I am encountering a strange problem all of a sudden. I have a shape file with a single polyline with a known direction. When I Create a Route with this line, the direction "Flips". Any ideas?
When you created the route was any of these checked?
The position from which measures will be accumulated for each output route. This parameter is ignored when the measure source is TWO_FIELDS.
These in cases will over rule digitized direction.
One way to get around it is to edit your line segment and chose the appropriate method of accumulating the measures:
Routes will only respect the direction of a line if you use From and To end measures on the original lines to build them, otherwise they change direction according to the orientation priority corner you chose with the Create Route tool. The original line did not flip, but the Route events for that line easily can flip if you built the route from the wrong corner. Anyway, post the Create Route settings you used if you want more help to get the behavior you want.
And if you need to flip lines, there is a tool for that, or you can do in an edit session manually.
Use case example (but not for routes), I use the flip line in a script whee I need to buffer a flight lines, based on the value in a field (left or right). Originally the lines are all contiguous and "in the same direction". I select all those with the on field = "right", flip them, then I can buffer them using the left option and they will buffer as I need them. I mention this to point out that it can be scripted if you have some way to select those lines that are "in the wrong direction".