Horse and cart never worked all that well, to the point that long distance travel by land was extremely rare. Deep ruts would form in the dirt roadways and mud

Horse and cart never worked all that well, to the point that long distance travel by land was extremely rare. Deep ruts would form in the dirt roadways and mud would cause constant obstruction. It was easier sail around South America than to travel by land between San Francisco and New York City. It wasn't until the railroad that travel by land was possible at scale. For the USA and Canada the railroad was seen as the only way to keep the vast land together as a single nation which is largely why it was subsidized. Further much of the subdisdy was land grants to land which would otherwise be economically useless without the railroad to allow transport of goods. While self driving vehicles may improve efficiency of our roadways it is unlikely to be anywhere near the same scale of improvement that railroads brought, and the need to connect the country for political reasons is not there.