Transition of control in highly automated vehicles

A literature review
Vlakveld, W.

The Directorate-General of Highways, Waterways, and Water Systems (Rijkswaterstaat in Dutch) of the Ministry of Infrastructure and the Environment (Ministerie van Infrastructuur en Milieu (IenM) in Dutch) has commissioned SWOV to conduct a literature review about transition of control in highly and fully automated vehicles from the perspective of the driver. Transition of control is the switch from fully automated driving to manual driving while in traffic. If the system of fully automated driving happens to fail or makes a mistake, or if drivers want to drive their vehicle manually, drivers have to switch from being driven to manual driving. This report presents the current knowledge about human behaviour in highly and fully automated vehicles and the psychological processes that influence task execution before, during and immediately after transition of control. The report also contains suggestions for further research and recommendations for policymakers.

The future development of fully automated vehicles cannot yet be fully predicted. Most probably an increasing number of fully and partly automated vehicles will enter into our roads and have to mix with other road users, including pedestrians and cyclists. Vehicles will not all be at once the fully automated vehicles in the way the Google car has been designed. There will be a mixture of vehicles with different levels of automation and this will gradually change into a mixture of vehicles with higher levels of automation becoming dominant. Only in the level-5 automated vehicles (see for a description Figure 1.1) that automatically drive their occupants to their destination, transition of control will not be necessary. This can only occur under the condition that these vehicles can master all possible traffic situations and weather conditions and that the automated systems in these vehicles will never fail. In all other situations transition of control is inevitable.

It is widely assumed that automation of the driving task will be beneficial for road safety, because automated systems do not speed, do not drive under the influence of psychoactive substances, never get tired, et cetera. On the other hand, automated vehicles may also have negative effects on road safety. For instance, crashes may happen when drivers have to switch to manual driving while they are not mentally ready to do this. Negative effects will most probably also occur when other road users have to anticipate fully automated vehicles not behaving according to expectation. This literature study will only discusses the first problem and not with the interactions between other road users and automated vehicles.

There is a distinction between planned moments, when the driver has to switch to manual driving (e.g. when fully automated driving is only possible on motorways and the driver wants to leave the motorway) and acute moments, when the driver has to resume to manual driving (e.g. when the system fails or the system cannot manage the traffic situation).

The mental processes that take place and the way these processes can affect driver behaviour while driving in a highly automated vehicle and during transition of control can be studied from different theoretical perspectives or paradigms. The dominant paradigm is the human factors paradigm. This paradigm focuses on human information processing. While driving in the fully automated mode, drivers will experience a loss of situation awareness, due to a low workload in which not much information related the driving task has to be processed. This diminished situation awareness will not be immediately reactivated after drivers have switched to manual driving. Indeed, a multitude of studies have indicated that diminished situation awareness occurs when task demands regarding the driving task are low, due to the automatization of the driving task. There are also some studies in which it was found that situation awareness was diminished in acute threatening situations, directly after resumption of the driving task. This is even more so the case when the transition of control occurs suddenly and the driver paid no attention to the driving task just before transition of control. Studies indicate that driving performance at the operational level (the longitudinal and lateral control over the vehicle) is also affected after transition of control.

Another paradigm is the motivational one: do drivers trust the automatic systems and what is the effect of trust on drivers in highly automated vehicles? This paradigm predicts that drivers will tend to rely too much on the flawlessness of the automated systems. When too much trust is placed in automation, a false sense of security will arise, which results in a loss of risk-awareness. The effects of trust and other psychological factors on the quality of transition of control have not been studied yet.

Again, another paradigm is dual processing. Dual process theories make a distinction between routine actions and conscious actions. While driving in the fully automated mode, the brain structures (schemata) that allow for routine actions will no longer be activated. These schemata create mental representations that help drivers to recognize the traffic situation, to predict what may happen and to act quickly. Dual process theories predict that after resumption of control over the vehicle, only slow and error-prone conscious actions can be executed before ‘normal’ driving takes over. This may result in poor hazard anticipation skills and behavioural habituation. For instance, it was found in one study that drivers kept driving too closely to a lead vehicle too long after the platoon of automated vehicles was lifted.

We have identified the following subjects for further research:

  • Duration of diminished situation awareness regarding acute threats, in particularly regarding latent hazards;
  • The influence of the driver state on the duration of the diminished situation awareness;
  • The optimum interface to turn drivers who are out-of-the-loop back in-to-the-loop, and
  • The influence of trust in the system on the severity and duration of loss of situation awareness regarding latent hazards in real traffic.

Preliminary recommendations for vehicle authorities are:

  • Allow only highly automated vehicles with fail-safe systems that function in all possible traffic situations and weather situations on roads where automated driving is possible. Only with these systems, abrupt transitions of control can be avoided;
  • Allow only interfaces of systems that effectively get drivers in-to-the-loop just before transition of control in planned transitions.

A preliminary recommendation for road authorities is:

  • When there are permanent sections on the road (e.g. when automated driving is possible on motorways and drivers have to switch to manual driving as soon as they leave the motorway) where drivers have to switch from automated driving to manual driving, circumstances should be created to avoid hazards from occurring. For instance, no other vehicles should be in the vicinity so as to avoid collisions with automated vehicles due to transition of control errors.

A preliminary recommendation for driving licence authorities is:

  • Facility training programs for learner drivers that teach them to drive in automated vehicles and that teach them to resume manual driving while in traffic.
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SWOV, The Hague

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