Five seconds can be a lifetime when you're heading for a crash. And that's official.
It's the timeline that's been set in the US for a driver to regain control of an autonomous car.
The benchmark time is the target for all sorts of long-range research and development work in America, including at the BMW think tank called Designworks on the outskirts of Los Angeles.
Designworks has been doing business for BMW since the 1970s and is now a fully owned subsidiary with operations in Munich, LA and Shanghai, although it has a degree of autonomy that's unique in the BMW world.
It is virtually self-funding thanks to design and development work it's done on all sorts of consumer products, from toothbrushes to railway carriages, for a huge number of companies led by the Sennheiser audio brand.
You cannot be ‘eyes off' from driving. You can be doing something else but you can't just fall asleep.
In the case of self-driving cars, Designworks is at the leading edge of development.
Designworks boss Laurenz Schaffer is among the most bullish when it comes to cars that take over the driving. "Semi-automated driving is 2020 or 2025," he forecasts. "Fully automated driving is 2025 and beyond."
He's not dipping into the arguments about giving up control of the road, or having computers make life-and-death decisions, or the number of crashes triggered in autonomous trials by cars which are far more risk-adverse than human drivers.
But Schaffer is solid on the five-second rule. "You will need to be able to take over in five seconds. But you cannot be ‘eyes off' from driving. You can be doing something else but you can't just fall asleep," he says.
Is 5 seconds long enough to regain control of a autonomous car? Tell us what you think in the comments below.