They may well be the most expensive pair of highway underpass wall coverings ever created.

Beneath the Brooklyn-Queens Expressway on Northern Boulevard at 69th Street construction workers are putting the finishing touches on two intricately decorated wall coverings made of bronze-inlaid granite, modeled using computer lasers and meticulously chiseled with diamond bits. In the last three years, the cost to taxpayers of the wall coverings and ancillary beautification has risen to $3.3 million, according to documents submitted by the general contractor to the state Department of Transportation.

The two walls’ coverings depict the history of the flora and fauna of Jackson Heights, the Queens neighborhood where the underpass is located. The pair of 240-square-meter coverings – they’re 150 feet long and 16 feet high – was added to the state’s BQE reconstruction project through change orders, contributing to a 17 percent rise in the project’s costs over the past four years. The rehabilitation project, which the DOT calls its largest construction project ever, is about a year behind schedule and is expected to have a final cost of more than $255 million.

The intricate designs of leaves and flowers are difficult to make out beneath the dimly lit underpass, which lies on Northern Boulevard across the street from a White Castle and a Burger King. The artist, Wopo Holup, said the work’s visibility would soon be greatly enhanced. State-of-the-art lighting will be installed along the walls to ensure that passing motorists and pedestrians are able to take in the scenes as they pass by, she said.

“It’s quite marvelous,” Ms. Holup said. “I don’t think anything of this scale has ever been created in the U.S. Sometimes I like to joke that it would have taken 300 Egyptians 300 years to do such a thing.”

Even so, some civic advocates expressed shock at the hefty price tag of the artwork and the state’s unusual choice of location for the installation. To view the new walls one day last month, a reporter and photographer from The New York Sun had to leave the BQE at Exit 38, Northern Boulevard; navigate past orange construction cones on a winding off-ramp; park in the Burger King parking lot; walk across the street through heavy traffic, and squint to make out the designs.

“The bottom of an underpass is where you put things you don’t want to show people,” the president of the public-interest group New York Civic, Henry Stern, said. “I know people have prints in their bathrooms, but they don’t cost $3 million.”

Mr. Stern, who was the city parks commissioner for 15 years, said: “If it’s true that $3 million has been spent to prettify the underside of an arch, it’s ludicrous.”

Some local activists said the underpass was not their first choice for the artwork’s location.

A member of Community Board 3, Tom Lowenhaupt, said local groups requested the panels after learning that the DOT sometimes sets aside funds for artworks as part of large projects. The DOT would allow the art’s placement only on, under, or adjacent to construction, he said. A nearby playground, St. Michael’s Park at 30th Avenue, was considered too remote for the installation.

In addition, the underpass lies at the junction of three different community board districts, representing Jackson Heights, Woodside, and Astoria. That, Mr. Lowenhaupt said, makes it easier to win approval from the disparate community groups clamoring for the funds as a compromise location, he said.

“I tried to get them to put it someplace else,” Mr. Lowenhaupt said. “But they said this is a very significant interchange. Apparently, when you get off the BQE, you can go left or right on each exit. Apparently, it’s the first in New York State and the DOT wanted to highlight their clever transportation project. It just looks like an exit to me.”

A nine-member neighborhood board formed to review the proposed art projects selected the entry of Ms. Holup, who lives in Manhattan. The entry is called “Common Ground.”

The community board’s Web site contains this explanation: “For the road traffic, Holup designed the upper tiers as a pattern of trees, with the top bronze band representing the sky and planetary system. For children walking by on their way to school, the lower bronze band and tier will reveal underground life such as roots, worms, and ancient fossils. The middle tier renders trees and grasses symbolic of Queens.”

One area of the artwork quotes Walt Whitman. Another contains text, all in capital letters, that begins like this: “These images of grass are dedicated to the people of Queens who have come here from everywhere on earth. Corn found in the Tehuaca Valley Mexico dating from 5500 BCE. Oats existed in Europe in the Bronze Age. Wheat domesticated in the Fertile Crescent of Asia 8000 BCE spread through Europe Africa India then to America Australia New Zealand.”

Ms. Holup said she worked on the design for five years. She declined to say how much she was paid.

“It really is a metaphor for the diverse population of Queens,” Ms. Holup said.

Much of the cost to transfer her design resulted from the choice of stone.

Residents of the area worried that plain concrete would be vulnerable to graffiti, so they lobbied for granite, which is more resistant to spray paint and can be cleaned more easily, Mr. Lowenhaupt said. To transfer Ms. Holup’s design into the granite, it had to be traced with lasers, put into a computer, and milled, over a year and a half, using diamond-tipped bits and water.

“Every five years you might get a job this big,” the director of Digital Atelier, the foundry that did the carving, John Lash, said of the wall coverings.

The wall coverings were not included in the original proposal put out to bid for the BQE project. The general contractor, Slattery Skanska, billed the state $1.2 million in its first change order, submitted to the state in August 2001, for the cost of hiring Mr. Lash’s company as subcontractor to fabricate and store the walls until they could be installed, records show.

Skanska added another change order on the Common Ground Holup creation, of $2.18 million, last February. That was for the cost of installing the artwork, setting up the light fixtures to illuminate it, creating simulated stone masonry around the wall, and coating it all with anti-graffiti protective polish. As the general contractor normally does, Skanska also included a markup – of 20% on most items – for overhead and profit.

The BQE reconstruction project covers the area between Broadway and 25th Avenue at the expressway’s eastern side, near LaGuardia Airport. The project includes 4 miles of new retaining walls and 3 miles of new concrete highway pavement.

It aims to realign the widely used roadway above the underpass, rehabilitate and construct 16 bridges and overpasses, and re-landscape adjacent streets. The project began in 2000 and is now expected to be completed this spring.