The study had 20 men and 18 women, all college aged, conduct a variety of tasks while driving in a simulator to measure just how much mental effort they had leftover for driving. The study includes a variety of metrics to quantify varying mental workload, but the graph [above] sums up the trends nicely.
Driving alone and without devices was given a baseline workload level of 1. You can see that adding in the radio or book on tape increases mental workload, while talking on a cell phone is about the same as talking to a passenger, while speak-to-text programs were even higher. (OSPAN is a specific cognition test you can read about here.)