Car radios, or should I say car stereo systems, are often high fidelity. As we drive we listen to music and talk radio, but we are not expected to participate. The content of car radios often functions as "background noise."
That cell phones can cause "tunnel vision" has been reported in the science press since 2002. This article from ScienceDaily notes that:
URI industrial engineering Professor Manbir Sodhi and psychology Professor Jerry Cohen used a head-mounted, eye-tracking device on volunteer drivers and concluded that the alertness of the drivers decreased considerably when they were conducting cognitive tasks, such as remembering a list of items, calculating in one's head, or using a cell phone.
Their research also found that the tunnel vision caused by cell phone use continues well after the conversation ends, perhaps because drivers are still thinking about the
"The debate surrounding cell phone use in cars has been directed toward concerns over holding the phone," said Sodhi. "Holding the phone isn't the main issue. Thinking is."
Links to scientific data can be found here. Cell phones are low fidelity and, in most cases, we are expected to carry on part of the conversation. Cell phones require more cerebral involvement, both because the poor audio quality forces us to mentally fill in the gaps and because the need to respond means that we need pay attention to the words being transmitted.
In these particular examples, I would designate car stereos as "hot" and cell phones used while driving as "cool." The emphasis here is on how the media are used, how much they require user involvement, both socially and cerebrally. I find McLuhan's "hot and cool" works better in specific human contexts rather than in attempts to categorize a medium in isolation.
And perhaps this leads us to a connection between McLuhan and French anthropologist Claude Lévi-Strauss. Lévi-Strauss's structural methodology relies on the analysis of an aspect of a culture by choosing terms in opposition and constructing a table of all the possible permutations of that opposition. Most famously opposing nature and culture, Lévi-Strauss was able to demonstrate a logical coherence within a body of seemingly disparate myths.
When used as structural opposites, McLuhan's notion of "hot" and "cool" may be more readily understandable.