We Don't Need Roads

"Roads? Where we're going we don't need . . . roads."

—Doc Brown, Back to The Future

Cars in fiction tend to be pretty awesome. Some just look cool and go fast. Others have weapons systems and active camouflage built into them. And then there are these cars, which can transform in order to traverse terrain meant for other vehicles. Like boats. Or planes. There are practical reasons for this to be the case, of course; after all, you wouldn't want to have to give up chasing the villains because they planned a water escape that you didn't account for. The primary reason, though, is the Rule of Cool. Because being able to drive off a pier in a sports car, transform on the way down, and hit the water in a speedboat is just plain Badass.

Less drastic modifications can be expected as well for different terrain on land, like retractable tire studs for driving on ice and other slick surfaces. Any weapon systems the car is equipped with will sometimes inexplicably change in order to fit the new environment; rather than missiles, the car will have torpedos.

The Gadgeteer Genius is almost certainly the one who made all these awesome modifications. A subtrope of Cool Car and a Super-Trope for Flying Car, Hubcap Hovercraft and Amphibious Automobile. Often overlaps with Cool Boat and Cool Plane, for obvious reasons.

Examples of We Don't Need Roads include:

Anime and Manga

Speed Racer's car can become a sub.
One story arc in Allison and Lillia shows an aerosled (see the description in the Real Life section), which is the only available vehicle capable of navigating through a blizzard.

Film

At the end of Back to The Future, Doc Brown travels forward to 2015 and gets some pretty major work done on the Delorean, including a "hover conversion" that's really more of a "flight conversion". At the end of Back to the Future III, he does the same thing to a locomotive.
- The fact that the Delorean is a time machine in the first place qualifies it for this trope.
Many James Bond examples.
- In Moonraker. Bond's gondola transforms into a hovercraft, allowing him to drive it onto the streets of Venice.
- Scaramanga in The Man with the Golden Gun has a car that can fly (with an attachable wing and engine).
The Last Starfighter. Centauri's car is also a starship.
In Blues Brothers 2000, the updated Bluesmobile could function underwater.
Quorra's car in Tron: Legacy can operate in the rough terrain off The Grid, unlike most ground vehicles in Clu's arsenal.

Literature

Chitty Chitty Bang Bang might actually be the Ur Example of this trope. The major premise was basically "Check out this car: it can fly."
The Robert Heinlein novel The Puppet Masters has "triphibians": cars that can drive on land, fly through the air and travel on water.
Possible example (depending on how loosely you define "car"): The Terror in Jules Verne's Master of the World functions as an aircraft, boat, submarine and automobile.

Tabletop Games

MegaTraveller Referee's Companion. At Tech Level 8 vehicle transportation includes triphibians, which can travel on land, in the water and through the air.
Champions. Vehicles can have any form of movement super characters can, including flight, swimming, tunneling and even teleportation. They can also have the power of Shapeshifting so they have an appropriate form for the movement mode they're in.
Game Research Group's Helltank Destroyer had Triphibious Cruisers, which could travel on land, in (and under) water and through the air.
Car Wars has "Grasshoppers", lightweight car/helicopter hybrids. Their armour tends to be rather poor, but if someone is chasing you too closely? Well, many of them come with bombs, you see...

Video Games

The Spy Hunter games are probably one of the best examples of this. In addition to being able to transform into a Cool Boat, if the armor takes too much damage, it will be shed to reveal a motorcycle (or a jetski) underneath). In the sequel, it can optionally convert into an off-road mode, where the ground clearance is raised, wheels widened and the tyres have spikes come out of then, which reduces speed but significantly improves handling when offroad.
One of the more expensive mods you can add to your car in Streets Of Sim City is a flight modification which, well, allows you to fly. Why is this useful, you ask? Dude, you can make your car frakking fly. Who needs a reason?
Certain Cybran Nation warships in Supreme Commander can deploy legs and turn into a Spider Ship, walking out of the sea and onto land to support their ground forces when assaulting a shoreline.

Western Animation

The Teenage Mutant Ninja Turtles' van could become a hovercraft of some sort.
In Double-O-duck, Launchpad McQuack is given a car that can turn into a boat or an helicopter.
Danger Mouse has a car that can also become an airplane. In fact, it's seen up in the air more often than not.
The Wallace and Gromit short "A Close Shave" features a motorcycle sidecar that doubles as a plane.
Superfriends (1973/74) episode "Too Hot To Handle". The aliens have a fire truck that can convert into a rocket ship and fly.
Batman the Brave And The Bold: Both the Batmobile and Green Arrow's Arrowcar can transform into planes. The Batmobile can also turn into a Giant Mecha.

Real Life

Hovercraft, though not the miracle originally hoped for, are effective ferries and possibly the only 'surface' vehicle that can cross minefields safely.
Aerosled (Russian: "аэросани") — a snowmobile with wide ski undercarriage and pushing aeroplane propeller (usually with engine pod attached to the top rear side of the hull). If you want something that can travel on deep snow with decent speed without high expenses and/or requirements of flying and hovering vehicles, and is resistant to most things it's going to face in a climate where such need regularly arises, that's aerosled. Invented by Igor Sikorsky in late 1900s, and produced in series in Soviet Union from 1920s, including several lightly armored military types, though that was much more of a challenge, of course. There are also distinct amphibious variants — vehicles with pontoons/skis and flat boats that move as airboats in water. Modern variants range from mini-buses on sleds and with airscrew to motorboat conversions (with skis as "outriggers" or sled as bottom reinforcement and different motor/rudder assembly attached), to wild variety of DIY vehicles, since it's not particularly demanding (doesn't have problems with intakes and skirt as a hovercraft could, does not need wings and powerful engines for huge lifting force like anything that actually flies, nor even solid frame and good suspensions of a wheeled vehicle, since snow surfaces are relatively smooth).
Many examples under Amphibious Automobile.
A few tries at Flying Car.