The reason that cell phones back up the highways is awful similar to why cars braking ahead does: little actions add up to big delays when the roads are full. The researchers conducted a study with 36 college students and ran them through a series of freeway scenarios. Reuters says that:
The drivers used a hands-free phone during half their trips and no phone in the other half. They were told to obey posted speed limits and use turn signals but the rest of the driving decisions were up to them. What they found is that when the drivers were distracted by a phone conversation, they made fewer lane changes, drove slower and took longer to get where they are going.
On average, people on cell phones finished the 9.2 mile courses between 15 and 19 seconds later than the\ drivers who were not distracted. Extrapolate this to the nation's highways, and we can see how each of those slowpokes on cell phones slows down traffic flow a little bit. With 10 percent of the drivers in the U.S. chatting while they drive, that adds up to a lot of 15- to 19- second delays. That means more time with the cars running and more gas wasted. So hey, hang up and drive greener.