BOULDER CITY, Nev. - With take-your-breath views on both sides, the narrow road over Hoover Dam is a rubbernecker's dream.
But the two-lane road has become a major bottleneck on the heavily traveled highway connecting Phoenix and Las Vegas.
Finding a new route over the Colorado River to connect Southern Nevada and Arizona has been a goal of a handful of federal agencies for at least a decade.
The increasing volume of traffic over the dam, especially heavy trucks, makes another route imperative, highway officials say. The question is, where should it go?
The federal government, with allies in commercial trucking and the state, wants to build a $200 million bridge south of Hoover Dam. Instead of bumper-to-bumper traffic inching over the dam and the nearby hard curves, they want a four-lane bridge that would span the river 250 feet higher than the towering dam.
The road across the dam has been designated as a primary transportation route under the North American Free Trade Agreement. Advocates for the new road argue that commerce shouldn't be blocked by sightseers.
Construction, depending on the availability of funding and the successful navigation of the design and environmental review process, is scheduled to begin in 2002 with completion in 2007.
But bridging the river demands an engineering feat almost as audacious as building the 726-foot-tall Hoover Dam. And unlike the 1930s, when the dam was built, environmentalists and American Indian groups are organized to stop what they believe will be an expensive mistake.
The Southern Nevada group of the Sierra Club is one of the foremost opponents. Fred Dexter of the Sierra Club wants to stop construction of the Federal Highway Administration's ''preferred option'' - a 2,000-foot-long span a quarter-mile south of the dam that would shear off the top of Sugarloaf Mountain before crossing the river.
The environmentalists' main concern is the impact the construction would have on the mountain and on Black Canyon, the stunning chasm that begins beneath the dam.
The Sierra Club has allies among American Indians, many of whom view Sugarloaf Mountain and the surrounding area as a religious and cultural icon.
The environmentalists have support from the federal Advisory Council on Historic Preservation. The council promotes ''full consideration of historic values in federal decision making.''
''Construction of the Sugarloaf alternative would introduce a significant visual intrusion into the dam's setting, which is unchanged in any respect from the 1930s,'' the agency said in a June update on the bridge project. ''Several archaeological sites along the Arizona approach also require further investigation.''
Dexter and other environmentalists favor putting the new highway across the river between Bullhead City, Ariz., and Laughlin, Nev. That alternative wouldn't infringe on the Hoover Dam National Historic Landmark or carve up Sugarloaf Mountain, and the plan has been endorsed by both cities.
''On numerous occasions we have asked that the Laughlin-Bullhead City alternative be considered, and it has always been rejected,'' Dexter said. ''The only one they ever considered was Sugarloaf.''
According to the Federal Highway Administration, three crossings were formally evaluated. One, about two miles downstream from the dam, was rejected as too difficult to engineer and because of its proximity to culturally important hot springs.
Another alternative to build a bridge across Lake Mead north of the dam was considered dangerous because of the possibility of a traffic accident spilling contaminants into Lake Mead, the source of Las Vegas' drinking water.
The Laughlin-Bullhead City alternative never was a formal option because it was never workable, says Jim Roller, Federal Highway Administration project manager.
''That alternative doesn't meet the purpose and need of this project,'' Roller said.
The problem is that bringing truck and other traffic through Laughlin adds about 25 miles to the Phoenix-to-Las Vegas trek. Those extra miles would prompt truckers and others to continue using the shorter route over Hoover Dam, Roller said.
Daryl Capurro, managing director of the Nevada Motor Transport Association, a trade group, says truckers support the Sugarloaf option.
The Federal Highway Administration estimates the Laughlin-Bullhead City route would cost $217 million, including substantial improvements to the highways leading to the span. Roller said there are other hidden costs - in gasoline and air pollution - in making the route longer.
The Laughlin-Bullhead City alternative would burn about 5 percent more gas for the Arizona-to-Las Vegas drive - about 150 million gallons of fuel over 20 years. Federal engineers say the Laughlin-Bullhead City alternative would cost taxpayers and road users $1.4 billion more than the Sugarloaf option over two decades - and still wouldn't relieve traffic over the bridge.
But Dexter argues that the federal agency is deliberately omitting the cost of a planned bypass around Boulder City, which could double the cost of the Sugarloaf alternative.
Indians also have raised objections to the project. Roller said they were only invited into the process for selecting a site for the new bridge over the Colorado one year ago - a decade after discussions began on the project, and after Sugarloaf was picked as the primary option.
Tribes are frustrated with the lack of consultation early in the project, said John Lewis, director of the Inter-Tribal Council of Arizona.
Lewis said consultation came after President Clinton issued a directive calling for ''full consultation'' between tribal governments and federal agencies working on projects that affect Indians.
The tribes in Nevada and Arizona also are concerned that the project will go forward despite their concerns, he said. But Lewis also said tribal representatives from both states will participate in discussions to ease any effects.
The federal engineers working on the project said they will ease the impact of a new bridge bypassing Hoover Dam, but they also believe that the bridge ultimately will benefit the dam, both practically by lessening traffic and aesthetically.
But Dexter and others remain unconvinced.
''I love Lake Mead,'' Dexter said. ''I don't want to see the lake trashed out by a four-lane road.''
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Las Vegas Sun reporter Lee Scrivner contributed to this report.
