The surprising length of time it takes the average human to re-take control from an autonomous car has blown the subject of the new technology’s safety wide open.

Autonomous cars can’t yet handle all situations, and sometimes have to ask the human behind the wheel to take charge for a while, sounding and/or flashing a warning that the vehicle is about to relinquish control.

Researchers at the University of Southampton have completed a study that found the median length of time between a test car’s prompt and the human taking over, even when the person in question was focused only on driving, was 4.56 seconds. Ouch.

The study, which used 26 male and female subjects aged between 20 and 52, saw each driver placed in an autonomous vehicle simulator set at 70mph. On the first run the driver focused only on the drive, but then on the second attempt read a newspaper instead.