enforcement of the bus lane restriction was long overdue because too many scofflaw drivers had been using the bus lane to pass scores of law-abiding drivers, and later would risk accidents as they forced their way back into a legal lane.

Another problem was mentioned by several readers: Many drivers want to leave 50 on the single lane that leads to the 395 interchange near the Pentagon. However, on many occasions that one lane can't handle all those cars, so a backup forms and sometimes extends for almost a mile.

The alternatives a driver faces are equally unattractive: either get into the right (bus) lane early and break the law, or stay legal until the last moment and then hope you can get into the right-turn lane without causing an accident.

Inasmuch as I do not commute on Route 50 each day, I am in no position to form an intelligent opinion about the advisability of maintaining a special bus lane on that specific stretch of highway. That much I will concede.

However, I still think it is sound policy to encourage the use of mass transit and to give priority to mass transit.

It might also pay to provide the public with more buses and trains, and with more extensive service than present patronage justifies. The hope would be that those newly added seats would fill up as fast as newly added bridges and highways do.