Why don't you divide your road in map entities parts and then remove in runtime all the parts that ate far away from the cars. Maybe some triggers to load the next part in front of the cars and remove the parts behind it. You only need the parts under your cars and a few parts ( more or less 2 ) in front of it. Perhaps this way you can save a few more fps.
Also, make all the blocks faces that are not visible to none.
And play also with the clip range. Decrease in a few areas where you don't need to see to far away.