The Schnellweg (en: expressway) system, a number of Bundesstraße roads, forms a structure loosely resembling a large ring road together with A2 and A7. The roads are B 3, B 6 and B 65, called Westschnellweg (B6 on the northern part, B3 on the southern part), Messeschnellweg (B3, becomes A37 near Burgdorf, crosses A2, becomes B3 again, changes to B6 at <answer>Seelhorster Kreuz</answer>, then passes the Hanover fairground as B6 and becomes A37 again before merging into A7) and Südschnellweg (starts out as B65, becomes B3/B6/B65 upon crossing Westschnellweg, then becomes B65 again at <answer>Seelhorster Kreuz</answer>).
When does the Messeschnellweg change to the B6?
Seelhorster Kreuz