It's no surprise that driving while texting (DWT) falls under the category of driving while stupid (DWS).
Drivers who text are about six times more likely to crash than those paying full attention to the road, this study says, and their reaction times are on average three times slower than the reaction times of drivers talking on cell phones.
According to the study's results published in the Human Factors journal:
Drivers apparently attempt to divide attention between a phone conversation and driving, adjusting the processing priority of the two activities depending on task demands. This requires drivers to switch their attention from one task to the other. When such attention-switching occurs as drivers compose, read, or receive a text, their overall reaction times are substantially slower than when they're engaged in a phone conversation."
The study was done on just 20 men and 20 women (ages 19 to 23) in a driving simulator, so it may or may not translate directly to what happens on the road. In the simulations, drivers tended to follow other vehicles more closely when texting and had worse reaction times, which includes being slower to hit the brakes.
Reading text messages also affected braking time more than did composing messages.