I am a student doing my Final Year Project where I need to build an android application for on the road navigation. The problem is the application has a feature that alerts the user of nearby incoming vehicles (that have the same system) when the user is near to a junction by updating and getting positions of vehicle in a database in real time. Hypothetically speaking, if there are millions of vehicle or user using the same system, what database can process these amounts of transactions very fast, let's say maximum response time is 1 second per application? 

I have literally no knowledge regarding the types of database.