When I went to watch Hertha Berlin at the Olympic Stadium, a tram pulled out just as I walked on to the platform. I thought to myself, now I'll have to wait ten minutes, but no, another one pulled in straight away.
Admittedly, the Olympic Stadium is the last stop on that particular branch line, so the trams can be stacked up in waiting and this is obviously not possible at the Etihad at present.
Also, many clubs in Germany include the price of the public transport to and from the town / city centre, thus encouraging supporters to use it, rather than using their cars.