Autonomous vehicles (AVs) are becoming more common and the market size is growing. In the truck industry, the deployment of a platoon of AVs is emerging as a response to existing safety and environmental problems. However, before autonomous technology can be applied to all vehicles in a traffic flow, mixed traffic containing both AVs and non-autonomous vehicles (NVs) is inevitable. In this study, using a driving simulator and the National Aeronautics and Space Administration Task Load Index survey, the behaviour of NVs at highway inflows and outflows was analysed to understand platoon operation management in mixed-traffic conditions. Unlike previous studies, this study considered an environment in which automated freight vehicle platoons occupy a designated lane and examined vehicle driving characteristics and driver psychology as NVs entered and exited the highway. Statistical methods (analysis of variance and multi-variable analysis of variance) were used to analyse differences according to the situation. It was found that, in entry, a platoon had an effect on drivers and the platoon size had a larger effect than inter-platoon spacing. However, among drivers with driving experience above a certain level, the platoon parameters did not have significant effects on driving. The results of this study provide basic information about the operation and management of a platoon of AVs on mixed-traffic highways.