Assuming there are no traffic lights, stop signs, yield signs, or any other signs, and that there is simply a zebra crossing / crosswalk painted on the road.

Are drivers legally required to stop to allow pedestrians to cross?

I understand that the answer to this question may vary across jurisdictions, but is there perhaps a general rule in the US? It seems that at least in Michigan, drivers seem to pay little attention to these crosswalks, which makes me wonder what purpose they serve.