I usually drive on the right lane of the expressway because I find that the traffic there is smoother. I keep to the speed limit of 90kmh, but occasionally encounter other road users who flash the high beam at me. Why do they do that?
There are traffic regulations and there is road etiquette.
Although you are not breaking any traffic regulation by moving at 90kmh on the right lane of an expressway, you are holding up others who may wish to overtake.
This can be frustrating to others - especially if the distance between you and the vehicle in front is vast.
You may think the motorist behind you can overtake you if he is in a hurry, but strictly speaking, overtaking on the left is not allowed.
So, to do the right thing, stay away from the right lane if you intend to keep to 90kmh. But, of course, if traffic is heavy all round, it is fine to use the right lane - even if your speed is below 90kmh.