"There is a very simple message I have on this -- to determine whether something works, you do the research on it," Greenwald said. "It turns out that almost all the products marketed as subliminal products have not been the subject of peer-reviewed research."
Halpern said he has tried the CD out informally on drivers in New York, Florida and Los Angeles. He said that those who listened to the CD reported that they felt they noticed more as they drove and were more alert and aware of their fellow drivers.
But beyond this, Hapern admits, there is no conclusive scientific evidence that the CD will actually work to quell aggressive driving.
"Can you test this in a lab? Yes you can, but it costs hundreds of thousands of dollars that I don't have," he said.
But Greenwald said that anecdotal evidence from driver testimonials is "pretty much the same as no evidence at all." And Epley said that a double-blind, placebo-controlled test for this music could easily be done on the cheap, simply by creating a version of the songs without the subliminal content and comparing the experiences of drivers listening to either version.
"There are almost an infinite number of ways to test this," he said. "If there was research, it would show that this would not work."
Greenwald, however, takes a less conclusive approach. He said that he and other researchers in the field have "published research showing some subliminal effects, so we're not 100 percent skeptical about the possibility."
Regardless of whether the subliminal content on the CDs lodges itself in the mind, both Epley and Greenwald said that those who purchase the album might still reap some of its purported benefits, thanks to what he refers to as the "expectancy effect."
"If [consumers] listen to the tapes and think they contain messages that reduce road rage, they may be effective -- but only because of the expectation you have for what's on the tape," Epley said.
Indeed, Greenwald co-authored a 1991 study in the Journal of the American Psychological Society that seems to back up this idea. In a study of 237 people who listened to tapes that supposedly contained subliminal messages designed to enhance either memory or self-esteem, all of the subjects who listened to the tapes after seeing their labels reported improvements in these areas -- what the researchers termed a non-specific placebo effect.
Plus, Epley added, "Calming or soothing music can calm people down without any sub-audible signals at all. ... You shouldn't delude yourself into thinking that whatever help you're getting from this particular method is from the subliminal content."
However, Halpern said sub-par subliminal audio products in the past have "perpetuated the message that all subliminals are a hoax." Specifically, he said, some companies in the 1980s sold tapes that they claimed contained helpful subliminal messages, when, in fact, they did not.
"Just because one company was a sham does not mean all companies were a sham," he said.
The DriveTime Rx CD will join about a dozen other titles Halpern has mixed with subliminal message-laced tracks. A spokesperson for Inner Peace Music, the company that sells Halpern's CDs, said the titles have racked up about 750,000 sales so far. Best-sellers have been a CD that Halpern said would help accelerate learning, and another aimed at helping people achieve an ideal weight.
And then there is one that he said should never be slipped into your car's stereo.
"The one that helps you go to sleep, you should never listen to while driving," he said.