It takes a really big runaway ramp to slow a semi that has been careening down a 5 -10% grade for 5-10 miles.
I remember kicking my vehicle (a Chevy Spectrum at the time) into neutral a few times just to see how quickly the speed would build. I could be racing down the hill at 85 - 90 mph within about 30-40 seconds, with my descent beginning at 55 mph.
Those runaway truck ramps are graded with deep layers of gravel and dirt so that the truck tires will dig deep into that to help slow the truck down. There's one of those along I-17 between Phoenix and Flagstaff, but it's not nearly as long or steep as the one pictured.
The Detroit area has got to have the worst. BTW...I think I saw a slope like that runaway truck ramp in Innsbruck, Austria....they used it for a down-hill ski jump at the site for one of the old winter Olympics!