For every 10 men over 65 years old in Canada, there are nearly 12 women over 65 years old. For every 10 men over 85 years old, there are roughly 20 women over 85 years old. This is true of most countries, more or less. Combined with the fact that older people tend to get shorter as they age, this means (among other things) that senior citizens, and especially older senior citizens, are on average much shorter than younger adults are. The average height of an adult in Canada is 5″7 (5″10 for men and 5″4 for women); the average height of a Canadian over 85 years old is probably 5″2 or 5″3.
Seniors, and children, also tend to be much more vulnerable to traffic than adults in general are. This includes not just auto traffic, but also a vulnerability to bikes in busy bike lanes, e-bikes, e-biking food couriers, e-scooter-wielding teenagers and tourists, etc. etc.
Seniors also tend to be less reckless on the road than younger people, particularly younger men. Men for example are much more likely to bike while drunk, and bike recklessly in general. Men are also heavier than women, so the impact they have when they do crash into a fellow cyclist or pedestrian tends to be greater than would be for a woman, child, or senior citizen.
Seniors and disabled cyclists also tend to use recumbent or semi-recumbent tricycles more often than adults in general do. (Though because of SUVs and trucks and road traffic in general, these recumbents are sadly not used very much on city streets). These recumbent bikes are obviously much shorter in height than conventional adult bikes are too.
I think you can perhaps see where I’m going with this. If we are going to turn our cities into much more bike-friendly places than they have been thus far, and especially if we are going to build higher-speed bike and e-bike expressways, then we also need ways of allowing the most vulnerable populations to bike without facing too many dangers and discomforts, like they currently do in our car-dominated cities.
The simplest way to do this is to create multi-lane bike paths, in which the outer lanes will tend to have lower speeds than the inner lanes. An additional feature that might be useful, however, is archway-bridge underpasses, in which the outer lanes of the underpass have lower ceilings than the inner lanes:
If the outer lane had a ceiling that was, say, only 7 feet tall, that might be short enough to nudge taller riders into using the more spacious inner lanes of the bike path, and so free up the outer lane for use by children, seniors, and recumbent bike or wheelchair users.
If the outer lane ceiling was only 5 feet tall, that would certainly do the trick, although it might also raise the risk of a taller rider accidentally using that lane and bumping their head…
Bikeways with archway underpasses and children-friendly low-ceiling outer lanes might be particularly useful in the developing world, where there are still lots of children, lots of bicycles and motorbikes, and, sadly, lots of road accidents, road traffic, and air and noise pollution. But it might also be increasingly useful in the rich world, where populations are aging, bike and e-bike ridership is rising, and car dependence (and in North America, obesity) is extremely high.