More than 4,700 SEPTA workers went on strike this week largely because they want an improvement to their traditional pensions - not because management wants to take them  away, as is often the case.

Willie Brown, Transportation Workers Union Local 234 president, said workers should demand retirement security.

"I think it's something we need to reinvest in," Brown said. "People want to leave here and have the ability to have a normal life."

To most workers in the private sector, where traditional pensions have all but disappeared as a major source of income for future retirees, any pension that guarantees a certain amount of money for retirement sounds like a great deal.

But in the public sector, where more than 70 percent of workers have a retirement plan from employers, mostly traditional defined benefit plans, how does the pension plan for TWU Local 234 stack up?

It's not easy to say, because no two pensions are alike, but one analyst said the TWU plan is less generous than average, compared with the 100 largest state and local pension plans.

"It's sort of like trying to describe a human being," said Greg Dash, of Dash & Associates, a Havertown company that assists management in the negotiation of collective-bargaining agreements, with a specialty in transit agencies. "Are we tall, are we short, are we black, are we white?"

"At least in transit, there is no standard approach to retirement benefits," said Dash, who described his firm as "the library of transit labor relations for the industry." He said he provided data to SEPTA this year, but has not been involved in the negotiations.

The differences start with the basic legal structure, Dash said. SEPTA negotiates pension benefits with the union. That's not always the case. In New York and San Francisco, transit workers are covered by city plans. In other places, such as Ohio, transit workers are covered by pension plans established by the state legislatures, similar to public school employees in Pennsylvania.

The key pension issue for SEPTA and its workers is a $50,000 cap on annual wages that count toward a worker's pension, even though the worker has to keep making 3.5 percent pension contributions on income beyond that. This caps a 30-year worker's pension at $30,000 a year, which the union considers unfair because managers face no such cap.

The union negotiated the $50,000 cap in 2001, according to a TWU newsletter. Adjusted for inflation, the cap would now be more than $68,000, which is about equal to average pay with overtime. That would result in a maximum annual pension of $40,800 for a 30-year worker.

On Wednesday, SEPTA management offered to remove the $50,000 cap and increase pension benefits by 8 percent. Further details were not available.

Historically, such caps were designed to remove an incentive for workers to amass large amounts of overtime toward the end of their careers in a bid to boost retirement income. In SEPTA's case, TWU members' pension is calculated based on the average of the three highest years of pay in the last six years before retirement.

Three years is a relatively short period, Dash said. "Four or five is more typical, which would tend to lower your average earnings," he said.

Stephen Herzenberg, executive director of the Keystone Research Center, in Harrisburg, who has conducted extensive studies of public-sector pensions, compared  SEPTA's TWU plan with the 100 biggest state and local pension plans.

A big knock on SEPTA's TWU plan is that it has no automatic cost-of-living-adjustment, Herzenberg said. He also compared the employee contribution of 3.5 percent and found it low, equal to the 12th lowest among the 100 plans. That translates to a generous pension. On the other hand, the multiplier used to calculate a worker's pension based on the final years of pay, at 2 percent, is in the middle of the pack, Herzenberg said.

Based on those three factors, the TWU pension is about average in terms of generosity, compared with the 100 largest public pension plans in the country.

But that is before counting the cap. Given average annual pay of $68,100, the effective, union-wide multiplier is just 1.47 percent. Assuming the final average salary at retirement is higher than the overall average, the effective multiplier would be even lower.

"So, overall, the SEPTA pension plans are less generous than most because of the lack of inflation protection and the cap on the portion of salaries to which the pension applies," Herzenberg said.

Staff Writer Jason Laughlin contributed to this article.