Swiss private-public managed road network is funded by road tolls and vehicle taxes. The Swiss autobahn/autoroute system requires the purchase of a vignette (toll sticker)—which costs 40 Swiss francs—for one calendar year in order to use its roadways, for both passenger cars and trucks. The Swiss autobahn/autoroute network has a total length of 1,638 km (1,018 mi) (as of 2000) and has, by an area of 41,290 km2 (15,940 sq mi), also one of the highest motorway densities in the world. Zürich Airport is Switzerland's largest international flight gateway, which handled 22.8 million passengers in 2012. The other international airports are Geneva Airport (13.9 million passengers in 2012), EuroAirport Basel-Mulhouse-Freiburg which is located in France, Bern Airport, Lugano Airport, St. Gallen-Altenrhein Airport and Sion Airport. Swiss International Air Lines is the flag carrier of Switzerland. Its main hub is Zürich.
How many passengers used the Zurich airport in 2012? (If the question is unanswerable, say "unanswerable")
22.8 million