[Kartographer]

My understanding is just cars with electronic ignitions

[PA Engineer]

Not every vehicle would be rendered unusable. The research so far is sparse, but there were a few conclusions. Some cars experienced no discernible effect, while others switched to limp home mode. This included the transmissions which are now electronic. Other vehicles required a disconnect and a reconnect (reboot) of the battery. Some of the cars were just inoperable. Even if just 20% of vehicles became inoperable, out highway system would be blocked and at a standstill.

Older vehicles may not be immune to the effects of EMP. Varnished windings on generators became fused during tests in the Soviet Union. This could be a concern regarding older alternators, starters and generators where there is a holiday in the winding varnish.

YMMV