Pet peeve: 30km/h = 20mph. 30.0km/h = 18.6mph. 50km/h = 30mph, not 31mph. It's so annoying when people don't do significant digits correctly when converting between metric and imperial, it completely breaks the flow.
No, because the imperial speed limit corresponding to 30km/h is 20mph, and imperial drivers would have encountered that and thus would have a clear and absolutely trivial frame of reference. Likewise 50km/h and 30mph.
18.6mph and 31mph is not correct, it's worthless pedantry completely missing the point to the disservice of the reader.
But I completely agree that from a practical perspective, 30 kph = 20 mph, 100 kph = 65 mph, and so on.
And this is exactly the kind of worthless pedantry that masklinn called out.
The real solution is separation of routing layers.
Also, 30km/h is used in the same places that would be 25mph zones in the US, i.e. residential areas and school zones on thoroughfares. The default speed on larger roads in cities is 50km/h, which is just below 35mph, a very common city throughfare speed limit in the US.
And on the flipside, many Swedish freeways have a speed limit of 120km/h, i.e. 75mph, which you're hard pressed to find in the US at all, and certainly not in California. It's only in the last decade they've changed the speed limit to 120km/h, but it's because freeways have been made safer, and cars have been made safer, so it makes sense to raise the limit.
Try getting the speed limit raised in California. I'm not gonna hold my breath waiting for that.