I ride a lot on highways (New Zealand highways) and have found that on my bents and trike cars do tend to give me more space than when riding my road or mtb bikes.
I figure being low isnt the problem so much because drivers manage to see lines painted on the road, I think the key is to be predictable and try to ride where drivers would expect to encounter traffic ie I don't hug the shoulder of the road, I ride partially in the lane but do move over when the car is approaching.
Flags, vests etc are good although I go with some Hi vis tape on my seat backs and bags.... Mirrors are a must.
Remember we are traffic so I try to act like it
