You are changing the problem at hand. It is not how to create a distance relation, it is how to add more data to an existing edge. To do that in RDF you need reification, which is notoriously painful[1]. Moreover <http://www.my_cool_domain.fr/prop/distance54> is not equal to "54" and 54 cannot be inferred from it because URI are opaques. In addition, you have now a totally different property than those of every pair of cities that are not 54 km afar. So, RDF have some strengths and good points but in the current case it is creating unnecessary problems to encode a very simple situation.

[1] https://www.safaribooksonline.com/library/view/practical-rdf...