It's often said that Berkshire Hathaway Inc. Chairman and Chief Executive Officer Warren Buffett's single favorite measure of economic activity is railway traffic.

Economists at Bank of America Corp. have also found the predictive power of Buffett's "desert island indicator" is especially strong relative to other weekly measures of real activity.

That makes the recent showing for rail traffic, whose four-week moving average is down 7.8 percent annually, especially troubling:

The common citation of Buffett's favorite indicator, however, is a bit incomplete. A full quote from a 2009 interview includes both freight car loadings and truck tonnage moved as what the Oracle of Omaha would focus on if he were abandoned on a desert island for a month.

For some reason — perhaps because Berkshire's railway business dwarfs its trucking holdings — the second half of Buffett's statement is typically overlooked.

And a recent report from Bespoke Investment Group underscores why it's important to broaden the definition of the Oracle's favorite indicator to multiple measures of freight activity in order to get a more accurate reading on the state of the U.S. economy.

Bespoke observed that there has been the tendency for trucking to be substituted in the event of a big downwards move in oil prices, and vice versa. In other words, trucking gets relatively less expensive when fuel prices decline because these vehicles use more fuel to haul freight a given distance than trains do.

"We can say quite confidently that relative shifts between intermodal rail and truck tonnage are in large part driven by changes in oil prices," wrote the analysts. "Going forward, we expect the shift towards trucking from intermodal rail to continue."

Truck traffic, their team notes, is up 5.5 percent year-over-year as of May, giving the impression of a solid expansion in the U.S.

"Keep this in mind when using either index as a stand-alone measure of economic activity!" conclude the analysts. "Neither is painting a clear picture."