We performed two experiments comparing the effects of speech production and speech comprehension on simulated driving performance. In both experiments, participants completed a speech task and a simulated driving task under single- and dual-task conditions, with language materials matched for linguistic complexity. In Experiment 1, concurrent production and comprehension resulted in more variable velocity compared to driving alone. Experiment 2 replicated these effects in a more difficult simulated driving environment, with participants showing larger and more variable headway times when speaking or listening while driving than when just driving. In both experiments, concurrent production yielded better control of lane position relative to single-task performance; concurrent comprehension had little impact on control of lane position. On all other measures, production and comprehension had very similar effects on driving. The results show, in line with previous work, that there are detrimental consequences for driving of concurrent language use. Our findings imply that these detrimental consequences may be roughly the same whether drivers are producing speech or comprehending it. Copyright © 2005 John Wiley & Sons, Ltd.