Yes, the transmission retarder is a helper brake.
Most of the LONG grades are in the western states but you will stop faster anytime you need the come down on the brakes hard using the transmission retarder in addition to the service brakes. There are numerous grades in the U.S. that are 6% or steeper and 5-22 miles long, some with sharp turns and even switch-backs in them. Looking at the Mountain Directory East, there are no listings for Louisiana, Mississippi, Alabama, Georgia, South Carolina or Florida. However, go up to Tennessee and you can find some good ones. An example is Tennessee Highway 142 from Roan Mountain, TN to Bakersville, NC. The Tennessee side is 7-1/2 miles of 7-9% grade. The North Carolina side (HWY 261) starts out with 6-1/4 miles of 7-9% grade with a short section of 10% then levels out then after a few miles continues at a 8-9% grade for a mile and a half. That kind of road will overheat your brakes quickly. A 9-10% grade looks like you are falling off the side of the earth from the driver's seat.
The Mountain Directory West lists over 150 pages of long steep grades and provides information on the location of truck runaway ramps versus about 87 pages for the Mountain Directory East.
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