We’ve already seen the first implementations of SAE Level 3 automation on new vehicles, building on the now-proven reliability and safety benefits of Level 2 ADAS systems such as lane-keeping assist (LKA) and lane-change assist (LCA). The deployment of L3 systems will soon become more widespread, especially in controlled environments such as on divided highways, but challenges remain in their implementation on less predictable roads and in moving to even greater levels of automation (Level 4 and above).

Indeed, there is a growing consensus that L3 and L4 automation will not always be active and is not for every situation. This means that interactions between driver and vehicle will require frequent transitions of control authority. The complexity of the handover process is already manifested in the varying degrees of torque resistance that drivers experience from the LKAs of different OEMs: how much the system ‘fights’ for what it thinks is the correct steering position and vehicle trajectory, versus how much it trusts the driver to do the right thing.

Driving simulators are extremely useful tools, both in researching driver behavior to inform new technology development in control handover, and in testing the technologies themselves in a safe environment.

In 2019, for example, a Cruden simulator was used by a team from the Delft University of Technology (TU Delft) to investigate variability in lane-change behavior between different drivers, and by the same driver. In this study, some of the variation in lane-change behavior was due to differences in what the team called, “spatio-temporal criticality”: logically enough, the same driver naturally performs the maneuver differently depending on the amount of time and space available.

As a result, the researchers proposed that vehicle trajectories when changing lanes should be based on steering behavior rather than lane-change duration. This could help to avoid steering-torque conflicts and improve customer acceptance when designing ADAS and automated driving functions using a shared control approach.

Other recent studies using the concept of traded control (TC) – instantaneous transfer from L3 automation back to manual control – have shown that these transitions may require a take-over-time (TOT) ranging from 1 to 20 seconds, when responding to a warning signal to resume control. Several studies show impaired driving performance as the vehicle overshoots the center of the target lane, and dangerous safety margins to the lane markings were reported.

An alternative approach to transitioning is proposed: After the warning to resume control, the steering wheel provides haptic support similar to an L2 LKA. In addition to lane keeping, this system is capable of supporting driver-initiated lane-changes as well. This is known as haptic shared control (HSC).

Recently, researchers from TU Delft worked with a Cruden AS2, 6DOF driving simulator to investigate to what extent the degraded vehicle handling from traded-control transitions was mitigated by transitioning back to haptic shared control. The team employed 30 driver participants to compare TC with HSC transitions during highway driving at 100km/h (62mph), with a forced lane-change scenario due to a stopped vehicle in the driving lane.

The experiment was performed with two levels of criticality, with the driver required to perform the take-over of control with a time budget of 5 seconds or 7 seconds. Researchers then analyzed take-over reaction time (TOrt), longitudinal and lateral safety margins to the lead vehicle, swerving and self-reported questionnaires.

The results were interesting. HSC yielded better safety performance compared to TC, in terms of increased minimum time-to-collision (TTC – safety margin) and decreased maximum lateral accelerations. In other words, assisted by HSC, drivers kept a larger longitudinal safety margin with respect to the car they were passing, and did not overshoot the center of the lane they changed into, suggesting that HSC keeps the vehicle under better control during the handover. No differences were observed in terms of take-over reaction time, nor in the drivers’ satisfaction with both systems on the self-reported questionnaires.

The results also showed that HSC seems to effectively reduce rapid steering movements, indicating potential to assist the driver in stabilizing their steering activity. However, in the 5-second, time-critical scenario, larger torque conflicts between the automated steering input and the driver’s own input suggest that careful steering calibration is essential in finding the right balance between the degree of assistance and driver authority.

To reduce the torque conflicts, TU Delft and Cruden are currently cooperating on a so-called ‘trial-by-trial driver adaptation of the HCR’ (HCR is the human compatible reference, one of four parameters considered when designing an automated driving system). In this research, the parties are investigating HCR implementations that adapt their trajectory based on the criticality of the take-over maneuver. In addition, the HCR could be made adaptable based on the driver’s intentions, to compensate for the intra-driver variability. The goal is to improve controller performance so that the driver experiences more assistance from the system or accepts the assistance as more natural.

In the future, driving simulators will continue to feature prominently in the further research needed into the control handovers between humans and automated systems. The benefit of a driving simulator for this research is twofold: repeatability of experiments and the ability to experiment with dangerous situations in a safe way.

The crucial issue at stake is the need to build drivers’ trust in ADAS and automated driving technology. Without it, the more advanced, more highly automated systems will struggle for market acceptance, no matter how great the advertised safety benefits. If drivers find a system too annoying to deal with, or fail to trust its accuracy, then it risks being switched off – at which point, any safety benefit is lost.

As a society we remain a long way from being able to always guarantee the safety of automated driving, wherever we’re driving, so solving issues around shared control and vehicle/human control transitions will remain key to progressing ADAS and automated driving technologies for some time to come.

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