A number of readers have asked for construction details on one of the most significant road projects in the metropolitan area: the widening of I-270 through Montgomery County. About 135,000 cars now use this road each day. In normal rush-hour traffic there's a seven-mile backup in the corridor between the Capital Beltway and Darnestown-Germantown Road (Rte. 118) that lasts inbound from 6:30 to 9:30 a.m. and outbound from 3:30 to as late as 8 p.m. And that's just the normal 5 to 10 mph creep and crawl. It can get worse. "I-270 is the main artery of Montgomery County, and the congestion is tremendous," said Michael Snyder, chief state highway official for the county. The I-270 project began in 1985 to expand the number of lanes in a 14-mile corridor between Tuckerman Lane and Clarksburg Road (Rte. 121). The good news is that the new lanes should help flow significantly. "There will be a freer flow of traffic during rush hour with travel time reduced from 30 to 45 minutes in the corridor between Rte. 118 and the Beltway," Snyder said. The bad news is that the project is not scheduled for completion until the fall of 1991. That's four more years of pain. "Bear with us," Snyder encourages.

Specifically, the improvements will be as follows:From Tuckerman Lane north to Quince Orchard Road (Rte. 124), a distance of 10 miles, the current three lanes in each direction will be widened to four in each direction. In addition, two more lanes will be added to the right side of the road, separated by a barrier, for getting onto and off the freeway. That will make essentially a 12-lane freeway for 10 miles. From Quince Orchard Road north to Clarksburg Road, a distance of four miles, the existing two lanes in each direction will be widened to three in each direction.

Currently, construction crews are working on widening and lengthening three bridges: Falls Road (Rte. 189), Montrose Road and Shady Grove Road. Those should be completed next summer. I-270 segments now being widened are from Montrose Road to south of Darnestown Road (Rte. 28) and from Shady Grove Road to Clopper Road (Rte. 117). The next segment to be widened will be Tuckerman Lane to Montrose Road. That should start next month. During construction, one lane of traffic in each direction is blocked off between 9:30 a.m. and 3:30 p.m.

Highway officials will shift traffic to different lanes while construction is in progress, but there will be no major benefit to traffic flow until the whole project is finished in 1991.

In addition, funding has been secured to expand from four lanes to six lanes each of the I-270 spurs between Tuckerman Lane and the Beltway. That project has not been scheduled, but probably will be started in the early 1990s, Snyder said.

Distraught drivers, and there are plenty of them around these days, might wonder why, in the face of all the development in Montgomery County this decade, the I-270 project wasn't started sooner. "Availability of money, state and federal," Snyder says. The project was begun after local and state officials made the commitment to do so, using money from an increase in the Maryland gasoline tax and federal funds.

While the project is four years from completion, there should be some comfort, Snyder said, "in that people are seeing it happen. Psychologically, that's got to help."

Hidden Entrance Ramps

Dear Dr. Gridlock:

When I am in downtown Washington, I often want to get on I-395 to begin my trip home to Bethesda. I would like to know where the entrance ramps are being hidden. I occasionally spot signs for 395, but after I start to follow the signs, I have gotten lost. A driver needs more cueing than has been provided.

NEIL Z. BEIN Bethesda

Tara Hamilton, spokeswoman for the D.C. Department of Public Works, says her department is aware that "some of our I-395 signs are missing and that a few more would be helpful." Therefore, she said, the department plans to install about 75 more signs, to I-395, Rte. 50 and Rte. 1, to guide people to those main roads. This should be done by the end of the year.

Get Ready for Adams-Morgan Day

Motorists driving around Adams-Morgan on Sunday should take note of traffic restrictions brought about by the Adams-Morgan Day Festival. In addition to difficult going in the area during the day, the following Northwest roads will be closed from 4 a.m. to midnight: 18th Street between Columbia Road and Florida Avenue; Kalorama Road between 18th Street and Champlain Street, and California Street between 18th Street and Florida Avenue.

Dr. Gridlock appears in this section each Friday. You can suggest problems by writing to GRIDLOCK, c/o The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Please include your full name, address and day and evening phone numbers.