Go and get new brakes. The squeaky noise is caused by when you are driving fast then you brake suddenly. Being on the highway alot causes squeaky brakes.
I bet you that those are semi-metallic brake pads. Semi-metallic brake pads tend to make noise, even when they're kind of new, because of the semi-metal design--and especially when the metal heats up do to excessive driving--and also, when the pads make contact with another metal surface like the disc. Metal to metal creates noise, and when those pads get worn out, the noise worsen. The best thing to do, is replace those brake pads with quality ceramic brake pads. It has ceramic material--no metal--that doesn't create sound, even when the pad has being in use for awhile.
Added point: Your vehicle only needs pads. But if it's a scrubing sound then that's a sign of bad rotors, which need replacing.
You'll need new rotors and pads, make sure to order wear sensors also. Mercedes rotors because of the aggressive friction material (Pads) are throw aways. Rock Auto.com has the best prices. Go with
an OE pad or Wagner Edge Pads and Rotors. You'll Like Them.
your brake pads are shot. get them changed. it's very inexpensive. also you need to have your brake system checked. my truck has rotor brakes on the back and disc on the front. each system has different components. regardless, i bet your pads are shot.
brake pads only last a couple of years. they are intended to wear out. they are made out of a soft material that gardually wears out. what you are hearing is matal on metal. over time, that will etch into your discs. so, it's part of regular maintenance that you replace your brake pads. so now you know!
also, you need to know that you can ruin your discs driving it that way. so get it checked asap. squeeking is not good!