There are other resources available about driver distraction. The Driver Distraction Internet Forum from NHTSA (National Highway Traffic Safety Administration) is an excellent repository of information on driver distraction up through August of 2000. The Ask the Experts forum is especially good, as well as the links to many valuable papers.
There are two papers at the forum (both in PDF format) especially relevant to using speech systems in vehicles: Driver workload assessment of route guidance system destination entry while driving: A test track study by Tijerina, Palmer and Goodman, and Speech-based Interaction with In-vehicle Computers: The Effect of Speech-based E-mail on Drivers’ Attention to the Roadway by John Lee et al.