[snaronikar]

I usually take the call and inform the caller i'am driving and inform them "i'am driving and will call you later". If the call is very important, then I will pull the car to curb and finish the discussions. If i'am travelling with my family, I will give the handset to my wife and ask her to take the message and I will call them later.

On a bike, I will never the take the call / sometimes I stop the bike and take the call (if it is urgent).