It's in the stats of accidents and fatalities that occur on highways without the separation medians, it is why the DOT has the regulation of space between opposing lanes.
Even if we discount safety entirely and we're going to argue that we can somehow "acquire" the land necessary to build a bullet train, be that beside, or in the middle of, roadways one has to look at the benefit of it. What is the benefit of a bullet train to "society"? And are those "needs" already addressed by existing infrastructure, are we better off to put any money into upgrading that existing transportation network, or expand out into a historically financially loosing proposition? What are the benefits of a bullet train that cannot be met equally, if not better, by the existing systems?