I regularly do 125 mph on 635 from Preston to Royal-Walnut Hill/635 and vice versa. It is 6 lanes wide plus 2 express lanes. If you get on the Express Lanes at Preston headed east you can really fly.
No one is ever one the express lanes. Nice clear stretch from Preston to Greenville. Speed limit is 70mph but you can easy do 110 mph to 120 mph and not worry about hitting anyone. Express Lanes have concrete dividers from the rest of the lanes.