When monorail enthusiasts asked Seattle voters for a new transit tax, they promised the new Green Line would be the first route of a citywide system. They would prove that a grass-roots movement could deliver 21st-century transportation.

Now that the contract proposal has been released, it turns out residents would pay 50 years of taxes — roughly twice as long as predicted — for a single, $2.1 billion line.

As the City Council prepares to vote whether to allow monorail construction, a furious debate has broken out over whether such a long tax is a good idea.

How did the Seattle Monorail Project stumble into this predicament?

The main reason is the tax base — vehicles subject to the monorail's citywide car-tab tax — has turned out to be too small to pay off the plain-vanilla, 30-year bonds governments generally sell. Also, the sole bid to supply tracks, stations and trains came in too high.

To cope, the SMP plans to stretch its dollars by selling 40-year bonds, deferring interest payments and selling some so-called junk bonds with relatively high interest rates. In all, payments would total $11.4 billion, including interest.

Monorail board member Cindi Laws said the agency focused too much on the project's possibilities and not enough on its possible problems.

"We got into this because every one of the bad things that could happen, has happened," Laws said.

The agency's former finance director Daniel Malarkey said the SMP is keeping its most important commitment, to deliver the monorail without going back to voters for a higher tax rate.

"The good story is, honest people trying to make a transportation project work within the constraints they promised the voters," he said.

Why a car-tab tax?

From the time tour-bus driver Dick Falkenbury drew his first monorail map in the mid-1990s, the supporters were outsiders, fighting the political establishment. Two pro-monorail initiatives passed, and state legislators voted in the 2002 session to allow a city monorail tax.

The Legislature allowed a property tax, a car-rental tax, a flat car-tab fee, or a tax based on vehicle value.

The state law exempted new cars during the first year of ownership, avoiding a political war between the monorail and auto dealers.

Monorail officials chose the car-tab tax, saying it was environmentally friendly because it discouraged driving, Seattle residents could deduct it on their federal tax returns, and it was fair.

"It ain't perfect as a progressive tax, but people who have more expensive cars are paying more for it. In my view it's a democratic tax, and people are going to be paying it for a while," said Steve Williamson, a monorail board member and local labor leader.

State Treasurer Mike Murphy this week called the SMP's financing strategy "ludicrous" and urged a shutdown.

He said the monorail's goal of 6 percent annual tax revenue growth would mean half a million cars in Seattle by the time the bonds are paid off.

"Where are you going to put all those vehicles?" Murphy said. "Isn't the point of mass transit to reduce the number of vehicles?"

Car-tax revenue has grown in the past, but a single tax source is less diversified than Sound Transit's base of sales tax, a smaller car-tab tax and federal grants.

In 2002, monorail consultants overestimated the number of cars, in part because Sound Transit gave the monorail group a flawed figure for a similar taxing zone encompassing Seattle, Shoreline and Lake Forest Park. The monorail group did not run its own check of the state's auto database.

In addition, the monorail lost money when thousands registered their cars outside city limits, avoiding the tax. The state has closed some loopholes.

Monorail tax collections gained 6 percent in the past year, but totals remain one-third behind the original plan.

If the monorail's original numbers had worked out, the SMP would reap surpluses by the 2020s. That money would have paid the bonds off early — or the cash could go toward additional lines, after a public vote. In 2002, the monorail group published maps of five routes crisscrossing the city.

Laws said it bothers her that other board members endorse a 50-year tax on the grounds future taxpayers will benefit.

"We need the next generation to pay for lines two through five, not line one."

Malarkey's predictions from 2002 showed a small chance of a 40-year tax, and a greater chance that the Green Line could be paid off years sooner.

"That part was always uncertain. We didn't know if it was going to be 20 years or 30 years out. It's going to be farther out than that, but not as long as the useful life of" the monorail, he said.

There are many ways to react when money gets tight on a megaproject; the monorail agency made commitments that limit some choices.

Shorten the line?

The 2002 measure to create the Green Line tax, requires all 14 miles from Ballard to West Seattle. It cannot be shortened without a public vote.

Monorail backers wanted to show they wouldn't follow the example of Sound Transit, which shortened its initial light-rail corridor because of cost overruns.

Raise more money?

The ballot measure fixed the tax rate at $140 per $10,000 of vehicle value.

A tax increase would require another bruising campaign, after the 2002 measure passed by just 877 votes.

Monorail officials have mentioned tapping a proposed "traffic mitigation" fund for the state's proposed Alaskan Way Viaduct tunneling project. Williamson said the monorail will serve commuters while the highway is closed.

Simplify the design?

Design cuts already are under way, yet the price remains roughly $400 million above the original $1.75 billion estimate.

Landmark stations, with elaborate windows and unique roofs, were scrapped in favor of standard open-air layouts. Two stations were cut, and SMP can afford only 13 trains for now, compared with the expected 19.

Falkenbury says the entire project should have been simpler. He faults SMP for introducing extra track switches and multiple column shapes, while Vancouver SkyTrain saves money with standardized trackways.

"What we've done is taken a simple process of building a Tinkertoylike system with two rails, and we've turned it into the opposite," he said. "The basic idea of getting up in the air is a good one — I would swallow right now, and approve the contract."

Stretch the payments?

This has become SMP's main option.

Voters approved a debt cap of $1.5 billion, in 2002 dollars. (As of this month, the cap is $1.7 billion.) The ballot measure put no limit on the overall dollars raised, nor an expiration date on the tax.

SMP intends to time the bond sales and defer interest payments to stay under the cap, which applies only to the principal.

The City Council's financial review is limited to whether SMP can afford to build the line and run it the first five years. Until this week, the council didn't envision debating the wisdom of a 50-year tax.

"That would have to be by the agreement of the City Council, if we are to go beyond the scope," said Council President Jan Drago, a monorail supporter. "I haven't thought about that."

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com