Translation:The safest road is that one on the left.
either safer or safest would be correct here because of structural ambiguity. 'più sicura' here is an adjective and because the subject already has a definite article none is added to 'più sicura' if it is intended as a superlative. the context of a conversation could make clear the intended use. "all these streets are dodgy at best. ma la strada più sicura è quella sulla sinistra." (superlative). "i've driven down both these streets before and la strada più sicura è quella sulla sinistra." (comparative) in the sentence that duo posted only two streets are mentioned, suggesting the best choice would be comparative, but without further context it's not clear.