A study at the University of Utah has concluded that people who talk on their cell phone while driving are just as impaired as someone who is drunk, even if they are using a handsfree phone.
For the study, researchers had 40 volunteers use a driving simulator four different times. In different trials, the volunteers drove undistracted, spoke on a phone while driving, spoke on a handsfree phone and drove with a blood alcohol level of 0.08 thanks to vodka and orange juice. In the 160 combined trials, drivers rear-ended vehicles in front of them three times. In all three cases, the drivers were sober and talking on a phone.
Drivers on either kind of phone drove slower than undistracted drivers and were 9% slower hitting the brakes. The drunk drivers drove slower still, but were more aggressive. Most interestingly, the phone users all claimed that their response and performance was unaffected.