Before self-driving cars are introduced on a broad scale, design solutions must be developed that allow cautious users to trust automation technology.
One way of increasing trust is to increase the user’s familiarity with automation by keeping them informed about what the automation will do. Skeuomorphic sonification, augmenting upcoming vehicle manoeuvres before they happen, can inform the rider about the car’s intentions in a non-intrusive way.
Timing of intention sounds
Playing intention sounds for 1–2 seconds before an event takes place allows the rider to mentally prepare for upcoming events. Longer time spans between the start of the sound and event may confuse the rider about what the signal indicates, while shorter time spans may not be useful.
Informing the user about what the automation is perceiving can positively affect trust. By revealing what the car is perceiving in the surrounding environment, such as pedestrians and cyclists, the user can feel relaxed about the car’s ability to drive safely. Skeuomorphic sonification, which resembles the sound of an object or road user, can make it easier for the user to understand what the car is paying attention to.
Audio signals resembling objects could be confused with objects inside the car. For instance, a cue that sounds like a mobile phone may be confused with a phone inside the car. This risk may be reduced by playing a soft preceding or concurrent sound before the informative cue, highlighting that it is a signal from the vehicle.
Combinations of intention sounds and perception sounds
When the vehicle takes action based on the behaviour of other road users, perception sounds can be followed by an intention sound. This can make it easier for the passenger to understand the vehicle’s decision-making process.
What we found in our research
As part of the Sonic Interaction in Intelligent Cars project, a research study was performed to investigate the usefulness of intention sounds and perception sounds in increasing trust towards the car. Twenty-eight participants experienced three intention sounds and three perception sounds in a VR setting containing a set of trust-critical use cases. All the subjects rode in a virtual vehicle with and without an auditory display. The results showed that the auditory display had a significant effect on trust in the majority of the use cases. The participants also found the car to be more intelligent with sounds on, and the acceptance towards the sounds was high, with 27 out of 28 subjects preferring to ride that car with audio signals on rather than off.
A second study was conducted with 20 participants on a test track. The evaluation examined intention sounds used to indicate acceleration, deceleration, and turns. All the subjects rode in the car while performing a reading task, experiencing the ride both with and without audio signals. This study also showed increased levels of trust in the audio condition compared to the no-audio condition. The acceptance of the sounds was again high, with 17 out of 20 participants preferring to ride in the car with audio signals on. The study is planned to be published in 2021.
Sound is well known to express and induce emotions in listeners. Sounds that express urgency could easily alarm the rider, making them uncertain whether they can trust the car to handle a situation or if they need to take action. To comfort the rider, sounds should be used that have pleasant, soft, harmonic, and slowly emerging characteristics. Sounds with sudden, loud, or disharmonic properties should be avoided. If using voice, a relaxed and confident voice and intonation should be used. The example below, which indicates a pedestrian on the side of the road, may be too alarming to create a sense of comfort and trust.
Associations that the listener makes as a result of the sounds can influence perceived urgency. It can be important to avoid signals indicating low build quality, insufficient technological level, or mechanical errors. The aim should be sounds that have a solid and modern expression. Sounds perceived as futuristic or old may indicate experimental or underdeveloped technology. Characteristics that may be especially negative for trust are rattling and temporal patterns, which may indicate that something is broken. Before implementing new sounds in the car, it is necessary to examine any associations users may have towards the specific design.
Provide a soothing atmosphere
A soothing sound atmosphere can make the user feel safe. Music can be an effective way to make cautious users feel calm and safe. For example, music can be used when the user gets into the vehicle before the ride.
Information about current actions
Clear and calm information about what the car has just done helps prevent the risk of confusion or feelings of insecurity among passengers in perceived safety-critical situations. Verbal signals, preceded by soft notification sounds may be the most informative solution. In some cases, skeuomorphic sonification can be used. For example, the sound of a locking mechanism might be used to indicate that the car has found a suitable lane after a motorway entrance. However, a simple, soft and non-urgent notification sound may be sufficient to create trust in such a situation.
It can be important to avoid urgent sound properties, such as dissonant or sudden and loud sounds, or other properties that are associated with warnings. Such cues can become annoying and are easily associated with unsafe behavior and the vehicle’s inability to handle the driving. The sound example below, indicating that the vehicle has found a suitable lane after a motorway entrance, may be too alarming to support trust.