It doesn’t need to be if you enforce the rules very stirctly at first using both officers on the ground and cameras to fine drivers who think they can just slip into the lane for a bit. Repeat offenders get stricter fines and even impoundment.

You could go one step further and not let cars on the tram line at all without grade seperation. You could do this by building dedicated seperate lanes for the trams or by not including any adjacent car lanes at all. And of course all of this should be done with transit signal priority to reduce the time they waste at red lights and help keep the tram on time with its schedule.