Five seconds seems roughly how long it would take for really bad choices to cause an accident. In heavy traffic you can cause an accident by steering into a car in under a second. I am sure someone has had two independent accidents in under a minute.
An hour on the other hand is completely arbitrary, why not a day or a minute?
Any fixed timespan chosen to analyze risk is arbitrary, and you can convert between timespans using probability math. I'd say it's typical to choose the average "mission" length, because that's the most relevant to the purpose. For a self driving car, an hour would make sense. For the first stage of a rocket, 5 minutes. You don't ride in a car appreciating every 5 seconds that you haven't crashed yet, do you? Maybe if it's a taxi ride in Los Angeles!
For a system that isn't "mission" oriented, you would probably think more in terms of mean time between failure.
The average American only drives ~35 miles per day. 1 hour may represent the average time driven per day but it’s much longer than the average trip length. Which doesn’t mean it’s a bad point of comparison but distance seems like the more common metric.
Failure intervals generally use multiple different metrics depending on context. You see statistics for fatalities per year or per passenger mile as both being relevant depending on the context. A life insurance company and a car insurance company would focus on different statistics.
The natural comparison for AI seems to be each individual situation. With City and Highway driving being different enough to be considered separately. In that context ~5 second intervals seem fairly natural because the car who accurately identified everything 0.001 seconds ago is probably going to avoid maying a major mistake for a few seconds. In much the same way a human driver can blink or look at a reed view mirror without significant risk.
Decision events probably correlate better with distance than time.
Like your self driving car shouldn't be meaningfully more reliable if it slows down by 5 mph (because if it is the operating parameters are poorly chosen!).
5MPH makes a dramatic difference in stopping distance so that alone is going to make a car noticeably safer.
However, safety isn’t the only thing people care about or it would be illegal to sell cars with the downright ludicrous top speeds currently available.
You're trying to talk about system reliability as a whole, while the others are talking about reliability of individual decisions or event handling.
It's like the difference between talking about all-causes mortality versus the reliability of a bullet proof vest. Some don't buy that the vest is more reliable when we are not being shot.
An hour on the other hand is completely arbitrary, why not a day or a minute?