You can get from one coast to the other entirely on a single interstate highway (e.g. I-80), and do it in dry weather and clear conditions. It's the easiest possible thing for an autonomous car to do and is basically just adaptive cruise control.
The hard part is making it work with crazy cyclists on city streets and blizzard conditions and defending against malicious adversaries who purposely try to confuse the car into crashing into something.
> Musk also promised a demonstration of a fully autonomous drive from Los Angeles to New York by the end of 2017
As in "without the need for a single touch from the driver". Doesn't really matter, AFAIK they are nowhere close.