diff --git a/include/extractor/travel_mode.hpp b/include/extractor/travel_mode.hpp
index 1de8c2266..af16bcaee 100644
--- a/include/extractor/travel_mode.hpp
+++ b/include/extractor/travel_mode.hpp
@@ -6,6 +6,8 @@ namespace osrm
 namespace extractor
 {
 
+// This is a char instead of a typed enum, so that we can
+// pack it into e.g. a "TravelMode mode : 4" packed bitfield
 using TravelMode = unsigned char;
 }
 }