Vehicle

Mechanical vehicles, especially those running on petrol, are generally non existent in the Fallout universe. The most common means of transport are brahmin carts. The super mutant army is also said to have used some steam-powered trucks.

The war was fought mainly for resources. The last resources remaining were probably depleted in the war itself. Without resources, vehicles couldn't be driven anymore. Furthermore, it is generally accepted that after the war, much of the human knowledge was lost. The knowledge of how to tend to the high tech fusion cars that existed before the war was lost, and with it the necessary skills to repair the cars.

There were no vehicles at all in Fallout 1 outside of the brahmin carts and a steam truck mentioned at one point in the game.

Fallout 2 featured a fully working car, the Highwayman, and high tech flying machines, known as Vertibirds. It is also mentioned that the NCR military includes mechanized forces.

Fallout Tactics Featured many vehicles that the player could operate, causing yet another point of controversy that contributed to the opinion of Fallout Tactics being non-canon although the cars do not appear to actually use gas as there are no emissions of any kind. Also, Fallout Tactics is set in what is the heart land of the automotive industry, giving some argument to the plausible presence of functioning vehicles.

However, since there are still untouched reserves of pre-war tech (like the Sierra Army Depot), it is reasonable to assume that vehicles will continue to have a presence in the Fallout universe. If the Den is capable of keeping a fusion car from rotting, imagine what a more advanced town might have squirreled away.