You're assuming that your self-driving car will not be slower than today's human-driven cars, but until all (or practically all) vehicles on the road are auto-driven, such cars are unlikely to be configured to exceed the speed limit (in addition to the intense legal pressure - internal (lawyers) and external (lawmakers) - to make sure speeding self-driving cars don't exist (I can already see that phrase "speeding self-driving cars" being used by the media to scare the unaware among the public).