This is a bit too technical for me, although I get most of what it says. It's a study done by researchers using three 50,000 watt radio stations in Baltimore to see what causes the most interference on AM in cars. Researchers chose a route that included both downtown traffic and places a bit farther away from built-up areas. 

You can read what they found at the link below, but here's the stat that stood out to me:

"The study also observed that noise levels tend to decrease at higher frequencies in the AM band."

That doesn't completely surprise me, but all things being equal, it means not all 50K AM signals are created equal. 

NRSC Releases Details of AM Band RF Noise Study