At either end of the aging Massachusetts Avenue bridge, police are waving off buses and heavy trucks. On the Boston side, an officer patiently explains to a Peter Pan tour guide there are other ways for his bus to cross the Charles River and get to Cambridge. On the four-lane bridge itself, cars whiz by the green and orange marker barrels that close off the span's outer lanes. As the wheels of a black pickup truck hit an expansion joint, the 92-year-old structure shudders.
The bridge's partial closing is an example of the concern that has been generated about bridges since the June 28 collapse of the Mianus River bridge in Greenwich, Conn. The Massachusetts Avenue bridge is getting special attention from state officials because it has cracks in two steel coupling pins similar to the ones that apparently failed in the Connecticut collapse, which killed three people.
But engineers and other experts stress that the concern generated over particular bridge designs should not obscure a deeper problem affecting the Cambridge-to-Boston span and many others throughout the United States: Many have simply worn out. And states are only beginning to face the challenge of financing their renovation.
Last year, a federal study concluded that nearly half of the nation's bridges were deficient or obsolete. In Massachusetts alone, more than a third of the 5, 000 bridges for which the Public Works Department is responsible need repair or replacement. An unprecedented 60 to 70 have simply been closed.
With so many bridges in need of repair, states are beginning to search for ways to finance their renovation. And here, Massachusetts may be helping to lead the way.
Three days before the Connecticut bridge accident, Massachusetts Gov. Michael S. Dukakis proposed a state central bank for repairing infrastructure - the groundwork of community facilities that includes water works, roads, and bridges.
MassBank, as it would be called, would loan money to state agencies and municipalities for construction projects. This would divert federal aid that now flows directly to state agencies and communities. With the federal money and its own resources, the bank would probably have a better credit rating and thus float bonds more economically than local communities.
Massachusetts' bridge problems have taken few by surprise. Most of the state's highway spans were built between 1950 and 1970 and were estimated to last 20 to 25 years.
As maintenance came due, it was often put off. The state's tax-cutting measure, Proposition 21/2, cut some funds for the Public Works Department, and personnel was slashed from 4,000 to 2,900. Of those remaining, only about 200 had engineering degrees.
''The lack of staff has hurt them in many ways, including bridge maintenance inspection,'' says Edwin Holahan, assistant divisional administrator for the Federal Highway Administration in Boston. In fact, he says, the state has been so slow in coming up with Interstate Highway design plans that it has forfeited about $70 million in federal highway aid for each of the past three years.
Another problem cropped up. Massachusetts and many other states began to allow heavier trailer trucks on its roads. The heaviest legal loads now exceed the specifications of many older bridges, structural engineers say. While the structures were built with a margin of safety, they add, repeated pounding causes faster deterioration.
''If loads were kept to the design load, there's no reason the bridge can't last almost forever,'' says Clifford D. Williams, a structural engineer in Augusta, Ga.
Legislation for MassBank has yet to be drafted. But the basic idea, variations of which are being proposed in New Jersey and New York, is already winning supporters.
''The concept of setting up a state bank to pool federal and state resources . . . goes a long way to address the financing of infrastructure repair,'' says Roger J. Vaughan, a state government finance expert who has written a new book on the subject for the national Council of State Planning Agencies.
Mr. Vaughan argues that states, not the federal government, should be renovating the nation's public works. The current piecemeal fashion of issuing direct federal grants actually wastes billions of dollars because it encourages inefficiency, he says.
One example is the fact that major - but not minor - highway construction projects are eligible for federal aid. Vaughan says states have sometimes neglected to pave roads, a minor project that would be paid with state funds, and waited for the road to deteriorate so much that it needed major renovation and became eligible for federal money.
Vaughan argues that a properly conceived state bank would cut the strings attached to federal aid, dole out money more efficiently, and, in exchange for its help, perhaps force communities to institute fees so that consumers would pay for the services they use.