believe it or not - they start with an a-z .... or at least the national Os.
these are scanned at ultra high resolution overlayed with long and lat info and the rest is automatic.
each mfr then writes the software that recognises roads - weights them and calculates the routs - just how they do that - just say you are about as likely to get the recipe for coke
there are a number of sites you can get maps and overlay info editors - for DIY editing
http://www.poieditor.com/
all the vans are for is to prove one way systems and tracks