Summary: Driver interaction with two production voice-command interfaces representing differing user interface design approaches were compared under onroad highway driving conditions. A sample of 80 drivers was randomly assigned to drive each vehicle (40 per vehicle). During voice-based phone contact calling and destination address entry, participants in one vehicle showed, on average, statistically significant "better" performance in terms of task completion time, mean glance duration, total off-road glance time, and total number of glances. However, these objective measures do not fully characterize the overall experience of participants. An analysis of error rates and subjective report of attitudes, effects on driving behavior, and behavioral intentions relative to their exposure to the two systems provided important, complementary and sometimes contrasting data about the relative advantages of each implementation.