My best guess is it's a busy road so it's dangerous and not really meant for bicycle and walking. I've done 13 miles yesterday to get a comic book on a bike and this right here is the distance between my house and a friend's house I told I can come on bike because what I just did gave me a feeling I could do it but my ass hurts so not right now. But yeah I want to see how this would play out. Before I would walk but I took a bike to a comic book store because it would've closed if I walk and I ended up getting there in time. Took longer than expected something that should've been an hour probably took 2 or 3 hours. So yeah I can do half of that for sure.
The warning makes sense when you consider gmaps randomly tells you to ride into the traffic on major highways.
I've never had that one happen and, until recently, that was the only navigational aid I had. It's still the only one I use on my motorbike.