Modern computerized vehicles offer the possibility of changing vehicle parameters with the aim of creating a novel driving experience, such as an increased feeling of sportiness. For example, electric vehicles can be designed to provide an artificial sound, and the throttle mapping can be adjusted to give drivers the illusion that they are driving a sports vehicle (i.e., without altering the vehicle’s performance envelope). However, a fundamental safety-related question is how drivers perceive and respond to vehicle parameter adjustments. As of today, human-subject research on throttle mapping is unavailable, whereas research on sound enhancement is mostly conducted in listening rooms, which provides no insight into how drivers respond to the auditory cues. This study investigated how perceived sportiness and driving behavior are affected by adjustments in vehicle sound and throttle mapping. Through a within-subject simulator-based experiment, we investigated (1) Modified Throttle Mapping (MTM), (2) Artificial Engine Sound (AES) via a virtually elevated rpm, and (3) MTM and AES combined, relative to (4) a Baseline condition and (5) a Sports car that offered increased engine power. Results showed that, compared to Baseline, AES and MTM-AES increased perceived sportiness and yielded a lower speed variability in curves. Furthermore, MTM and MTM-AES caused higher vehicle acceleration than Baseline during the first second of driving away from a standstill. Mean speed and comfort ratings were unaffected by MTM and AES. The highest sportiness ratings and fastest driving speeds were obtained for the Sports car. In conclusion, the sound enhancement not only increased the perception of sportiness but also improved drivers’ speed control performance, suggesting that sound is used by drivers as functional feedback. The fact that MTM did not affect the mean driving speed indicates that drivers adapted their “gain” to the new throttle mapping and were not susceptible to risk compensation.
Drivers use their vehicles as more than just a means to arrive at their destinations. As explained by Rothengatter ; road user behavior is to an extent governed by the “pleasure of driving fast” (p. 605). Indeed, a portion of road users appears to be attracted to sporty driving, as evidenced by the sales of sports cars or vehicle models that offer high engine power and agile handling characteristics . As an alternative, several manufacturers produce vehicles that can provide a sporty driving experience via a sport mode the driver can select. The sport mode has gained a substantial presence on the car market today [3–8].
According to manufacturers, the sport mode “permits an increased responsiveness from the engine and the gearbox”  and offers a “sporty driving style” . The sport mode may encompass technology that increases the throttle sensitivity, road holding, and agility of the vehicle [9–11]. This includes the active drivetrain, for example, changes in engine mapping and gear shifting [12, 13], active suspension, and four-wheel steering [14, 15]. Additionally, sport modes can be accompanied by mechanical sound enhancement, which concerns the adjustment of physical elements of the drivetrain and the active control of valves that redirect the engine airflow and influence the exhaust sound [16, 17].
In recent decades, several techniques have been developed to increase perceived sportiness without altering the vehicle dynamics and without requiring costly components or mechanical adjustments to the vehicle. Two of such techniques are Artificial Engine Sound and Modified Throttle Mapping.
1.1. Artificial Engine Sound (AES)
Artificial Engine Sound (AES) refers to a system that produces synthetic sounds through the cabin speakers. AES has been proposed for electric vehicles (e.g., [18–22]). However, current research on sounds for electric vehicles mostly focuses on pedestrian safety (e.g., [23, 24]). Considerably less research is available that focuses on the experience of drivers inside the electric vehicle.
Psychoacoustics research has shown that perceived sportiness can be increased by adjusting characteristics of the sounds, such as loudness, roughness, sharpness, and tonality [25, 26]. However, a limitation of psychoacoustics studies is that they are typically conducted in listening rooms. As Jennings et al. (, (p. 1263)) argued, “perception of the sounds of on-road cars is affected by stimuli for other senses (e.g., visual and vibrational), and the fact that an assessor is also concentrating on driving.” To illustrate, research in a listening room by Park et al.  found that loudness was predictive of perceived sportiness (r = 0.84) but negatively predictive of perceived comfort (r = −0.83), consistent with the generally accepted “trade-off hypothesis of pleasantness and power” ( p. 1203). A driving simulator study by Hellier et al. , however, found that drivers regarded no engine noise at all as uncomfortable. Hence, it appears that sound perception may be different in listening rooms as compared to active driving.
Very little research on perceived sportiness in real vehicles is available. An exception is Zeitler and Zeller , who let acoustical experts rate the interior sounds of different vehicles on a test track. Their results showed that perceived sportiness was strongly correlated with the sound volume increase during engine load (i.e., while accelerating). However, engine performance (e.g., actual sportiness) and acoustic feedback were confounded; that is, the vehicles that delivered more power were also those that produced a sporty sound. In a follow-up experiment, they tried to disentangle these two effects using AES and found that vehicle sounds and engine torque independently contributed to perceived sportiness.
Apart from investigating the effects of AES on perceived sportiness, it is essential to examine the extent to which AES influences driving behavior. Previous research suggests that the presence and volume of vehicle sound affect driving speeds. More specifically, it has been found that a reduction in engine volume or the lack of engine sound causes drivers to drive faster [30, 32], underestimate their speed [32–34], and show poorer speed control [35–37]. These findings are consistent with the notion that engine sound acts as an information source that facilitates perception and control, or as argued by Hellier et al. ( p. 598), “engine noise can be characterised as “feedback” rather than “noise.””
In summary, although the above-mentioned studies indicate that the presence and volume of sound affect driving behavior, there appears to be a lack of research about how drivers perceive and respond to sound enhancement techniques that could be applied in electric vehicles, such as AES. Furthermore, research on vehicle sound has to date been predominantly conducted in listening rooms, a setting that cannot provide information about drivers’ speed adaptation.
1.2. Modified Throttle Mapping (MTM)
A second approach that may increase perceived sportiness without requiring mechanical components is Modified Throttle Mapping (MTM). MTM is defined as the software-based adjustment of the relationship between the driver’s throttle input and the engine throttle input. Through MTM, for a given driver throttle input, the engine produces more torque while the maximum torque (i.e., the torque for 100% driver throttle input) remains the same. Note that MTM is not the same as modified “engine mapping,” that is, the adjustment of engine characteristics through changes in fuel injection, air charge, ignition timing, and valve timing and other factors that influence engine performance [38, 39].
Research describes different ways of changing the throttle mapping and the corresponding effect on vehicle performance (e.g., [10, 40, 41]), but only a few studies have investigated the effects of MTM on driving behavior. The few studies that did investigate human-in-the-loop effects of MTM used intelligent controllers, such as a throttle pedal for regulating the desired engine torque and desired wheel torque  or a throttle pedal that caused the vehicle to decelerate more strongly upon releasing the pedal in critical car-following situations .
1.3. Aim and Hypotheses
Little is known about how drivers perceive and respond to vehicle parameter adjustments that intend to provide a sporty driving experience for electric vehicles, such as MTM and AES technology. It is important to investigate this topic with a view to road safety. If such systems reduce vehicle controllability and increase driving speed, this could be seen as undesirable.
The current study aimed to investigate how drivers perceive and respond to AES and MTM, two systems that intend to provide a sporty driving experience for electric vehicles and do not change the vehicle’s performance envelope in any way. The individual and combined contributions of MTM and AES were compared to a Baseline condition and a vehicle that offered increased engine power (“Sports car”). The Sports car was included to investigate how the results for AES and MTM compare to a car that offers actually increased sportiness. The combined condition (AES-MTM) was included to examine whether or not the effects of MTM and AES are additive.
The expected effects of MTM and AES can be explained using theory from the field of manual control (e.g., ). Figure 1 shows a model of human driving behavior in a speed control task, based on Weir and Chao  and McRuer et al. . Here, the human outputs a foot movement (“throttle driver”), which via the throttle mapping (a variable gain, i.e., a multiplication factor) results in an input to the vehicle model (“throttle engine,” describing how much torque is requested from the car). The car model outputs the current driving speed, which is fed back to the driver via visual and auditory pathways. The driver perceives these two feedback sources with a time delay. Additionally, the driver is represented by a gain, which describes how strongly the driver responds to the difference between the perceived speed and the desired speed. The desired speed represents the speed at which the driver wishes to drive at a particular moment; it is dependent on many factors, including the environment (road curvature; road width), the driver’s personality, and the driver’s risk assessment based on the visual and auditory information received.