Driving
Driving
[Black Hat, carrying a rock, is walking toward Cueball, leaving a trail of sand.] Black Hat: Would you guess this weighs as much as a small adult? Cueball: What? Cueball: Uh, probably.
[Black Hat walks past Cueball who turns to look after him.] Black Hat: Great!
[Black Hat has walked out of the frame. Cueball is looking in the direction he left in. Several noises and voices are coming from off-panel.] Off-panel: *Thump* Car voice (off-panel): Please fasten your seatbelt. Off-panel: *click* Black Hat (off-panel): Take me to Anchorage, Alaska. Car voice (off-panel): Navigating Off-panel: *slam*
[Black Hat walks back in the panel towards Cueball.] Car driving off: Vrrrrrrrrrrrr rrrr rrrrr rr Black Hat: I love self-driving cars. Cueball: ...Whose car was that? Black Hat: Dunno, but they shouldn't have left it running.

A self-driving car is a car that requires no human interaction to navigate streets to a destination. Thus, when Black Hat places a rock that weighs "as much as a small adult" into the car's seat, he begins the process of fooling the car into thinking it has an occupant when it does not. His purpose in doing so appears to be to send the car to Anchorage , Alaska , which is presumably quite far from where Black Hat and Cueball are standing, thus taking the car far away from its owner with relatively little effort on the part of Black Hat. This is yet another evil prank from xkcd's resident classhole .
The title text references the fact that driving to Alaska from the contiguous lower 48 states requires two border crossings, once into Canada from the mainland, and once from Canada into Alaska. The car apparently begins some distance from the Canadian border, since it will likely run out of gas before reaching Canada. Title text expresses regret about this probable failure; perhaps Randall was looking forward to the encounter between the border guards and the vehicle's "occupant."  However, even if the car does not get to Anchorage, Black Hat will have created a serious problem for its owner who will have to report the car as stolen.
Shortly after this comic appeared, Tesla released footage of a robotic charger that can connect itself to a Tesla automatically. If this kind of technology becomes common then a self-driving electric car might be able to make a transcontinental journey without human intervention.
At the time of the release of this comic there were no places where self-driving cars were for sale to individuals. However, several corporate-owned test cars are frequently seen on public roads (such as those operated by Google among others). Nevada, Florida, California and Michigan were the first states to allow the testing of self-driving cars on public roads, and this legality is quickly spreading to many other states, as well as several countries in Europe. Alternatively, Randall might be setting this comic in an idyllic near future, wherein you could drive all over the country (and Canada) with these cars!
It could have been Beret Guy's car - see 1493: Meeting . Self-driving cars are a recurring topic on xkcd.
[Black Hat, carrying a rock, is walking toward Cueball, leaving a trail of sand.] Black Hat: Would you guess this weighs as much as a small adult? Cueball: What? Cueball: Uh, probably.
[Black Hat walks past Cueball who turns to look after him.] Black Hat: Great!
[Black Hat has walked out of the frame. Cueball is looking in the direction he left in. Several noises and voices are coming from off-panel.] Off-panel: *Thump* Car voice (off-panel): Please fasten your seatbelt. Off-panel: *click* Black Hat (off-panel): Take me to Anchorage, Alaska. Car voice (off-panel): Navigating Off-panel: *slam*
[Black Hat walks back in the panel towards Cueball.] Car driving off: Vrrrrrrrrrrrr rrrr rrrrr rr Black Hat: I love self-driving cars. Cueball: ...Whose car was that? Black Hat: Dunno, but they shouldn't have left it running.
