Driving while talking on a cell phone is extremely hazardous for most people.

Only a tiny fraction of “supertaskers” can do both simultaneously without ill effect - 2.5% of people studied could successfully drive and use a cell phone at the same time.

It took most people 20% longer to hit the brakes and increased following distances 30%, meaning they failed to keep pace in the simulator with virtual traffic. Also, their memory performance dropped 11% and their ability to do the math fell 3%.

Video: Stop texting while driving (http://bit.ly/S6osm). Terrifying. All drivers should watch this. Warning: This video may not be suitable for minors.

References:
97.5% Can't Drive Safely Using Cell Phones. WebMD.
Video: Stop texting while driving. Terrifying. All drivers should watch this.
Texting while driving increases crash risk 23 times