William Forschen is generally regarded as an expert in this field, and much of my information comes from what he's written.
William R. Forstchen, author of
One Second After, "is an American author who began publishing in 1983 with the novel Ice Prophet. He is a Professor of History and Faculty Fellow at Montreat College, in Montreat, North Carolina. He received his doctorate from Purdue University with specializations in Military History, the American Civil War and the History of Technology."
By the way, I spent four summers at Montreat.
Back to reality... Most of his work is fiction, although he's written some non-fiction. Although his novel was based "upon several years of intensive research and interviews," I would not call him an expert in EMP. By comparison, I would most certainly label military scientists who've studied EMP for sixty-five years as "experts" because that's precisely what they are.
I would also refer to the Federal Association of Scientists as experts. I linked to a paper by them in my previous post.
An EMP would absolutely disable a modern vehicle. All the computers and such would be fried and the vehicle simply cannot run without them.
While it certainly could disable a modern vehicle, that's not a given. The chips in modern vehicles are designed to withstand serious electrical surges, including the kind caused by lightening and, yes, EMPs. High voltage can take them out, but again, it's not a given. If it were, you'd see cops equipped with rifle-like tasers to take down cars. Doesn't exist as it's not a given.
Furthermore, a vehicles chips are spread all over the vehicle. They usually reside in the engine bay, often in a single box, and sometimes all on one chip. That makes servicing easy.
Finally, these chips aren't high-tech. They have about as much processing power as an HP-41C, although they're faster. What they are is rugged and easily replaced. For around $50 to $150 you can buy a factory-spec replacement chip you can keep in a Faraday cage in case of an EMP. That won't save your built-in GPS, but at least you'll be able to start your car.
As for Forstchen's depiction of what would happen sociologically, you have to have spent some time in Black Mountain, NC, in order to understand where he's coming from. The folks there are nice, but they're not exactly swift or terribly resourceful. They're certainly not a fair representation of folks elsewhere in the United States.