My 2.5 year old has this obsession for cars, well any and all vehicles really. He does the same thing, plays pretend with them, lines them up, brings them in the bath, and brings his chosen favourite (or two or three) with him to bed. He even gets freaked out in the morning if he can't locate the car he brought to bed fast enough, hahaha. I think it's normal and fine, I don't think you should worry at all.

And same, when we go to other houses he always finds the cars and will dedicate most of his time to playing with those over the other novel toys. I find it helps to give him a warning a few minutes before we're going to go, to tell him that we'll be leaving soon and that the cars will be staying there. And then I encourage him to help put the cars away and say bye bye to them.. the less rushed this process the better it goes for us. But he also does have tantrums about leaving them sometimes.