Well, nukes are pretty hard to build tbh, especially with the distribution of nuke building materials being watched pretty carefully. It is not impossible, but would require a lot of organisation.

I am not sure that this issue is a new one though, more people than you might expect would be willing to sit in a car which a nuclear bomb in it for the service of their country or ideology. The fact that we can now get driverless cars to do it by themselves doesn't make a nuclear car bomb any more likely or any scarier.

If you think it's scary now, think how scary it'll be in 20 years. I heard some guy on a podcast somewhere say that right now, the internet is the size of a golf ball - in 20 years, he said, it'll be the size of the sun. The number of devices connected to the internet will increase similarly. Every device (i.e. cars, planes, drones) is hackable. The terrorists will take advantage of this. We need to upgrade our security architecture. We're all so fucked right now, cyber-defense wise.

Terrorists may take this as a chance to say "OH! A chance to destroy this country and the other yes!" That's why you shouldn't have active bombs or nuke's anywhere near them! And should really keep missiles and anything explosive at high security so high you should basically have nearly a dozen of tank's right at them. Don't even let a finger touch them is what the country should do.

You speak of bombs, nuclear or not, as if that is the worst weapon mankind possesses atm...

If damage is the goal of a attack today, it's quite obvious that the game has stepped up since the World war, as we have once again developed the experiments from the Great war into real abominations; Toxins, viruses, generally speaking biological warfare. Infecting a major airport in the US with smallpox, or one in Europe would not only send that country back decades or so years, also the whole continent would take a timeless of at least five decades years back in time.

Blowing up a dirty bomb is probably not the weapon of choice today as seen of the superpowers, economic warfare is the new "legal warfare". Starving citizens seems to be the no 1 play of the US and western collision when it comes to handling Russia and the middle east, striking the labouring man so that change can be brought from inside the country...

Political warfare today is the same as during the cold war without the population being indoctrinated that it's the US against Soviet, where for some reason the Soviet were the bad guys while the US kept torturing its own citizens and killing innocents in Vietnam and Korea while the Soviet tried holding the US interference in check in those regions alongside their allies. (And for a very brief period of time the opposite in Afghanistan.) And reading the first post I take it you still believe it will come to nukes being fire like rounds at the other...

The answer to your question is the only thing holding people back is that they have something left to protect. As long as you don't push people to their limits so that they welcome the afterlife by doing something like flying a airplane into a high building or something else you have nothing to fear. Then again the US has proven that it can't seem to not cross that bride as history has showed us not once, but twice.

Their will always be people who's limits are easier to step over than others, we are not all reasonable people who will consider their actions logically and thoroughly. If someone does a terror attack on your country it isn't always your countries fault. The real human threat to life is not from politically respected or recognised countries but from those who are members of small groups with strong convictions.

Chemical weapons are terrifying, but require knowledge of how not to infect your own people while preparing it and how to avoid uncontrollable spread behond the intended victims.

Well in all honesty, other then airdropping the car in, I dont think you would get very far with this plan.

I was actually thinking fitting one or two inside a torpedo, and fire it off a submarine. Or construct self-driving torpedoes and launch them off of ships or aircraft carriers from a distance.

Might as well just save yourself the trouble of designing them to be self-guided and guide them yourself if you're going to blatantly launch them off ships or planes.

No nation is going to fire nuclear weapons at another nation because they know there are a bunch of groups or organizations that will fuck them up if they do. And no nation is going to fire nuclear weapons at alot of nations because that will effect their economy n shit aswell.
And no militant group is going to get their hands on the materials required to make nuclear weapons because 1)people track these materials, 2)they're expensive, and 3)I seriously doubt that even black market people or w/e would be willing to sell these materials to a group of extremist noobs.

Also I doubt people will use biological weapons due to tourism because the disease or w/e would end up in the country that released it aswell as everywhere else, and if they closed their airports before releasing it everyone would know who did it and fuck them up.

Also no on actually wants to commit genocide so stop being so damned paranoid.

It's not as unlikely as you think. Groups like ISIS and AQ get funded from Saudi Arabia, and they're friends with people who can give them that stuff. State-sponsored terrorism. It's scary shit.