I've done probably 20 shirts with Siser Glow in the Dark - out of a batch of a dozen, I had one weird, inexplicable failure where all the letters came off of the shirt - front and back while the other 11 were fine - all brand new shirts (purchased by the customer from Wal-Mart - I removed the stickers myself). Other than that I've had no problems.
It takes quite a bit of light to charge it up. I have a UV LED flashlight that will charge it up quickly, as will any black light or even a bright light bulb, but it takes a little longer. The glow will last for a while, although it gets dim pretty quick - in complete darkness it will be visible for a couple of hours, though.
If you're thinking it's going to be a safety improvement, any sort of lights from cars or streetlights is going to be brighter than the glow - heat transfer reflective material would be a much safer choice - but it won't be as cool to kids as glow in the dark...