I use the same analogy when asked 'how fast?' Either 'about as fast as a world championship Marathon runner' or 'about the speed of a languid cyclist'. 12.5 mph or 20 kph doesn't mean much to most people, probably better if they want numbers to say 5.5 metres/sec or 18 feet/sec- about the length of a small American Car
or a couple of feet more than two Smart cars...