Using a series of hub gears
The HP Velotechnik Scorpion is available with three front chain rings, three-gear internal hub, 8-speed rear derailleur setup. This has the additional advantage that you can shift while stopped, e.g. after emergency stops or similar.

I found it rather tedious to keep the overview over which gear I'm currently in. My Velomobiel Quest has triple front, 9-speed rear derailleur and that covers enough ground for me. Walking pace is not a problem and I spin out at about 70km/h, which is already pretty hard to keep up, even with race fairing.

Depending on where you live, riding 70km/h is already pretty dangerous taking into account that trikes cannot brake too well because the rear wheel will lift off pretty quickly.