What I do is when I get into my car is immediately plug my headset into my phone and place my phone somewhere that I can easily get to it when I'm driving. If calls are so important that I need to answer them then I think its important enough to take the extra effort to make it easier and safer for me to answer the call. Its become a habit and I no longer even notice the extra effort.

And yeah, most speakerphones do suck in cars. They aren't designed to be used in that situation.