In countries where speed limits are in km/h rather than mph, we need
a way to know that (probably a new field in the COUNTRY_CONFIG settings dictionary)
to convert any speed limit values that are provided in the OSM extract in km/h into mph
HOWEVER... because speed limits are generally defined in round numbers but the conversions don't map very cleanly, we want to round converted values to the nearest 5. So e.g. 50 km/h -> 30 mph (not 31) and 30 km/h -> 20 mph (not 19).
In countries where speed limits are in km/h rather than mph, we need