Currently, active steering systems are implemented in production vehicles to assist the driver by varying the steering response, where the adaptation of the steering response is either initiated by the vehicle or by the driver. Though studies have shown that these steering systems affect the steering performance positively, the effect on driving behavior and driver acceptance is not taken into account. Moreover, the effect of machine-initiated and driver-initiated steering systems on the driver has yet to be investigated. The aim of this driving simulator study was to examine the effects of machine-initiated and driver-initiated steering systems on driving behavior and driver acceptance. During the experiment, the machine switched between a slow and fast steering response on predetermined locations based on the traffic conditions and road curvature, and the driver could switch the steering response by pressing a mouse-button which was attached to the steering wheel. The expectation was that the preferred steering response would be dependent on the steering task, and that between machine- and driver-initiated steering there would be a trade-off between effort and acceptance. Twenty-four participants drove with a constant velocity on a two-lane road with three sections i.e. overtaking traffic vehicles, driving on a straight road and driving on a curved road. Four conditions were completed in a counter-balanced order i.e. passive slow steering response (PS), passive fast steering response (PF), machine-initiated steering (MI) and driver-initiated steering (DI). A post-experiment questionnaire showed that the participants had a preferred steering response for each of the three sections: between slow and fast for overtaking, slow on the straight road, and fast on the curved road. Furthermore, a lower effort and higher acceptance was achieved with the active steering systems compared to the passive steering systems, where there were no significant differences between MI and DI. For future research, it is recommended to further investigate the use of a range of steering responses and free driving speed.

Conventional steering systems in passenger vehicles have a mechanically fixed steering ratio. The steering sensitivity, defined as the amount of vehicle response to the driver's steering wheel input, remains fixed with changing road environments. Research has shown that driving comfort and safety can be improved when the vehicle's steering sensitivity is adapted to the road curvature profile. Current vehicle models can adapt the vehicle's steering sensitivity based on vehicle's speed and driver's steering wheel angle (i.e variable gear-ratio systems), or on individual selection of driving mode (i.e sport, comfort). It is hypothesised that adaptation of the steering sensitivity based on frequency measures of individual drivers' steering behaviour could improve driving comfort and safety. In a fixed-base driving simulator experiment involving 24 participants, real-time adaptation of steering wheel sensitivity based on individual drivers' steering behaviour was compared to three different fixed steering sensitivity settings on a road with changing road curvature. Here I show that intermittent switching frequency in drivers' steering movements can be used to adapt the vehicle's steering response to a varying road curvature. Significant differences in intermittent switching were found between different road curvature sections and between different steering sensitivity settings. Driver's positional control and comfort ratings did not significantly increase with the steering sensitivity adaptation strategy.

Many modern vehicles are equipped with a sport mode, which intends to increase drivers' perceived sportiness of the vehicle, via e.g. power-train enhancement (PTE) or engine sound enhancement (ESE). However, to the best of authors' knowledge, no studies are available that investigated the individual or combined effects of PTE and ESE on perceived sportiness and driving behaviour. Therefore, this study aimed to investigate the effects of ESE, PTE and their combination on perceived sportiness and driving behaviour.
In a within-subject driving simulator study, thirty-two participants drove under five conditions: no enhancement (Off), PTE, ESE, PTE and ESE combined (PTE-ESE) and a control condition (Control) with a physically sportier car (i.e., more engine power and a sports car sound). PTE provided a more sensitive pedal-to-throttle mapping and ESE an engine sound associated with increased engine speed. Both implementations did not increase engine power. Perceived sportiness was measured using a questionnaire, whereas driving behaviour was retrieved from the simulator.
The results showed that ESE contributed significantly to perceived sportiness and perceived engine responsiveness, whereas PTE had no to limited effect. Furthermore, ESE created the impression of enhanced engine responsiveness, more so than PTE. PTE resulted in increased acceleration during acceleration from standstill, whereas driving behaviour was not significantly affected by ESE compared to Off. In addition, PTE significantly influenced control behaviour: it led to a decreased mean accelerator pedal depression angle and an increased mean throttle reversal rate compare to Off.
We conclude that ESE increases perceived sportiness to the extent it approaches the perceived sportiness of an actual sportier car without altering the driving behaviour or decreasing safety margins. The findings of this study support the use of ESE in sport mode. PTE should be further explored in an experimental setup that provides vestibular feedback.

Variable steering systems have the ability to change the ratio between the steering wheel and the front wheels while driving. These adjustable steering systems have led to an improvement in traffic and road safety and decrease in driver’s workload. A previous study concludes that driver steering behaviour is significantly dependent on vehicle speed and road curvature (number and sharpness of bends). Interestingly, variable steering ratio systems often depend on speed but not on road curvature. Variable steering ratio dependent on road curvature possibly influences driving behaviour and might be desirable for safety and driver acceptance. The goal of this research is to investigate driver acceptance and driving behaviour for two separate steering ratios (1:12 and 1:40) and two different road profiles (i.e. specific curvatures straight highway and curvy country road) at a constant speed. We hypothesize that on a curvy country road low steering ratio (1:12) leads to higher safety margins and subjective ratings, whereas on a straight highway a high steering ratio (1:40) leads to higher safety margins and subjective ratings. Therefore we conducted a within-subject driving experiment (N=24) in a fixed-based passenger vehicle simulator at constant speed. The results show that on a country road a vehicle with a low steering ratio increases time-to-line- crossing (TLC) safety margins and increases self-reported subjective ratings compared to the high steering ratio setting. Likewise, on a highway, a vehicle with a high steering ratio leads to higher safety margins and comfort rating compared to a low steering ratio. Thereby it can be concluded that steering ratio variable to the road profile improves safety and acceptance. These results provide promising evidence to make steering systems adaptable to road profile (e.g. steer-by-wire and active rear wheel steering).

Lateral acceleration is a key aspect of the vehicle response perceived by the driver. Assistance systems such as Active Rear Steering (ARS) or Torque Vectoring (TV) are developed to modify the lateral acceleration response such that the vehicle has an improved stability and an extended linear handling region. With this extended linear handling region the vehicle abruptly reaches tyre friction limitations (representing an entry into the vehicle handling limits (VHL)), this can potentially lead to dangerous situations. This thesis aims to quantify driver behaviour when being forced to drive near the VHL, in terms of how often the VHL is entered and what happens after entry. To assess this, a human factors experiment (N = 18) in a fixed base driving simulator was performed. In this experiment three different vehicle configurations were compared: (1) a conventional vehicle (Passive) (2) a vehicle with an extended linear handling region (Active) (3) a vehicle with an extended linear handling region and increased yaw response (Active Sport). In these configurations, the drivers need to drive with a fixed velocity on an oval track with Electronic Stability Control (ESC) switched off. It was expected that a conventional vehicle enters the VHL more often compared to the vehicles with an extended linear handling region. However, when entering the VHL, the vehicle with an extended linear handling region was expected to be more difficult to control due to the abrupt change in vehicle dynamics and corresponding steering feel. The results indicate that the Passive vehicle entered the VHL more frequently compared to the Active and Active Sport configurations. However, when the Active Sport configuration entered the VHL, significantly more road departures and an increased steering reversal rate compared to the Passive configuration resulted. Therefore, it can be concluded drivers enter the VHL less frequently in a vehicle with an extended linear handling region (caused by systems such as ARS or TV). However when the VHL is entered it is more difficult and dangerous for a driver to control compared to a conventional vehicle.

Drivers continuously adapt to the different needs and constraints in the driving scene. Literature has provided evidence for two adaptation strategies in response to an increased risk (decreasing the road width or increasing the driving speed) while lane keeping: decreasing driving speed and increasing endpoint arm stiffness. However, so far these studies did not investigate the interaction between the two adaptation strategies. The aim of this study is to find the interaction between drivers’ speed and neuromuscular adaptation for different risk durations. We hypothesize that the speed reduction is larger when the narrow road is longer which allows for a lesser increase in arm stiffness. Additionally, when the narrow road is shorter the increase in neuromuscular stiffness is larger and allows for a higher speed.
Twenty-six participants drove in a driving simulator experiment in a 1.8m wide car on a 35 km long road. Different levels of risk durations were imposed to the drivers on straight road sections by a road narrowing (from 3.6m to 2.2m) with a varying length (10m, 100m, 250m, and 500m). During the experiment speed reduction was measured and neuromuscular adaptation was quantified by measuring the grip force. Additionally participants subjectively rated their experienced effort from 1-10.
The results show that participants adapted to the road narrowing both by speed reduction as well as increased grip force, without significant impact of the length of the road narrowing. Only on the 10m narrow the speed reduction and increase in grip force was smaller compared to the other three cases. Interestingly, although drivers increased their subjective effort, no differences in speed and grip force adaptation were found between the three longest narrow roads. These results suggest that for narrow road lengths up to 500m drivers adapt their driving style to road width rather than road length. Future studies should identify if the identified speed and grip force adaptations also hold for longer and different durations of risk.