The problem with self driving cars is that in order for it to be truly safe everyone on the road has to be in one.

I don't see that as a problem. Right now regular cars aren't "fully safe" and that doesn't prevent them from being widely adopted. I think it'll be easier for even independent self driving cars to be much safer than a human driven car, and that might be one of the biggest advantages.

Another reason is security. All the cars will need to communicate with each other. Who is holding the "keys" so to speak? It's going to be a security nightmare.

The things self driving cars need to communicate are things that are already public - how fast they're going, when they're going to break, etc. They're just things that can be communicated faster and more reliably instead of relying on old fashioned lights and cameras to figure out what's happening.

Computers are everywhere already. Computers are trading in the financial markets and making predictions. It already happened.

Having lots of computers is a condition for AI, I don't think it's a tipping point because we've had them for a long time and progress on general AI hasn't made much progress. It might be that we need to surpass a certain amount of computing power, with a certain amount of inter-connectivity? but I think self driving cars are going to be the leading cause of growth in both those areas.