On December 3, 2001, Dean Kamen unveiled his self-balancing, 'human transporter,' the Segway

[RockRaven]

5. Their lack of catching on is probably due to branching/diverging variations...

Everything gets smaller and smarter and smoother and other brands take over... As much as we tap out stuff qwerty style on our phones these days, few of us use BlackBerrys anymore...

Two wheels horizontally aligned -- those "hoverboards" (which infamously caught fire) and such

Two wheels vertically aligned -- various electric scooters

One wheel in line with user's body -- various electric unicycles like Inmotion, Solowheel, etc.

One wheel perpendicular to user's body -- various single wheel skateboard like Onewheel, etc.



Just a couple days ago I spotted a person using one of those electric unicycles on a nominally 35 mph surface street during commute time (they were only doing 15-20, but in the middle of the lane with cars ahead and behind).