@superbetsy It's a warning there's train/streetcar/something tracks in the pavement whose gaps are wide enough to catch a bike tire (if you hit it wrong and it wrenches your front tire sideways).

This happened to my wife one time many years ago: she flew off and landed helmet first (thank goodness for helmets!) and got a bunch of scrapes and a really nasty black eye. For a week or so every time we were in public - starting with the nurses at the ER - I got the worst looks from people making unfortunate assumptions about the cause of said black eye. But she was ok.