Trials of self-driving cars in Bristol have shown that the handover process between the car and driver is a safety critical issue.
“The results of these tests suggest that autonomous vehicles on highways should slow to a safe speed before handover is attempted”
The first trials in the VENTURER project set out to investigate ‘takeover’ (time taken to reengage with vehicle controls) and ‘handover’ (time taken to regain a baseline/normal level of driving behaviour and performance) when switching frequently between automated and manual driving modes. This is important for self-driving cars that still require the driver to take over (called SAE level3), which is the most popular option for such vehicles at the moment.
Fifty people used a simulator and an autonomous vehicle on roads on the UWE Bristol campus. The tests were at speeds of 20, 30, 40 and 50 mph in the simulator and 20 mph in the autonomous vehicle.
During the trial, the driver was aware that they might be alerted to take control of the vehicle at any moment, either due to the decisions made by the driver, or the capabilities of the vehicle in particular situations.
Early trials
A key finding is that it took 2 to 3 seconds for participants to regain manual control and resume active driving after short periods of autonomous driving in town driving.
The trial also found that participants drove more slowly than the recommended speed limit for up to 55 seconds following a handover request, which suggests more cautious, but not necessarily safer, driving. This could be important for traffic management – if drivers on the road replicated this behaviour it might impact the flow of traffic.
“Initial trials show that there are some risk elements in the handover process”
“Designers need to proceed with caution and consider human performance under multiple driving conditions and scenarios in order to plot accurate takeover and handover time safety curves,” said researcher Dr Phillip Morgan at UWE Bristol. “In the time it takes for drivers to reach their baseline behaviour, the vehicle may have travelled some distance, depending on the speed. These initial trials show that there are some risk elements in the handover process and bigger studies with more participants may be needed to ensure there is sufficient data to build safe handover support systems.”
In addition, participants returned to their baseline manual driving behaviour after handover within 10-20 seconds, with most measures including speed, stabilising after 20-30 seconds. This was not the case within the highest speed simulator condition where stabilisation did not seem to occur on most measures within the 55 second measured handover period.
“The outcome of this research for trial one is significant and must provide food for thought as the market develops for driverless cars and how we progress through the different levels of automation,” said Lee Woodcock of engineering firm Atkins which is leading the project. “Further research must also explore interaction not just between vehicles but also with network operations and city management systems.”
Simulation success
One useful result was that there was no difference between control of the simulator and the self-driving car which allows simulators to be used in the future for the development and testing of autonomous vehicles.
“The results of these tests suggest that autonomous vehicles on highways should slow to a safe speed before handover is attempted. Further research is required to clarify what that safe speed is, but it would be substantially slower than the 70mph motorway limit, and somewhat lower than the highest speed (50mph) considered in our simulator trials,” said Professor Graham Parkhurst at UWE Bristol.
“The results of this trial have been very useful as we consider the issues that the handover process raises for insurers”
This is the first trial to directly compare handover between human drivers and the car’s autonomous mode, and is important from a legal and insurance perspective – the length of time it takes people to regain full control of the vehicle represents a meaningful risk that insurers need to understand.
“The results of this trial have been very useful as we consider the issues that the handover process raises for insurers,” said David Williams from insurance company AXA. “Although some motor manufacturers have said they will skip SAE Level 3, some are progressing with vehicles that will require the driver to take back control of the vehicle. The insurance industry will need to assess the relative safety of the handover systems as they come to market but VENTURER’s trial results show that with robust testing we can properly assess how humans and autonomous vehicles interact during this crucial phase of the technologies’ evolution.”
The technical report is at www.venturer-cars.com/trial-1-results/venturer-trial-1-planned-handover-technical-report/
