That would increase accident rates. Parent was positing a scenario where self-driving cars don't cause fewer accidents than humans. This could mean a more or less equal amount.
I mean, Germany does this on the autobahn. I can't really find any mortality data for the autobahn though. Also, German driving laws are fairly more strict than in the US.
Germany: About 1 fatality per 10,000 cars per year, 4.1 fatalities per billion km driven per year, 38 fatalities per million people per year (2017). Relatively few accidents on the Autobahn, but the proportion of fatal accidents is higher. All in all, the Autobahn is fairly safe, in particular in relation to distance traveled.
USA: About 106 fatalities per million people per year (2013).
I suspect the difference is due to more public transport, less distance driven per person, and possibly more stringent driver education in Germany.