As a Metro train sped into Arlington's Ballston Station this week, Nicholas Evans reflected on what he considers a devil's bargain facing him at the polls Tuesday.

Does he vote for a sales tax increase that would raise money for what he loves -- Metro -- when it would also fund what he hates -- more road building in the region? His answer is a thoughtful, but emphatic, yes.

"I'm really interested in the public transportation money that will help the commutes around here," the 29-year-old consultant said, referring to the revenue that would go to Metro and other transit projects. "I'm less excited about the road construction and what that means for allowing more cars on the road. But we've got to do something to make everyone's commute easier. . . . This is a tough choice either way."

It's a quandary faced by many voters in Alexandria and Arlington this final week before Election Day, when voters across Northern Virginia will decide whether to raise their sales tax from 4.5 cents to 5 cents per dollar, an increase that would bring in $5 billion for transportation projects over 20 years. Forty percent of the revenue would go to transit projects, supporters say.

Many residents in the close-in suburbs say the additional money would help pay for things the city and the county desperately need: new Metro cars, light rail on Columbia Pike and Route 1, not to mention vital repairs on aging bridges. But they also worry that if more money goes to build and widen roads elsewhere in the region, more commuters will cut through their neighborhoods heading into the District from the outer suburbs. Although almost all elected officials in the inner suburbs have enthusiastically endorsed the tax increase, voters appear more ambivalent, even when they are supportive.

"It's a trade-off," said David King, 27, a lawyer from Arlington, adding that he will vote for the increase. "I guess we're going to get wider roads for more rail, but I think it's worth it."

Poll numbers released last weekend indicate that most voters in the inner suburbs favor the tax increase. A Mason-Dixon poll of 400 registered voters in Alexandria and Arlington found that 61 percent were in favor of the measure, while 29 percent were against it and 9 percent were undecided. These results largely mirror polls done earlier this fall by advocates on both sides of the issue.

The numbers also appear to reflect the conventional thinking that Alexandrians and Arlingtonians are more dependent on public transportation than those elsewhere in the region and are less likely to share the aversion to any tax increase that has been expressed by voters recently in Prince William and Loudoun counties.

"We're looking to get solid support from the inner counties," said John Milliken of Arlington, president of the Citizens for Better Transportation. Together, Arlington and Alexandria voters make up 19 percent of the electorate that will vote on the measure. "I think the improvements that are in the plan for more Metro cars and light rail are resonating with people there," he said.

Yet when the package of projects to be funded was first made public, many inner suburbanites worried that the proposal -- even with its projected 40 percent allotment for transit -- was more about road building in faraway places than about meeting their needs.

The Arlington County Board delayed its endorsement until there were guarantees that the county would not be vulnerable to road widenings imposed by other jurisdictions, particularly along Interstate 66 inside the Capital Beltway. Not until local leaders secured guarantees that each jurisdiction would have veto power over any project did the board publicly support the tax increase.

"I understand the concerns that many people have, but there are protections to assure that we get our fair share and we don't get what we don't want," said Chris Zimmerman (D), the board chairman, who helped develop rules giving jurisdictions the right to nix local projects. "This revenue cannot be used to widen Interstate 66 if we don't want it."

Tax increase opponents say that inner suburban ambivalence could well turn into "no" votes by Election Day. They are reminding those who drive along King Street and Clarendon Boulevard, for instance, that the higher sales tax would fund few projects to make their commutes easier. Much of the opponents' strategy has been to characterize the tax increase not as a boon to walkable communities served by transit but as a way to provide more roads and encourage sprawl in the outer suburbs.

"We've been telling those in Arlington and Alexandria that the Metro improvements they're being promised are a Trojan horse," said Stewart Schwartz, executive director of the Coalition for Smarter Growth, which has led much of the opposition to the increase. "We hope [they] understand that this tax hike is about building roads and inducing sprawl, not helping people closer to D.C."

That message seems to have resonated with some voters. Although in dozens of interviews, residents showed considerable support for the tax increase, they also worried that the measure may not provide enough benefit to Alexandria and Arlington.

"I don't think it's really going to help us the way that we need it to," said Milika Franklin, 41, a teacher who lives off Duke Street in Alexandria. "When I hear about widening roads anywhere around here, I think about more traffic, more cars cutting through our neighborhoods, and that is no good for any of us."

Such is the long-standing concern of inner suburban residents about the effects of traffic generated in the outer counties.

"Residents in the inner suburbs have never been excited about projects that move more vehicles from the outer counties through their neighborhoods," said Robert D. Holsworth, a political scientist at Virginia Commonwealth University. "But with the strong Democratic base there, I think you can expect to see solid support" for the tax increase.

"Everyone is concerned about the quality of life in their particular region," said Sen. Patricia S. Ticer (D-Alexandria), who favors the increase. "The fact of the matter is, this is a groundbreaking opportunity for all of Northern Virginia."

Many seemed to want to strike a balance as they talked about the referendum this week.

"A half-cent is such a minor expense for something that would help a lot of people," said Richard Littell, an Alexandria writer and lawyer who describes himself as an moderate Republican. "I think there's a greater good that can be accomplished here."