PROVIDENCE — The Sierra Club says it has found ways to help get the state transit system out of its financial hole and enable it to grow.

The organization, a longtime supporter of a stronger state transit system, has issued a study that tries to answer the question hanging over the financially troubled Rhode Island Public Transit Authority — how to pay for its existing service and expand it.

The proposals apparently wouldn’t help with the immediate crisis, which the authority says could force it to eliminate up to 20 percent of its bus service in January.

But the Sierra Club study says that the whole underlying system for financing the bus system, mainly the state tax on gasoline, is a failure and won’t improve. That revenue is fading as high gas prices drive down consumption. The result is that, paradoxically, RIPTA’s revenue is declining when demand for its service is increasing as drivers try to shift to the authority’s buses.

The authority, meanwhile, is facing the latest and deepest in a series of financial crises. It has scheduled public hearings for late this month and early October on a broad set of service cuts it says it needs to balance its budget.

The Sierra Club study says that RIPTA, like other transit agencies, will never pay for itself by raising its fares. To pay RIPTA’s operating costs in 2006 the fare would have had to rise from $1.50 to about $4.35, the study says. To cover other expenses, including borrowing and administration, the fare would have to be even higher, so high that most riders couldn’t, or wouldn’t pay it, the study says.

The Sierra Club says it looked at transit systems across the nation and found a number of financing mechanisms that are supporting strong transit systems. Most of the financing plans depend on increasing government taxes or fees, that would, directly or indirectly, increase the cost of driving. Also, some of the plans are also being looked at by the Blue Ribbon Panel on Transportation Funding, which is focused on bridge and highway projects.

They include:

•Tolls, which the study says are “appropriate where one goal is to displace some portion of personal motor traffic into mass transit.” For example, it says, New York City “aggressively” tolls inter-city traffic, putting much of the money into its huge transit system.

A toll system on bridges and major highways in Rhode Island, the study says, could shift rush-hour traffic in Providence and summer traffic in Newport toward a strong transit system. (The study says that new technology would cut down on traffic delays that toll booths might cause.)

•Employer taxes, like those levied in a defined area in and around Portland, Ore. The TriMet system, created in 1969, collects a payroll tax of $6.62 per $1,000, or less than 1 percent. That pays more than half of the district’s expenses. It operates buses, streetcars and commuter rail service. TriMet says its weekly ridership has increased for 18 consecutive years and that 70 percent of its riders “either have a car available that they left at home or don’t have a car available because they prefer to use TriMet.”

•A statewide tax on free parking. Customers of businesses in urban areas have to pay to park, while the customers of suburban businesses don’t. The study says that a statewide assessment on commercial properties that provide free parking could both produce transit income and help “level the playing field” between businesses inside urban areas and those outside.

•A sales tax for transit. The study cites the Cap-Metro system, centered on Austin, Texas, which collects a local, voter-approved 1-percent sales tax, and the Charlotte Area Transit System in and around Charlotte, N.C., which collects a 0.5-percent sales tax, also voter-approved, that pays more than half of its expenses.

•A surcharge on motor vehicle registration fees.

•A real-estate transaction tax. The study says that the Syracuse transit system, CNY Centro, which serves four counties in central New York, including Syracuse and Utica, relies in part on a one quarter of one percent tax on real-estate transactions.

•A real-estate subdivision tax, where developers would help pay for transit service along with the other “impact” fees they pay, for water and sewer service.

•Support from businesses that transit serves. The study says “business improvement districts” help pay for street cleaning and trash removal, and could also support transit. It says that Portland is using a system like that to support a street car system.

The Sierra Club rejects, for the moment, one strategy it turned up — an added, excise tax on parking. The cost of parking in Providence is already cited as a reason for businesses leaving the city, the study says, and it’s important not to do anything more to discourage businesses.

“We need a transit system good enough to be a plausible substitute for driving before a system of excises like this would work here,” the study says.

blandis@projo.com