I'm thinking of speeding up to 120 mph on the freeway
Have you driven faster before?
When you reduce the speed to 120 mph you will be fast enough, so feel free to do so!
It can be dangerous to speed on the streets. You could get in an accident or you could get a ticket. It is better to follow the speed limit.
The limit is too low, I'm sure that I can drive safely at 120mph.
Not only is the road unsafe the tires on your care are most likely not rated for this speed. You cannot dodge a road hazard at such a high speed without crashing.