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The tram line has been open for about two years now, but people still have trouble remembering the tram has the right of way. Maybe this will help, the audible warning doesn't seem to be enough since many people have headphones (understandable, noise cancelling is a must if the city has any car traffic).
I'm pretty sure the real reason it's projected is so it doesn't get covered by snow in the winter. At least that's the reason for projected street signs elsewhere in Finland, such as in Oulu.
That's partly the reason, that's why they're used e.g. for marking bike paths. This has the added benefit of being very noticeable when you're looking at the ground.