Maryland Transportation Secretary William K. Hellmann has earmarked $4.2 million in state funds to ease rush-hour traffic jams on Rte. 118 (Darnestown-Germantown Road) in Montgomery County, a major commuter road that was the focal point of a recent squabble between leaders of the General Assembly and Gov. Harry Hughes.

While the widening of the road may be months or even years away, Hellmann's decision should bring eventual relief to Montgomery commuters who use Rte. 118 to travel to and from I-270.

The dedication of money in the state transportation trust fund marks a departure from past practice in Hellmann's own department, which until now had not included the Rte. 118 improvements in Maryland's long-range highway plan and had denounced the method by which the legislature had attempted to fund the project.

Hellmann, who is on vacation, could not be reached for comment yesterday. State and county transportation officials said the secretary's decision should be announced officially sometime this month, perhaps as early as tomorrow at Hughes' weekly news conference.

"I say hurrah," Montgomery transportation chief Robert S. McGarry said of the decision.

The actual construction timetable is still uncertain, but Hellmann's decision clears the way for crucial planning, engineering design and right-of-way acquisition, McGarry said.

He added that he and County Executive Charles W. Gilchrist were informed of Hellmann's intention to set aside funds for Rte. 118 early last month, shortly after Hughes' May 2 announcement that Prince George's County would be the beneficiary of $25 million worth of accelerated road projects.

That Hellmann took pains to inform the Montgomery officials of an upcoming announcement on Rte. 118 further underscored the politics of highway construction in the fast-growing Maryland suburbs.

Montgomery County legislators, who made new highway money their priority in this year's General Assembly, won a concession from leaders of the House of Delegates and state Senate to place $3.5 million in the 1986 state budget for road projects throughout Maryland. Half was earmarked for work on Rte. 118.

Hughes, Hellmann and their aides promptly rejected the compromise as illegal, deriding the budget language as blatant "pork barrel" and the result of unsavory "back-room deals."

Several legislators noted at the time that a key consideration was which politician would be able to take credit for any planned improvement on Rte. 118, a two-lane road whose condition is so bad as to make its widening all but inevitable.

By providing the $4.2 million funding for a project that eventually will cost three times as much, Hughes will be exercising the executive's traditional prerogative concerning road projects and drawing attention to his office on a bread-and-butter issue that is important to voters