As MikeP has said, sound isolation is all about density. It should not be confused with sound absorption, which is another matter altogether. You would need an enormous quantity of rockwool or rigid fiberglass to provide isolation from traffic noise, probably several feet thick. And even then it wouldn't do well at low frequencies, which tend to be conducted through building materials such as steel and wood.
One of the cheapest and readily available isolation materials also happens to be one of the best: filled cement hollow block. Eight inches of hollow block will provide between 40 and 60db isolation, depending on the frequency. Perhaps it's possible to construct a wall between you and the highway?
The thing to remember about walls as sound barriers is that they don't actually block sound, they create a sound shadow. The horizontal extent of the effective shadow will be about equivalent to the wall's height, so the wall should be constructed close to your existing wall, rather than out by the road.
The one thing concrete can't shield you from is very low frequencies, which unfortunately is a component of traffic noise, whenever a large truck passes by. Those will be conducted right through the ground and your floor, so the cure there is isolating yourself from the floor. A false floor on rubber spacers will work, but that's expensive. You can probably do a lot simply by constructing risers to set your microphone stands upon, and decouple the risers from the floor with rubber.
You might still need to filter out those low frequency rumbles, which is usually not a problem for vocals, since they can usually roll off below 120-160Hz with no loss of quality.
