Best database for a real time application
I don't think the database server will be your bottleneck. Many databases can do sub-second responses to queries provided your database is scaled and designed correctly.

I think you need to be asking yourself questions about how the android devices will connect (mobile internet can be slow sometimes), how your middle tier logic will work and how the devices will communicate with the middle tier.

If you are serious about having millions of vehicles on the system you need to be thinking about how often each of them will update their location and how often the other cars will ask for nearby cars.

Let's say you have a million of cars, and each device has to get a sub-second status update about the nearby cars, that would mean that cars need to update their status every half a second and every car requesting statuses every half a second.

That's millions of requests per second from the devices to your web services and millions of queries per second from your middle tier to your database server.
Keeping in mind that your web service will need to calculate proximity, take into account maps and streets and the like, i think your database platform will be the least of your worries.