How do we know that 6 seconds of uniform acceleration

at 60 mph is equivalent to 6 seconds travel at 30 mph?

1 answer

Acceleration is defined in terms of changes in velocity. You can have equal acceleration values at any velocity.

The acceleration produced by a given force upon a given mass DOES depend slightly upon velocity, according to the special theory of relativity. However, at the low speeds that you have mentioned, the difference is negligible.