Car crash reaction time
Some of the numbers might seem plucked out of thin air but they aren't


Still better have them explained. I can understand 3.6 and even 19.6 (it is \$2g\$, isn't it?), but I have no idea what 98/490 stands for. Declare them as symbolic constants with meaningful names.
I don't see a need for abs in

    reaction_time = round(abs(diff.total_seconds()), 2)


Here diff may not be negative. If it is, I'd rather throw an exception, because the result is meaningless anyways.
You are rounding too early. There is no penalty computing Rt with reaction time not being rounded, but rounding leads to loss of precision. Only round when printing results.
You have too many parenthesis. Sd = Bd + Rd is much cleaner than Sd = ((Rd)+(Bd)).