Slowing

[Oleanda]

Is it more correct to say 'slow down' or 'slow up'?

[Ewood27]

This is just a personal opinion, but I suggest 'slow down' is more appropriate where one has the option of NOT slowing, e.g. in a car, while 'slow up' is more for where one does not have the option, as in ageing.

[Hippy]

Suggestion here from observations: "Slow down" is British usage, and "Slow up" is American.

[Quizmonster]

Phrases like this often change over time. In the past, when something was someone's responsibility, we used to say: "It's up to you." Nowadays, it's probably more common to hear: "It's down to you." As in the example you give, these would seem to be opposites, but they are - effectively - synonymous. Neither, now, is any more 'correct' than the other in general use.

[RCP]

Another case of words appearing to be opposites but actually meaning the same is flammable & inflammable. Why do we need both?

[honkytonkman]

I'm with Hippy on this one. 'Slow up' does appear to more the American usage. Same as Americans prefer to 'fill out' a form where we would say 'fill in'. But I think it's down to personal preference.

[OnlyMe]

Just an observation, but since both mean to decellarate, it would seem more logical to use the British 'slow down', as your speed does indeed go down, not up.