I'm not sure you can argue that these are the same thing.
When you're talking to someone in the car, they can react to traffic and act as a second set of eyes for you. And, you're not struggling to hear them over all the noise injected by our 1990s-era cell phone networks. Finally, passengers in your car do not suddenly ring, giving you only a few seconds to acknowledge something potentially important (distracting you from the road). I've seen people when their phone rings; they go into crisis mode and mentally drop everything until they've answered the phone or decided to ignore the call. When you're driving, you can't do that. And it's something a passenger wouldn't require you to do.
As for the radio; you have the opportunity to adjust it at your leisure, typically without even moving your hands off the steering wheel. It's simply not as distracting as a phone call.
Passengers and the radio are dangerous, but not as dangerous as talking on your phone.
Road accidents are expensive for the public finances - think of all the mergency workers and medical expenses multiplied by the large number of road accidents. That really adds up over a year, which is a big reason we have such a thing as a 'national transportation safety board' in the first place.
The most recent stats (2009) show about 30,0000 fatalities a year (http://www-fars.nhtsa.dot.gov/Main/index.aspx). Thirty thousand deaths! Even in a country with a population of ~300m, that's a lot. On the plus side, it's fallen by about a third over the last 15 years. The most recent fall is probably linked to a loss of economic activity and the rising rice of gas, but regulation of vehicle construction and the like has played a big part too. Consider the loss to the economy of 30,000 people a year, plus the cost of people who are 'just' injured; there's no #s on that FARS page but I've seen estimates of $250-350 billion a year, which is near 2% of GDP. Put another way, if the ~30% drop in fatalities shown at the link above is a good proxy for costs, the US economy benefits to the tune of $100 billion a year. Changing driver behavior only accounts for a fraction of that, but the obvious place to start would be looking at DUI statistics.
When you're talking to someone in the car, they can react to traffic and act as a second set of eyes for you. And, you're not struggling to hear them over all the noise injected by our 1990s-era cell phone networks. Finally, passengers in your car do not suddenly ring, giving you only a few seconds to acknowledge something potentially important (distracting you from the road). I've seen people when their phone rings; they go into crisis mode and mentally drop everything until they've answered the phone or decided to ignore the call. When you're driving, you can't do that. And it's something a passenger wouldn't require you to do.
As for the radio; you have the opportunity to adjust it at your leisure, typically without even moving your hands off the steering wheel. It's simply not as distracting as a phone call.
Passengers and the radio are dangerous, but not as dangerous as talking on your phone.