Researchers at the Texas A&M Transportation Institute tested 43 drivers on a closed course.
Drivers made the first lap without using a cell phone. They then drove while using two different text-to-voice apps, as well as manual texting.
"We instructed them to drive 30-miles-per-hour and respond to a light that would turn on periodically."
Transportation researcher Christine Yager headed up the study.
"We found that the reaction times for drivers that were texting were two times slower compared to when they were not texting, and it didn't matter what method they were using to text."
Yager says study participants reported feeling safer using voice-to-text over manual texting. But the voice method still requires users to respond to prompts, and that can get a driver distracted.
"You may be holding the phone with your hands, but if your eyes are on the road, your brain may still be focused on what you're trying to say and think of the response."
Yager says drivers who were texting, no matter what the method, spent a lot less time looking ahead at the roadway. They also took twice as long to respond to sudden hazards.