Tworking
We do not really know what ‘tworking’ means, we think that it is closely related to ‘twerking’. So in this context, this vandal was trying to say that these men were not working, they were ‘tworking’ as in dancing by shaking their butts. Well played street vandal, you got them good.
It is simple humor like this that just makes the world go round and in general a better place to live in. Imagine if you went your entire life without seeing signs like these posted around town. If everyone respected the rules of each sign and didn’t try to vandalize it, talk about a boring place.