Mainly because of its heritage as Polish town, Hamtramck developed a sense of separateness from Detroit, whereas the African-Americans moving into Highland Park were mainly from Detroit. Even as the Polish population in Hamtramck, dwindled, subsequent generations of immigrants bought into the idea that Hamtramck was truly a world unto itself.
Because of this, Hamtramck citizens truly stuck together during the hard times and the city was very aggressive in policing its borders "against" Detroit. In fact, Hamtramck got into some trouble in the early 1970s with the federal government when it was found that the city's urban renewal plans were purposefully designed to try and reduce the influx of African-Americans from Detroit.
Still, Hamtramck did lose half of its population over that time, so it's not like the city didn't experience some decay. The area west of I-75 isn't looking so hot nowadays, and same with a few other spots.
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