Drivers' emotions significantly affect their driving performance and thus are related to driving safety issues. The objective of this study is to examine how taxi drivers' on-duty emotional states are associated with their driving speed in real driving situations. An experiment was conducted among 15 taxi drivers in Hiroshima, Japan for 15 consecutive days in 2019. A biometric device was used to track drivers' emotional states while on duty; the five examined states included happy, angry, relaxed, sad, and neutral. Random effects panel regression results revealed that negative emotions of taxi drivers (angry and sad) have significant impacts on increasing driving speed. In contrast, a neutral emotional state is related with decreased speed, while happy and relaxed emotional states show no significant impact. Moreover, we found that factors such as driving with customers, driving long hours, and number of break hours are significantly associated with driving speed. This study contributes to the literature by providing empirical evidence on the roles that emotional states play in explaining driving speed in real-life driving situations, in contrast to studies that use simulated driving or mood induction procedures. (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).