I am not a professional, but I would start with total population (Demand) and vehicles (Supply). Total number of people, segmented through different location zones (center, suburb...) and availability of vehicles for each of the zones. By vehicles I mean land transportation such as cars, buses and railways / subway and also other types such as boats (if the city is on a river) and helicopters.
You also have to be aware of some constaints such as road infrastructure capacity, time and prioritize -> maybe one area needs to be evacuated first or there are some people that need to be evacuated first (such as POTUS or ladies and kids first).
I don't know the context, but it is also important to understand why do you want to evauate Madrid. You're not going to use subway if there is a huge earthquake.