According to Doppelmayr and as has been posted several times the top speed of the D-line is 7 m/s which is 15.65 mph not 17. And as we know now the Skyliner system is currently running at 5 m/s.
D-Line | Products
D-Line. A product overview of Doppelmayr products can be found here. Get information about cable cars, ropeways and aerial tramways.www.doppelmayr.com
That may be advertised max speed for ordinary ops, but during the initial week of testing with gondolas, they were clocked going faster than 17 mph.