I think that makes sense, yeah. I would rather not flash other digits (55) as that could be confusing for someone who sees that number go by (was that the bus number or the number of minutes?)
I don't think that it's as common to write or read
I'll be there in 5'
Also, since this is a sign seen from about 10-15 meters away, using an apostrophe might lead to confusion (it might not be clear to read).
The aesthetics of the big flashing words are also intentional. Way, way back in my brain there was definitely [0] playing while I was creating this. :-)