Depending on the types of messages and population density, capacity could be enough for sending messages to individual cars.
FM radio with computer messages could be a backup system for bidirectional cellular connections. By the way, one frequency could also serve drones, self-driving boats and even trains and small planes like cessnas. Vehicle does not have to be self-driving for some of this information to be useful for the driver.
May be best to use public-key signing of data blocks to prove integrity.