Thats driving on what is known as the "curtilage," the space on a private property that is effectively "partially public". FedEx can drive up a driveway, and use your walkway to deliver a package. That is okay. FedEx can not jump your fence and enter your backyard to leave your package somewhere safe. The backyard is off limits. As such, an irrigation access road is NOT within the curtliage of any property and thus is more of a "backyard" to those not permitted. There are signs and gates which state that yet FedEx directed its drivers down that road several times and they complied.
I imagine the 'curtilage' for many properties had quite a few 'private property' signs. If GPS directs a driver into a road marked as such, a driver may easily assume that there will be a delivery address on that private property, which would warrant ignoring the signs.