"When imports are sent to market, they usually stop at a few places along the way, where they are switched from rail to truck or from larger trucks to smaller ones," Suran explains. "But exports usually go straight from the place of manufacture to the port of exit with no need for intermediary transfer sites. Imported consumer goods in particular can end up at multiple warehouse or repackaging centers before they reach their destination with stores or individual buyers."

Consequently, she says, the recent surge in exports due to the declining value of the dollar will not compensate for a reduction in imports due to lowered consumer sales. But while this rule applies to most markets, it is not necessarily true for markets with a strong manufacturing base. "Detroit and Ann Arbor, for example, show a reverse correlation because of the number of products manufactured for export. What's important, though, is that these goods won't be stored or repackaged again till they get overseas," she notes. "It appears as though all trade flows are not created equal, at least when it comes to warehouse demand."

In addition, the economist tells GlobeSt.com, conventional import and export measures also provide inadequate gauges for determining industrial space demand. "Most trade data is quoted in dollar terms," Suran points out. "But this presents serious issues because the value of a product has very little if any relation to the amount of space required to store it. I could have a million dollar diamond ring, but it's going to require a lot less space than a $20,000 automobile or $600 couch. It's the merchandise not the cost that matters."