Core: Priority vehicle: blue light + siren, both together

Blue flashing/strobe beacon AND two-tone siren, both at the same time. Without that combination, a vehicle is not a priority vehicle, however urgent it looks. Priority vehicles are broader than just police, fire brigade, and ambulance: the book also names 'other emergency service vehicles and Rijkswaterstaat' (Book p.116). The status depends on the signals carried, not on the organisation. One hard limit: causing danger to give way to a priority vehicle — for example by driving through a red light — is not allowed. You look for a safe way to make room, not a dangerous one.