ABS only needs to sense when the wheel locks up, or stops spinning. You only need one when for that. Traction control usually uses multiple sensors, maybe you’re thinking of that?
Except a wheel doesn’t need to actually lock up for braking to cause loss of traction. Modern ABS systems don’t just pulse the brakes when the wheel stops spinning, it does so upon loss of traction. To determine whether traction has been lost, one wheel isn’t enough of a sample size. The ABS works in tandem with traction control.
Regenerative braking cannot lock up, which is where the whole idea of calling it E-ABS got started, but it absolutely can still brake hard enough to cause loss of traction. I experience it all the time when slowing down on gravel with my electric skateboards, for example. It simply doesn’t, and cannot, work the way ABS does on a mechanical braking systems, where braking is pulsed to regain traction the instant it is lost.
Yes, all ABS really means is anti-lock braking system, but what most people expect it to DO is to guarantee that traction is maintained while braking. (Even though that isn’t technically correct, either)
