  Although partially autonomous driving (AD) systems are already available in
production vehicles, drivers are still required to maintain a sufficient level
of situational awareness (SA) during driving. Previous studies have shown that
providing information about the AD's capability using user interfaces can
improve the driver's SA. However, displaying too much information increases the
driver's workload and can distract or overwhelm the driver. Therefore, to
design an efficient user interface (UI), it is necessary to understand its
effect under different circumstances. In this paper, we focus on a UI based on
augmented reality (AR), which can highlight potential hazards on the road. To
understand the effect of highlighting on drivers' SA for objects with different
types and locations under various traffic densities, we conducted an in-person
experiment with 20 participants on a driving simulator. Our study results show
that the effects of highlighting on drivers' SA varied by traffic densities,
object locations and object types. We believe our study can provide guidance in
selecting which object to highlight for the AR-based driver-assistance
interface to optimize SA for drivers driving and monitoring partially
autonomous vehicles.
