ON THIS sultry summer morning, it might be refreshing to recall how people drove to work a year ago and compare it with the way a growing number of Washington area commuters are riding to work on Metro today. In June of last year, Washington simmered through the worst month ever in the crawling gas lines and then began climbing aboard Metro trains and buses in unprecedented numbers. It will be interesting to see what effect, if any, the recent fare increases will have on Metro's ridership. Now, even though Metro has failed to keep all the riders it added during the first half of last year, the figures show that a substantial number kept their seats and that Metro, like other mass transit systems across the country, so far has been experiencing steady monthly increases in ridership.

In March 1980, for example, the last month for which figures are available, the average weekday total of people who rode Metro trains and buses was 633,021, a 14 percent increase over the previous March and a 36 percent increase over March 1978. There are several reasons Metro's ridership is growing. One, according to Metro General Manager Richard S. Page, is the opening of the new Orange Line between Rosslyn and Ballston in Arlington County. Another factor is that more Metrobuses now feed into subway stations. And then, since last fall, people have been able to ride the subway on Sunday. Obscured in the near-exclusive focus on the glamorous new subway system is the impressive increases in bus ridership -- 19 percent -- between March 1979 and March 1980.

Still, for those city and suburban residents who use Metro daily, mechanical failures, long waits and over-crowding on certain subway and bus lines remain a tiresome fact of their commuting lives. In addition, the higher fares will disproportionately affect some people -- the elderly and the poor in particular. Despite the recent fare increases, a safe prediction is that Metro's ridership will continue to grow. Fine. But as more people ride Metro, Metro's responsibilities to them also grow. This means ensuring that trains and buses strictly follow schedules, receive regular maintenance checkups and are cleaned each day. And now, in the sweltering heat of summer, Metro has a special obligation to all its riders to keep trains and buses cool; and it ought to worry about the excessive crowding that has begun developing during rush hour in particular. Metro has proved so far to be a popular way of moving in and around metropolitan Washington. But Metro has to keep earning the trust and support of its growing ridership.