Sentence:  When the automobile was in its embryonic stage , such roads as existed were pretty much open roads with the tacit understanding that horses should not be unduly terrified <WSD>being</WSD> about the only rule governing where , when and how fast a car could go . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)