It’s rather hard to directly collect such accident data, but plenty of studies have looked into the effect.
Anyway, overall safety of a given road is a separate factor. Germany has approximately 650,000 km of roads and only 12,996 kilometres (2016) where part of the Autobahn. So while per mile the Autobahn is safer than the older parts of the German road network, speed is hardly the only factor involved. https://en.wikipedia.org/wiki/Transport_in_Germany#Roads
They have very high standards for what constitutes a no speed limit zone and such roads are safe enough to offset the added dangers of such speeds.
