Arbitrary

Beau is exactly right. They're trying to make you NOTICE the sign. It worked.

You know, I thought my apartment complex in college was bad. It was listed at 8mph! Never seen a half though! Nice.

My guess is that they're counting on the novelty of the sign to make people pay attention. I've seen a few odd signs like this around Atlanta - but none with a 1/2. I've also seen a couple of homemade signs that play on the same effect (if they work) to get people to recognize them and pay attention to their speed rather than just blending into the background noise.