In a letter to the editor the other day, downtown Washington merchant Joseph E. Threatt contended that low-cost municipal parking is needed in downtown Washington to make downtown shops competitive with suburban malls.

Threatt may find this surprising, but Metro Scene agrees wholeheartedly. The downtown area has long needed several well-located large ramp-type garages with prices tilted to favor shoppers as well as tourists who park for relatively short periods but -- mark this! -- with prices rigged to limit their use by commuters who park all day.

In his letter in Saturday's paper, Threatt asserted that this column "has inveighed repeatedly" against parking projects that would benefit merchants and their customers.

To the contrary, this column never has focused upon that specific issue. We would support such projects, while opposing absolutely all free or subsidized parking for government or private-sector employes who work daytime hours in the downtown area and projects that would disfigure the cityscape.

In my view, commuters who drive -- whether they work for the White House, Congress or for government agencies or for private employers, including mine -- should pay the going commercial parking price in return for the convenience of driving to work. (To his credit, President Carter tried to do this to civil servants, but was shot down in the courts.) If commuters car pool, splitting the tab is by no means ruinous. Full pricing would help balance the equation between driving and the use of public transit.

Alas, Threatt's wish for low-cost municipal parking for shoppers is a forlorn one. There is no momentum to make it real. Land costs have skyrocketed, and previously available sites are being gobbled up for building projects.

The city was moving toward Threatt's goal in the late 1950s, squirreling away millions in parking meter revenues with the idea of spending them to build downtown parking facilities. But members of Congress, who then enjoyed such perquisites as permanent gold-colored parking passes from the city's parking lot barons, forbade the city to go into cost-cutting competition with the big boys. The parking meter money was transferred into the general highway fund.