Recently I've seen many videos of planes landing on car road. Is this legal? Does this only apply to only USA or even other countries?
In the USA, when an airplane is in the air, it is governed by Title 14 of the Code of Federal Regulations, also known as the Federal Aviation Regulations (FARs). FAR 91.3(b) gives the pilot in command authorization to deviate from any flight rule necessary to meet an emergency: if that pilot needs to land on a road to meet an emergency, it is certainly legal as far as the FAA is concerned.
In fact, for private flights in small airplanes, the FAA doesn't bother much at all about specifying where you choose to land. There are no regulations about it at all.
The FAA's authority to regulate stops when the gear touches the ground, though. The road, or field, or waterway, or whatever you might be landing on usually belongs to someone or some government entity. Government entities often have laws against landing aircraft in certain places like roads or waterways, but just as often, they don't. It depends on which government. But in all cases I have ever seen, if such a regulation exists, so does an exception that starts, "except in cases of emergency...."
If you are in an emergency situation and think landing on a road is the safest option for everyone, you might as well do so. Usually grass areas or fields are preferred due to the fact that roads usually have more obstructions (traffic signs, street lights, bridges, cars,...) than fields, which might only have a fence or similar.
Example: If you lose engine power over a city or densely populated area, roads are probably the safest option. When losing engine power above a agriculture area, you most probably would avoid roads as best as possible and pick a field with little vegetation.
I'm not sure about the legal side but I think if you have an emergency and need to land immediately, nobody will prosecute that emergency landing legally. However, if you land on a road without having an emergency, it probably will be prosecuted in most countries.
In most western jurisdictions, wherever a pilot decides to land is considered by the regulator to be an "aerodrome", from an aircraft operating perspective. A lake (on floats), a field, a back country road; the regulator doesn't care unless say, the operation violated a regulation about putting putting people or vehicles or buildings in danger.
The rub is who owns the "aerodrome" and what they may have to say about it. You may decide to land on dirt road, and the national regulator probably doesn't care either way (nothwithstanding any specific regulations banning certain operations), but the municipality that owns the road may not be too pleased. This gets a lot of Ag operators in trouble from time to time because they operate from dirt roads a lot.
Landing on a highway? If it's an emergency, anything goes and worry about the consequences later. If it was discretionary (non-emergency), and you came close to cars or people, you will likely be busted on endangering life and limb. On the other hand, if you do a discretionary landing on a highway with nobody around, it might just be dealing with the state/province, or whatever jurisdiction looks after the road, who decides to impound your plane... if you get caught.
Necessitas non habet legem
Necessity knows no law
This principle is known pretty much all around the world. In an emergency, you may choose to do things that are normally forbidden. However, that does not mean that you are exempted from any rules. You are still responsible and liable for your emergency actions.
Assume a simple case: you are pilot of a small plane. The engine dies. You cannot find an airport within your gliding range. There is no free area. The only possibility is a highway. You decide to land on that highway.
If you are in an emergency, and if there is no other - better - alternative, you may choose to land on the highway.
If everything goes well, and you cause no further damage: congratulations! There will be no prosecution for landing on the highway.
What could possibly go wrong?
Maybe there is an investigation, and authorities conclude that there was an airport in reach, or that you could have recovered your engine if you had followed the check list, or they believe that you faked the emergency for the sake of getting Youtube clicks, etc. Either way, if your case is not credible, you might loose your certificate.
There may be damage on your plane. Anything from a few scratches to a total loss. Hopefully, this damage is covered by an all risk insurance. The insurance company will scrutinize your case and try very hard to find anything that you could have done better and avoid paying.
You might have caused material damage to third parties. Your 3rd party liability insurance will have to cover - unless they can show that you have caused the damage through gross negligence. In that case they can try to wriggle out of their liability. Another financial hazard is the liability ceiling. If you cause a damage of several million $€¥, you might become broke for the rest of your life.
Now to the worst part. In your emergency landing, you are touching some cars or pedestrians. You did not foresee or want that, but unfortunately you killed them. You will face a trial. The court knows that you had good intentions and possibly be mild with you. It is also possible that the court is convinced that your decision was a bad one and that they will punish you for killing these people.
For the last point, let's assume you are PIC of an airliner. You have 200 people on board. You are convinced that you can save all of them, but you have to sacrifice a very small number of uninvolved people on the ground. This is not legal - you are not allowed to sacrifice anyone else to protect your passengers, no matter how odd are the numbers. You would face a trial, possibly with a tough outcome.
A pilot in a dead or alive situation will not care about losing his certificate, some fines, or financial consequences. Surviving the flight and bringing the passengers down safely would be everything. However, an emergency situation does not exempt one from responsibility and liability. Sacrifying uninvolved bystanders is not acceptable.
Legislation differs from country to country.
This post is for your entertainment only. Do not quote me after landing on the highway. Don't refer to me after an emergency landing on the Red Square.