There was (is?) a commercial station in the bay area that broadcasts simultaneously on AM and FM. I could tune the car radio to the correct frequency on each and then compare the sound quality by flipping the AM/FM switch.
FM is WAY better sound quality. It's not even close.
FM is absolutely better sound quality than AM. That's why music is mostly on FM. (On the other hand, AM uses less bandwidth and travels farther, since it's in the MF band and can bounce off the ionosphere. That's the tradeoff.)
At the same time, the highway information stations the parent poster is talking about are low power AM stations designed to play a road condition message within a distance of 3-5 miles from the antenna. The audio is often very poor quality, even for AM, because of the transmit power restrictions placed on them.
That said, it's worth noting the places you need these highway information stations most are also often places with some of the worst cellular coverage. If you drive in the mountains, they can be essential.
Although FM has a higher bandwidth, from a listeners perspective its more likely that you were experiencing a narrowband monaural AM - hence the massive difference to your ear.
Could have just been the stereo tricking your brain into thinking it sounded better. For example, in the late 1980's my family had a car that had an AM-only radio. One day while channel hopping one station just sounded so much better than anything we'd ever heard on this radio. Someone finally noticed a little STEREO light had lit up, because that station had recently started stereo broadcasts. Eventually we moved and later got a different car and I've never again seen a little light like that on an AM station.
I don’t live in the bay anymore but know for a fact sports radio knbr 680 also has an fm station, think it’s 104.5
I’ve actually taken a bit to AM lately to listen to different sporting events when outside of fm range
Not vouching for it and I wouldn’t care if am was deprecated as a vehicle radio feature (my vehicles are ‘88 and ‘02) but it still serves a purpose for sure
FM is WAY better sound quality. It's not even close.