The speed limit has little to do with safety. There is no reason for the ridicules limit on the Palisades, for example. Most drivers know what the safety limit is, and that limit is often observed even when it is 15-20 over the posted limit.
And please inform us, which Mitzvas Assei or Lo Sasei is Dina D’malchusa Dina? The only reason to observe the posted limit is to avoid a ticket.