Regalia is accessible after completing the first few quests in Final Fantasy XV. It is your main medium of transportation, especially for a long-distance travel. Noctis can drive it manually or ask Ignis to drive for him.
The automatic driving mode is only available during the day from 6am to 7pm. A player can ask Ignis to drive to specific quest locations, parking spots, or even manually assign the destination. During the driving mode, a player can park Regalia at any time or even switch to a manual mode.
For the driving mode, Noctis will drive Regalia himself. It is pretty straightforward by holding R2 for an accelerator, L2 for brake/reverse, Square for U-Turn, and park the car with Circle. It is impossible to go off-road even with the manual driving mode.
Fast travel to any destination (while in choosing a destination for Regalia auto-driving mode) is available after a player already drives to that location once. This can significantly decrease traveling time and help complete side quests faster and easier.
In addition, Noctis can fast travel back to Regalia or the last camping location at any time as well.
Similar to the real world, Regalia needs gas which can be filled up at any gas station. Running out of gas during the journey leaves two options: pushing the car to the nearest gas station or paying Cindy to tow it back to the Hammerhead (with the cost)
To prevent this kind of situation, it is better to refill fuel every time you reach a gas station. Or at least, make sure to have more than a half of fuel in a gas tank. Fuel gauge can be seen on the bottom left corner while driving Regalia.
Regalia can be customized by Cindy and even upgraded into a flying ship at a later point of the game.