"In one scenario, it appears, a driver is looking for a parking space and drives over a giant QR code on a street or in a parking garage. The QR code might either have exact directions to a specific parking spot, or it might link to a parking garage database of which parking slots are open." This strikes me as ripe for abuse.
Surely the technical part is no more open to abuse than normal parking signs in cities listing the amount of available spaces in certain car parks. The QR codes would contain very little information I would imagine but rather a key that is used by the car as an argument for further operations. Leaving all the directional data on the road would be way too easy to abuse.
"it might link to a parking garage database of which parking slots are open"
Imagine creating a QR code, (or copying one that already exists) which tells the car to fetch information from http://your.competitor.example.com/. Then imagine going out late at night and sticking it on a busy motorway.
DOS by traffic.
Also. You better hope that your car doesn't have any vulnerabilities that can be abused by a specially crafted QR code, or by fetching specially crafted data that a QR code prompts it to fetch.