Meaning:
The literal translation would be “broadness!” or “wideness!” which obviously does not make much sense in English. “Szerokości!” is a contracted form of the traditional wishes directed at somebody leaving to drive a long way: “szerokiej drogi!” which means: “may the road be wide (for you)” and might have something to do with Polish roads being in general of a low quality and dangerous. The shortened version created probably in the early 90s still sounds very slangy and far from official language. It is primarily used among truck drivers, less often by motorcycle and car enthusiasts to stress the emphasize that they are a club of their own.