SAN BRUNO, Calif. — An ominous theme has emerged from the wreckage of a deadly pipeline explosion in California: There are thousands of pipes just like it nationwide.

Utilities have been under pressure for years to better inspect and replace aging gas pipes – many of them laid years before the suburbs expanded over them and now are at risk of leaking or erupting.

But the effort has fallen short. Critics say the regulatory system is ripe for problems because the government largely leaves it up to the companies to do inspections, and utilities are reluctant to spend the money necessary to properly fix and replace decrepit pipelines.

“If this was the FAA and air travel we were talking about, I wouldn’t get on a plane,” said Rick Kessler, a former congressional staffer specializing in pipeline safety issues who now works for the Pipeline Safety Trust, an advocacy group based in Bellingham, Wash.

Investigators are still trying to figure out how the pipeline in San Bruno ruptured and ignited a fireball that torched one home after another in the neighborhood, killing at least four people. Pacific Gas & Electric Co., the pipeline’s owner, said Monday it has set aside up to $100 million to help residents recover.

Experts say the California disaster epitomizes the risks that communities face with old gas lines. The pipe was more than 50 years old – right around the life expectancy for steel pipes. It was part of a transmission line that in one section had an “unacceptably high” risk of failure. And it was in a densely populated area.

The blast was the latest warning sign in a series of deadly infrastructure failures in recent years, including a bridge collapse in Minneapolis and a steam pipe explosion that tore open a Manhattan street in 2007. The steam pipe that ruptured was more than 80 years old.

The section of pipeline that ruptured was built in 1956, back when the neighborhood contained only a handful of homes. It is a scenario that Christopher Hart, National Transportation Safety Board vice chairman, has seen play out throughout the nation, as suburbs have expanded.

“That’s an issue we’re going to have to look on a bigger scale – situations in which pipes of some age were put in before the dense population arrived and now the dense population is right over the pipe,” he said.

Thousands of pipelines nationwide fit the same bill, and they frequently experience mishaps. Federal officials have recorded 2,840 significant gas pipeline accidents since 1990, more than a third causing deaths and significant injuries.

“In reality, there is a major pipeline incident every other day in this country,” said Carl Weimer, Pipeline Safety Trust’s executive director. “Luckily, most of them don’t happen in populated areas, but you still see too many failures to think something like this wasn’t going to happen sooner or later.”