Those are called "detectable warning surfaces" and the knobs are not for traction but for the visually-impaired to be able to find their way down the sidewalk ramp and locate the place where they can safely stand without stepping into the traffic lane.
I just happened to notice this suburban intersection today:
It has 8 of these recently-installed "detectable warning surfaces" and given where they were installed [[probably in violation of the ADA design requirements) I fail to see how a blind person will ever find them. Common sense should have told the city engineers and their cement contractor that this was a total waste of time and money.
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