"Rhedwch yn araf!"
Reminds me of the Latin phrase 'Festina lente' which means that you should take urgent things not too fast, lest mistakes are made; or something to that effect
Yes, 'Mae'r gwasanaeth bws yn rhedeg yn araf' would be a very good sentence.
Although whether you want to command a slow service?
Not me; but looking at the current service, someone has passed the word. Possibly not in Welsh, however.