According to Cheyer, researchers of the study asked drivers to use Siri improperly, leading to instances where the drivers had to read the screen which shouldn't have happened. He says the researchers seem "to have misunderstood how Siri was designed to be used." Going on to say:
The study didn't use Siri's car mode by admission of TTI representative Christine Yager. Had the study used Siri's car mode, Cheyer says that almost all of the instances of drivers needing to look at the screen or press a button would have been eliminated. In car mode, "Siri is even stricter and will not execute certain commands that would require looking at a screen", says Cheyer.
It sounds like TTI may need to recreate the study, because other voice command apps like Vlingo also offer car modes, so Siri may not be the only app being misrepresented in the study.