Special Relativity

What happens if a ball is traveling at a universally constant speed, say measured to be 50 mph straight up relative to the people on a bus thats moving 40 mph relative to a sidewalk, but also measured at 50 mph at a 3-4-5 triangle relative to the people on the sidewalk?
In that case the ball exhibits a "special relativity" as it travels at a constant speed relative to all observers, which means either time is technically dilated or the bus length is technically shortened:
This phenominon of Special Relativity time dilation has been shown to exist for the decay times of particles in atom blasters:

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