There's a big problem with your idea of letting cars drive faster: the drivers.
German drivers are simply not like American drivers. American drivers have almost no training at all, and certainly don't have the training or discipline necessary to drive at Autobahn speeds. American drivers can't even practice proper lane etiquette (staying right except to pass), and really don't understand it.
The last thing American roads need is any assumption that American drivers are remotely like German drivers in ability.
Remember, in America, almost everyone drives because there's no public transit in most places, and even where it exists, it's kinda lousy at best. In Germany, it's perfectly possible to get around without a car, unless you're going to small villages or towns or rural places. Because of this, not all Germans drive! It costs a lot of money to get the private training required to obtain a license there, and driver testing is strict. None of this is true in America.
Finally, if you want bullet trains, build bullet trains. Bullet trains are very safe at speed because they're on rails, and are maintained by professionals on a strict schedule. You don't get that with privately-owned vehicles. And cars are horrifically bad for pollution and fuel economy compared to trains when operating at high speeds. We're spewing way too much carbon into the atmosphere as it is. If you want to move people around faster, you need to build the proper infrastructure for this, like the Japanese have with the Shinkansen.