I cannot disagree more strongly with the advice to carry a gas can in the trunk (or on the rear bumper). If you are rear-ended, a collision which you can do almost nothing to prevent, at even moderate speeds, that gas can become a highly incindiery bomb. That only has to happen once to spoil your whole trip.
I am not aware of anywhere in the United States or Canada where, if you keep your gas tank at least half full, you cannot get to the next gas station down the road. I am aware, on the other hand, of a whole lot of people who have burned to death in rear end collisions.
The states and federal government have also done a great job in marking highways where you are about to enter a longer-than-normal stretch where gasoline and other services are not available. My advice is to keep your gas tank topped off and leave the jerry cans at home.