The distance varies mainly with approach speed I think, which may be why it seems further on rural roads than town roads.
You could try playing with Off Road Transition - which lurks in a very obscure place, in the Routes page and left key (menu key). The default setting is auto. If you change this to Distance you can then set a distance, say 50m - set 50 then alter the units to metres -
this may achieve something (no promises, not really explored it)