I just copied what I saw online. I'm certain the writer converted kilometers per hour (kph) to mph. 31 mph is 50 kph, so the speed limit was probably posted at 50kph.
I remember as a young kid my family made a road trip down to Ensenada in Baja California. My dad saw a speed limit sign on the highway reading '100' and decided to see what our new '61 Oldsmobile would do. We found out when we got to Ensenada that the speed limit was in kilometers per hour. We could have ended up in a Mexican jail forever, LOL! Back then nobody even had a hint of, or cared, what the conversion factor to metrics was.