ABS does increase stopping distance. it's designed to prevent the tires from locking up on the pavement, which will cause all available traction to be used for slowing down the vehicle, rather than ~80% stopping capablility and ~20% turning.
think about it, what has the potential to slow down something faster, a big chunk of rubber scrubbing against asphalt with thousands of pounds of weight on it, or the same chunk of rubber rolling to a stop?
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