I unlocked it for you.
Both the 'from' segment and the 'to' segment must be unlocked or locked at your level or lower in order for you to modify the turn arrow for that route.
Whenever you lock a segment or modify it in any way, if it is already locked, it now becomes locked at your level. If you get promoted later, the lock will 'float up to your new level'.
Some things SHOULD be locked. See http://www.waze.com/wiki/index.php/Auto ... g_of_roads