WITH ALMOST AS MANY reservations as Washington's hotels have on Inauguration Day, Secretary of Transportation William T. Coleman Jr. has approved construction of I-66, the controversial Northern Virginia highway corridor between the Capital Beltway and the Theodore Roosevelt Bridge. "This decision has been a most difficult and troubling one," Mr. Coleman states, which is most evident when you read through the backing-and-hauling explanation the secretary presents on behalf of his action. In essence, he has now reasoned that today's I-66 plan, for four lanes instead of six or eight, and with certain conditions set by him, is "a substantially different one from last year's" - which he disapproved for a long list of sound reasons.

Though Mr. Coleman goes to great lengths (71 pages) to present and repeat his problems and conclusions this time around, he doesn't really come out and say what followers of this controversy know so well: That Virginia Gov. Mills E. Godwin had him over a barrel.

The deal was this: Either approve I-66 - or watch the Metro rail system suffer. Gov. Godwin, who's fond of highway projects but hasn't been impressed with Northern Virginia's need for subway support, had all the cards, too. For one thing, the I-66 corridor contains a median strip that Metro needs for its planned line above ground from near the Glebe Road station in Arlington to the Dunn Loring station just outside the Beltway's intersection with I-66. So: No road, no right-of-way. Also, the state of Virginia has federal highway money that could be transferred to Metro. But: No I-66, no transfer. In short, Gov. Godwin's offer (shades of Natcher) was not your most refusable one.

Thus it is a case of a governor forcing a federal decision, which is then written with "conditions" that attempt to cushion the damaging effects, including a stipulation that the governor must agree to do what he threatened not to do unless he got his way: help Metro with the median and transfer those other highway funds. In addition, the decision does specify that highway lanes in peak hours would be restricted to carpools, buses, emergency vehicles or those using Dulles Airport.

Well, never mind that on Aug. 1, 1975, Mr. Coleman wrote after lengthy study and reflection that "the improvement of existing roads and highways in the corridor and the extension of the Metro line to Vienna, Va., are prudent alternatives that will meet the transportation needs of the metropolitan area in a manner more consistent with metropolitan development goals and planning objectives and has fewer longterm adverse consequences." And you might as well label irrelevant his conclusion then that "considerations of energy conservation, air quality, noise, conservation of urban and community resources - such as public parks and recreation areas - all suggest the need for a lesser emphasis on automobile use in urban areas, particularly for peak-hour radially-oriented commuting."

In his latest decision, Mr. Coleman concludes, among other things, that - yes - I-66 will "still have adverse social and environmental impacts," but that "the transportation benefits which I-66 would provide in conjunction with Metro, combined with the extensive efforts to be undertaken to minimize the adverse impacts, outweigh the net adverse social and environmental effects of the proposal." So, after more than 20 years on the drawing boards, the I-66 corridor may now be on its concrete way to becoming what surely ought to be named the Mills Godwin Highway.