Pretty hard. Reading a license plate is already difficult and those use large symbols. Unless you wanted to plaster the car with a code the size of the door, it will be very hard to read that from a distance and on a moving car.
QR codes are robust but they are also fairly "dense" and not meant to be read from a large distance (they were designed as a replacement for a regular barcode). If you wanted to implement something like you are implying, it would be vastly simpler to enter my license plate into the database to make the system ignore it instead of adding an extra code scan.
And if you are talking about speed cameras, those actually don't do things in real time, the radar only triggers the camera to take a picture if it detects a vehicle going faster than a certain limit. So it would photograph your car regardless of any codes - the device is pretty "stupid" in this regard - the camera doesn't do anything else but take pictures for evidence purposes once the speeding has been detected.
At least here in France the data from these cameras are then downloaded periodically (the older ones require a guy to physically visit the installation and to retrieve the data, the newer ones send the data over GSM automatically) and then the photos are reviewed by actual people who generate the tickets, as appropriate. It isn't fully automated (and can't be, because the radars often generate unusable pictures for various reasons and such tickets would get overturned by the courts), so any "code" wouldn't help you there.