The only way to do something else while also driving seems to me to shorten the amount of time my eyes are off the road, doing the other thing, including pipe smoking. Because of the attention required pipe management is the worst activity of pipe smoking: filling, tamping, lighting. But if I rapidly glance up at the road and down at the pipe, back and forth, then if I need to attend only to the road, the time during which I am not reacting to the hazard, because I'm tending the pipe, amounts to a perceptual gap of perhaps only 10-20%. But also to be managed, which management is impossible in the flash of handling the obstacle, is that the pipe is in my hands and the pouch in my lap, and thus it seems to me that added to the perceptual gap above is the motor gap of bringing hands to the wheel and feet to the proper pedal.
What seems true to me, then, is that even if I manage the pipe well, if an obstacle on the road needs to be handled, I'm still at about a 20-30% disadvantage of handling it well if I'm smoking and doing pipe management. Once the pipe is lit then probably more like 5%.
These are rough estimations, mind you.