Index: /trunk/data/validator/relation.mapcss
===================================================================
--- /trunk/data/validator/relation.mapcss	(revision 12772)
+++ /trunk/data/validator/relation.mapcss	(revision 12773)
@@ -34,2 +34,8 @@
   assertNoMatch: "relation type=enforcement enforcement=maxspeed";
 }
+
+/* #15208 */
+relation[type=route][route=mtb    ] > way[bicycle=use_sidepath],
+relation[type=route][route=bicycle] > way[bicycle=use_sidepath] { 
+  throwWarning: tr("way with {0} is part of a bicycle route relation", "{0.tag}");
+}