It's very much tied to
System "Security Level".
For example, I took 7 supercruise trips last night, of 7.5 minutes each, with the throttle at the very bottom of the blue zone. Flying a hauler, neutral reputation, cargo hold full of 16t of shiny, valuable cargo. I was not interdicted even once. That was in a
High Security Communist system.
Contrast that to a test in the same ship, same cargo, same duration flights. In just 4 flights, I was interdicted 8 times. That was in a
Low Security Anarchy system.
Apparently, the ship you fly can make a difference. The reputation you have with local factions can make a difference.
But... "all else being equal"... "High Security" seems to be safer than "Medium Security"... and don't think of using the words "Anarchy" and "safe" in the same sentence.