Adding a New Route

I use another service to log my training but thought about moving to BT but when I tried to enter my ride from yesterday I came across an issue.  When mapping out a new route I noticed that it doesn't automatically follow the road and instead just draws a straight line between the 2 points I pick.  Is there a setting I need to change to get it to do this?  Some other sites (e.g. Map My Ride) will follow the rode which makes entering a 60 mile ride pretty easy.  I'm probably missing something very simple so please let me know.

Thanks