This is one of those polls where it'd be useful to have two votes. I agree with both the 4th and 6th options. I think it's arbitrary, but like any other speed it is okay when safe and too fast when it's unsafe. However, right now I feel the arbitrary way 100mph is sometimes presented as being especially dangerous (e.g. reading "in excess of 100mph" in the media and the way the law deals more severely with >100mph offenders than 99mph or below) is perhaps more important, not least because it reinforces the idea that a numerical speed by itself can be dangerous and detracts from the key point of a safe speed for the circumstances.
I think 100mph gets the attention it does for psychological and historical reasons. We've had cars capable of double digit mph speeds available for many years - since not long after cars were invented in fact, so certainly for the entire life of almost everyone alive in the world today. In any event, horses have given people double digit mph speeds for millenia and even a lardarse like me can put on a pair of skis and get over 10mph with no assistance. Basically double digit MPH speeds are the norm for all of us and have been for many generations. OTOH no wheeled vehicle has ever reached a four digit mph speed, and it's probably not going to happen any time soon. So for some people the first speed that really has a bit of a "wow" factor ends up being 100mph simply because it has that 3rd digit and was unattainable by many cars until 15-20 or so years ago (my first two cars couldn't even downhill with a tail wind, but every one since then can break 100).
But it certainly is entirely arbitrary. What if we had a different counting system? What if we used base 8 or 12 for some reason? What we call 100mph would end up being a fairly unremarkable looking number, but it's still the same speed. (Just as well we don't use binary or we'd all get banned for doing 4mph
). Alternatively, what if there was no such thing as the mile but we still used furlongs? 100 furlongs per hour is a very sedate 12.5mph, but maybe society would be getting its collective knickers in a twist about 1000 furlongs per hour, which would give us an extra 25mph before ton-of-bricks time. What if we'd stuck with the Roman definition of the mile, which would chuck everything out by a few percent (dunno how much, but I know it's not the same as a mile today)? What if the statute mile didn't exist and all miles were nautical miles, chucking it out by a different amount again? What if we had 20 hours in a day instead of 24? We could go on indefintely, but the point is that there is nothing particularly special about 100 miles per hour in either the number 100 or in what we think of as a mile or an hour.
I also think that perhaps there's a human need to hang a label on something to say "oooo, that's too fast" and 100mph just so happens to fit nicely. We then end up with people thinking it's special because the law treats it differently and the law treating it differently because people think it's special. It's almost worth giving up and going metric just to force an official rethink - obviously 62 and a bit mph isn't particualrly worrying, so they couldn't treat 100 Kph differently, and 161 Kph isn't exactly right either. I think they'd have to pick 150 or 160 Kph as their magic speed, but I can imagine a lot of head scratching while they try to justify not using the same actual speed as before