We investigate factors that affect speaking rate in conversation, using large corpora of conversational telephone speech in English and Chinese. We find that speaking rate as a function of "turn" length rises rapidly for turns from one to seven words; remains level (when final words are included) or falls gradually (if final words are excluded) for turns of medium length; and rises slowly for longer turns. When talking with strangers or discussing certain topics, people tend to use longer turns but slower speech rates. In general older people have a slower speech, and males tend to speak slightly faster than females. Finally, we find that the effect of L1 (native language) on L2 (second language) speaking rate is L1 dependent.