I have been reading One Second After by William R. Forstchen.
It discusses the result of an Electro Magnetic Pulse (EMP) attack on the United States. Without going into the merits of the actual story, it does portray a frightening scenario of an attack on the United States using nuclear weapons burst 200 miles above the United States.
The effect of one of these bombs would be to release a very quick electromagnetic pulse over a very broad area. As noted in the book, just three bombs would cover the area of the United States. The effect as portrayed by the Author would be to knock out all electrical and electronic systems (not using old style vacuum tubes) immediately. What is immediately obvious in the story is that all of the modern cars are and trucks (built after 1980 with their on board electronics) are fried.
But the Official Report goes into details as to the immediate effect on vehicles. Automobiles that were turned off showed no damage, automobiles that were running cut off 10% of the time, but were immediately able to restart after coasting to a stop. In the case of trucks, one of the thirteen tested did have to be towed. While obviously disruptive, this is a far cry from the thousands immediately stranded on the highways. Further, it notes that the effects on most electronic medical devices would be limited.
As noted in the report, the primary damage from an EMP attack would be to knock out the electrical system. This would be devastating of course, but requires a different set of protective measures than trying to build a Faraday cage around your car. The books website does reference the report on a separate tab. Possibly it was not available to the Author at the time the story was written.