How make custom route just 1-way (not Double Back, Closed Loop, or Reverse).

How make custom route just 1-way (not Double Back, Closed Loop, or Reverse).

Sometimes, my custom route is automatically made a Double Back. 

Other times, i can start my route as 1-way and it STAYS 1-way - but if i then click the buttons for Double Back or Closed Loop, my route stops being 1-way and I can't get it back to 1-way.  

I need this 1-way option for when I want to measure the steepness of a route.