That’s a great question. Many places claim to have “one of the longest runs in the world” but most of them are combinations of off-trail (sometimes off piste even) an that gets difficult to measure accurately. A groomed run on the other hand is more straight up. If it is groomed there is usually only one way to ride it, so just measure that. As for the longest groomed tun out there, it would be hard to beat the Sarenne run at Alpe d’Huez in France. It is almost 10 miles long!