Whether motorists are humming "Born to Be Wild" and looking for adventure and whatever comes their way or just commuting to the office, one thing's for sure.

Like the song says, we're in an interstate state of mind.

President Dwight D. Eisenhower probably wasn't thinking of changing the nation's way of life -- for better or worse, depending on whether or not your town got an interchange -- when he signed the law creating the federal interstate system on July 29, 1956.

But that's what happened. Americans have always been enthralled by the romance of the open road, but interstates had us head over heels in love. Four lanes of pavement stretched from one end of the country to the other, with no stop signs.

Forty years after they were signed into law, interstates have changed just about everything. The trucking industry exploded, the rail industry shrunk. Retailers were able to reconsider their inventory needs because goods could be shipped farther and faster.

Driving cross-country didn't require a month-long vacation, and automobile sales and designs rocketed.

Cloverleafs blossomed, and suburbs, urban sprawl and the commuter culture sprung up.

But Eisenhower didn't have a cultural revolution in mind when he created the interstate system, said David Snider. Ike's intentions, like his background, were firmly rooted in military concerns.

"Remember, the goal was to provide an interstate highway network for defense purposes," said Snider, the assistant chief engineer-operations for the Missouri Highway and Transportation. "We just finished the Korean War. We had just finished World War II."

Eisenhower envisioned a series of highways connecting the country's major cities and military installations, a network military vehicles and troops would have no trouble navigating.

"That was his real goal, to accomplish that feat," Snider said. "A secondary goal would be better movement of people and goods across the United States."

Over the years, moving motorists, not soldiers, has become the primary function of the interstate system, but the military's needs are still factored into highway design.

Federal regulations mandate that interstate bridges be at least 16 feet, 6 inches high "in order to accommodate military vehicles," Snider said. "It's not to accommodate high trucks."

The grades and curves built into interstates are also designed with military vehicles in mind, he said.

But the traffic cruising the interstates has turned out to be minivans and 18-wheelers.

"When we started building the interstates, if we'd gotten 10 percent trucks on there, we'd have thought we were doing good," Snider said. "We're now at 28 percent trucks on all of our interstates."

From 9 p.m. to about 6 a.m., he said, big rigs make up a little more than half of interstate traffic.

Interstates haven't just changed how we live, Snider said; they've changed where we live.

"Before interstates, the culture was live in the city, work in the city, die in the city," he said.

Now people live where they want and take the interstate to work. A commute of an hour won't raise an eyebrow, and it isn't unheard of for someone to spend three hours on the interstate getting to work.

Snider doesn't see the interstate system falling by the wayside any time soon, and six-lane highways are already on the drawing board in many areas, "even though the proponents of rail and mass transit are right, and we are going to have to have those systems."

But while there's fuel -- and electric cars aren't just the stuff of science fiction anymore -- Americans will still be hitting the highway.

WHAT DO THE NUMBERS MEAN?

Even numbers run east-west

Odd numbers run north-south

Three-digit numbers are connector routes and make loops around metropolitan areas