Can the post office refuse mail delivery on a private road?

Yes. I live on a private dirt road that has fallen into disrepair during the winter months. I received a letter from my local post office telling me that my delivery services are suspended until the road has been graded for safe travel. Ironically that letter was delivered to my mailbox, proving that my road is drivable. Yet it was the last letter I have received since. I can't find regulations regarding what the USPS uses as guidelines to determining whether or not they will deliver mail. But I know for a fact that they can just decide one day that they no longer will deliver mail to your house.