In October, the federal government will begin enforcing new rules for highway bridge design meant to make new structures more efficient, more reliable, safer and longer lived — and implicitly better than hundreds of thousands of existing spans across the nation.

The new rules have been in the works for two decades, and their enforcement by the Federal Highway Administration is unrelated to the Minneapolis bridge collapse, officials said. The rules will act like housing codes and not be retroactive. Many old bridges will suddenly be out of date but will be grandfathered in, as is the case with many old homes as community standards rise.

States have been slowly applying the new rules for years, some better than others. Minnesota is at the forefront, with 100 percent enforcement on new bridges as of last year. New York has been slower, and California slower still.

States that fail to follow the new rules after October could jeopardize their federal financing.

The rules, highly complex, call for major improvements in the design of bridge steel, concrete and foundations. They are more conservative, for instance, in their assessment of the sturdiness of driven pilings, and in some cases will raise the cost of bridge foundations.

Officials called the rules the biggest change in federal regulations for bridge design since 1931. Behind the change, they said, are such factors as more realistic assessments of highway traffic as well as advances in the statistical analysis of past bridge failures and their causes.

Among other things, the rules will produce bridges better able to withstand peak traffic stresses and severe weather, as well as “extreme events” like ship collisions and earthquakes.

“It’s going to increase reliability, which translates into bridges that are safer, more cost efficient, and have longer design lives,” said Kelley C. Rehm, program manager for bridges and structures at the American Association of State Highway and Transportation Officials, which wrote the new rules.

“It doesn’t mean existing bridges are going to fall down,” Ms. Rehm added. “In the past, some were overdesigned, and some didn’t have as much of a safety factor as we’d like to see. Today, we know a lot more about how loads are doing to affect the bridges, and why they fail.”

Surprisingly, experts say most bridge failures involve not structural failure of the span itself but of its foundations. For instance, churning water can undermine driven pilings, causing the whole bridge to collapse suddenly.

The rules were actually written seven years ago, during the Clinton administration. In June 2000, David H. Densmore, director of bridge technology at the highway administration, ordered them applied to all bridges built after Oct. 1, 2007.

In theory, the rules affect just federally financed bridges, but experts said the effects would eventually include all state and local bridges because the old rules are no longer updated and adhering to them could hurt bridge trustworthiness.