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. :-)

[0] * * * WARNING A lot of Flashing Lights! * * * https://www.yhchang.com/SAMSUNG_MEANS_TO_COME_V.html

5' is also a notation of 5 Feet..