So I went on the highway for a bit and got my radar out but no one ever speeds do I need a mod or something?

There are some that speed, on the Highway the Speed Limit is 40~ roughly, and u will see people speed from time to time, but remember the AI is mostly coded to follow the rules, there is no random chance of them speeding, and as of right now, I don't believe there is a mod to make them speed.

Usually, they speed up when going down a hill, so watch out for that.

wow I caught a guy and he ran away then my game crashed!!!!!!!!stupid gta lol

Well there is one mod that will have speeders. That is the new version of the police helper mod. So far those speeders are very reckless