iD’s implementations of the roadheight and roadspeed field types share a lot in common. Meanwhile, other fields would benefit from unit dropdowns too but would need a different set of units, for example the Max Weight field: openstreetmap/iD#6707. This category has more examples of measurement-typed keys. We should replace the roadheight and roadspeed field types with a more general measurement field type, along with a dimension property that can be set to one of several values like area and duration.

For dimension naming ideas, this CLDR file lists industry-recognized measure units, each of which starts with a category subtag.