How to eco-drive?
The reduction in emissions deriving from the private transportation sector can be achieved by promoting ecologically sustainable driving behaviors. Huang et al. (2018) summarized factors influencing fuel saving in eco-driving technologies for sustainable road transport. These factors are described in the following.
While driving, a constant speed should be maintained. For instance, El Shawarby et al. (2005) found a considerable reduction in emissions per unit distance by maintaining the speed into the range between 60 and 90 km/h; while Wang et al. (2008) identified a narrower range of 50–70 km/h. Of course, these ranges are still pretty large: more specific values can be identified depending on engine type, vehicle mass, and many other factors. Moreover, the values are well below the highways’ current speed limits, so the observance of these limits would be limited to urban roads.
Acceleration and deceleration
From a fuel-saving perspective, it is generally recommended to avoid aggressive driving styles in favor of smoother ones. This can be achieved by keeping a reasonable safety distance to the car in front, anticipating traffic lights and unexpected events, avoiding unnecessary accelerations/decelerations, and sticking as much as possible to a target speed. Nevertheless, it is not always true that small accelerations are preferable. Several studies (e.g., Saerens and Van den Bulck 2013; Xia et al. 2013) reported that harsher accelerations to reach the target velocity in a shorter time could lead to fuel and emissions savings. Moreover, it is not easy to assess a universal model for any vehicle and situation. For instance, in their study regarding liquefied petrol gas (LPG) car consumption, Choi and Kim (2017) differentiated between accelerations starting from zero velocity and the ones occurring when the speed is not null, identifying different critical values for the two situations.
Defined as the act of remaining still in neutral gear, it should be avoided or minimized, being it a zero-fuel efficiency situation. Besides, while idling, a larger amount of pollutants are produced, like carbon oxides (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM). In the last few years, many vehicles integrated the “start and stop” functionality to automatize the switching on and off the engine, obtaining good results, especially when driving in an urban environment. It was estimated that this technology might allow a 20% CO2 saving for vehicles with no start and stop (Fonseca et al. 2011). This is also allowed by the fact that modern motors do not suffer from multiple ignitions and thus idling times above 10 s can be substituted by a turned off phase.
Despite not being precisely an eco-driving tip, it massively influences consumption and emissions. Navigation systems available on the market suggest to users the shortest or the fastest route, but none considers the amount of fuel used to reach the destination. Ericsson et al. (2006) found that in 46% of cases, suggested pathways were not the most environmentally sustainable and that an 8.2% saving could be achieved with a fuel-optimized navigator. Analogous results were also obtained by Zeng et al. (2016), who developed an eco-routing approach resulting in 11% CO2 emission reduction with a 10% travel time buffer (i.e., allowing a 10% increase in the travel time considering the shortest trip possible). The importance of route choice is strictly related to the road slope. This aspect was analyzed by Gallus et al. (2017), who also correlated the consumption with the acceleration values. Results showed that for acceleration values up to ± 0.1 m/s2, CO2 and NOx emissions have a linear dependence with road slope for urban, rural, and motorway conditions.
The air-conditioner compressor might use up to 5–6 kW of power. Compared with medium-size cars, it is equivalent to drive steadily at 55 km/h approximately. This means that it is convenient to roll the windows down at low speed because the aerodynamic drag is not too high. Instead, the drag increases and turns the air conditioning on for higher velocities, resulting in lower consumption.
Extra loads should be reduced to a minimum, considering that 45 kg might bring a 1–2% rise of fuel use onto a small-sized vehicle. A similar increase derives from driving with under-inflated tires. 4% is the possible effect of a poorly tuned engine, and worse behaviors could be caused by a faulty oxygen sensor (up to 40% increase; Sivak and Brandon Schoettle 2012).
Among these factors, in this study, we focus on acceleration and speed since these are the factors mostly related to the driving style itself. Other feedback and suggestion systems could be designed to improve car users’ habits and decisions (e.g., providing advice on the most ecologic way to achieve thermal and general comfort according to the situation).
However, delivering information and stimuli to the driver effectively to change their behavior without provoking sensory and cognitive overload is not a simple task. In the following, we discuss which aspects to consider in designing a feedback system and interface for eco-driving.
Eco-driving and human–machine interaction
When dealing with driving, we need to consider the profound changes affecting the way we interact with vehicles. The increasing amount of technology inside the car requires the driver and occupants to pay attention to many stimuli and increments the number of different actions we perform inside the cockpit, modifying our driving behavior. Moreover, new drivetrains are now spreading, like the hybrid and electric ones. While driving style always affects fuel or energy consumption, its influence may vary depending on the motor type (Thomas et al. 2017). Moreover, our driving behavior is also affected by the lack of engine noise in electric vehicles than traditional ones (Knowles et al. 2012).
It is the duty of interface design (or ecological interface design—EID—see McIlroy and Stanton 2015) to study and develop new and more efficient interaction channels to deal with this complexity. As stated by McIlroy and Stanton (2017), “the challenge is to develop Human–Machine Interaction (HMI) design guidance that not only deals with the novelty and complexity inherent in modern, non-conventional drivetrain vehicles, but influences drivers to choose more energy-efficient driving behaviors.” HMI design’s progress increases the possibilities of interaction and information to be conveyed, allowing the examination of depth fields that have remained unexplored so far. The fact that a person’s driving style has a strong interconnection with energy consumption and emission is not something new: Evans already studied this correlation in (1978), observing a 14% fuel saving thanks to the adoption of gentler accelerations. Other experiments were conducted during the 1980s, confirmed the correlation between acceleration patterns and fuel consumption (Hooker 1988). In those first stages of eco-driving analyses, people were simply asked to drive economically, without any kind of feedback, and this was sufficient to achieve a considerable result. However, suggestions and requests can be sufficient to make people behave in a certain way during a test, but they are insufficient to establish a constant and durable habit over time or in any situation.
For this reason, the feedback was introduced to create awareness and stimulate a specific behavior. Nevertheless, feedback should be carefully designed not to be misleading and head to an utterly undesired result. For this reason, HMI design must guarantee clear communication, accurately selecting the information to be sent, and the more appropriate channels to be used depending on the specific temporal and spatial context surrounding the vehicle itself. Users need to feel comfortable when using such a device otherwise, they are likely to ignore it or, if possible, turn it off. The stimulus must be perceived as advice, as something safe, non-contradictory, and not invasive during the driving task. Direct intervention support might be perceived as annoying or even dangerous if it requires a high portion of the driver’s attentional resources (Ho and Spence 2007; Spence and Ho 2008). The designed interfaces should thus adapt to the user’s driving style, nudging him/her toward the desired behavior, and still guaranteeing a high-quality driving experience. It is fundamental to select the proper sensory channel to deliver the feedback (without provoking sensory overload) and manage a series of parameters (e.g., intensity, position, timing). Moreover, designers can use a combination of multiple ones, as discussed hereafter.
Use of multisensory feedbacks to promote eco-driving
For decades, information about the car state (e.g., fuel availability) and, in some cases, about the driving style (e.g., abrupt accelerations) has been provided to the driver through visual stimuli. As a result, car dashboards have been filled with icons, whose meaning may also vary depending on color changings (e.g., green-to-red) sometimes. This large amount of information may generate a confusing and distracting effect on the driver instead of being informative or helpful. Since vision is highly overloaded when driving, the haptic channel can be considered a valid alternative (e.g., Gallace et al. 2011; Ho and Spence 2017; McIlroy and Stanton 2017). Furthermore, beyond being less saturated, the tactile system has some intrinsic features which can make it even more convenient than the visual one (e.g., Gallace 2012; Gallace and Spence 2014; Spence and Gallace 2008). For instance, haptic stimulation allows for private communication, as it can be perceived only by the driver.
Moreover, it can take advantage of the body’s large surface, which comes in contact with the car equipment (e.g., the hands placed on the steering wheel, the feet laying on the pedals; Gallace et al. 2007; Spence and Gallace 2007). The idea of using the haptic channel already came out about ten years ago, with the development of the Nissan Eco Pedal (https://www.nissan-global.com/EN/TECHNOLOGY/OVERVIEW/eco_pedal.html). This system worked as a force-feedback device, then exerting a reaction force to oppose the gas pedal pushing when a certain angular threshold of the pedal itself is overcome. However, its diffusion was minimal, probably because of its cost (it was only mounted on a few expensive vehicles—Infinity Q60, QX50) and its low user-friendliness. From a human perspective, the interaction with pedals is strictly connected to a “safety sense” while driving, and this pedal-resistance exploited by force-feedback devices might be perceived as dangerous, eliciting in the driver the feeling of not being in complete control of the vehicle.
In these years, research focusing on the use of tactile stimuli to promote eco-driving is increasing. For instance, Birrell et al. (2013) investigated the effects of a haptic accelerator on the user’s driving performance and cognitive workload. The haptic stimuli consisted of vibrotactile feedback, delivered when a throttle threshold of 50% was overcome. The study results showed that the haptic feedback induced a better use of the throttle, with the additional advantage of reducing the subjective workload compared with when testers were only asked to drive economically. McIlroy et al. (2017) assessed the effect of the presentation timing of vibrotactile stimuli delivered to the accelerator pedal about the coasting behavior when approaching stopping situations. They found that medium and long times (i.e., 8 s and 12 s before the situation) were more effective and more accepted by users than shorter (i.e., 4 s) time-to-event stimuli and as compared with baseline (no feedback).
Other studies assessed the effect of multiple sensory feedback, in isolation or combination, to reduce fuel consumption. In Staubach et al.’s work (2014), an eco-driving support system composed of a visual display and a force-feedback pedal was designed to deliver information regarding economical gear shifting and acceleration/deceleration behaviors. This system was shown to guarantee better results in terms of eco-driving performance than a condition with no support. However, the haptic system’s acceptance was not high, as the pedal’s counter pressure was considered too strong by some users. McIlroy et al. (2016) delivered unisensory visual, tactile, and auditory, as well as their combination when coasting. The results seem to suggest (despite not being statistically significant) that visual information alone is less effective in inducing eco-driving behaviors than when combined with auditory or haptic stimuli. Regarding the users’ acceptance of the system, the conditions, including auditory feedback, were always appreciated less than the others.
This work aims to use an immersive VR simulation to compare different eco-driving feedback (visual, haptic, and visuo-haptic) and collect some preliminary data regarding their acceptability and effect additions to user’s stress. We hypothesized the use of haptic stimuli to be more effective than visual feedback alone, as found in the previous literature and discussed above. The goal is also to verify if haptic feedback alone is more effective than a multisensory one, including visual stimuli as well.
Moreover, we aim at evaluating the usability and efficacy of the developed VR system as a testing tool. The use of HMDs for driving simulations is less common than desktop display or multiple screens setups, and it has been reported to cause more simulator sickness (Weidner et al. 2017; Cao et al. 2020). However, immersive systems (HMDs, CAVEs) increase the feeling of being in the virtual environment (i.e., sense of presence; e.g., Kilteni et al. 2012) and emotional arousal compared with non-immersive ones (Kim et al. 2014): This may be a relevant feature since users’ stress condition is fundamental to understand for the development of useful feedback. However, Cao et al. (2020) compared game driving simulations using HMDs and desktop displays, did not found a significant difference in engagement levels. Hence, more research is needed to investigate the use of HMDs to test eco-driving feedback in simulations: To the best of our knowledge, there is a lack of studies on this topic in the extant literature.