Question:  So,there must be a traffic engineer out there who can answer THIS question. If 10 percent of people check their email on iphones or droids or blackberries while at stoplights, how much does that slow down traffic overall in a typical large city?
Answer:  In Traffic Operations class, we learned that reaction time (about 1/2 second to recognize that the light turned green and another 1/2 second to move a foot to the gas pedal and start pressing) accounts for about 1% of the typical traffic signal cycle. Using phones might be pretty similar (and additive) but would be pretty variable, and one of the very unpredictable variables would be how long the person behind the texter would wait before honking at the rat bastard.  ;-)  Seriously folks, every now and then take the time to remember that your car is not a toy, it can be quite lethal if you are inattentive at the wrong time, remember why you got behind the wheel (to drive somewhere, most likely), and take your responsibility seriously.