It’s not a viable defense if you get a ticket for speeding but in practice the speed limit is really the prevailing speed of traffic plus X mph, where X adjusted for the state. I.e. in my experience Texas is more strict about the speed limit even on their desolate highways, LA is about 10 mph faster than San Francisco, in Seattle it depends on the weather, you’ll never hit the speed limit in New York anyway, and in Florida you just say the gator ate the officer who pulled you over.
A lot of laws aren't enforced consistently in practice, sure. The implicit point is that while that may be so, it is nonetheless enforceable and nonetheless the law. So while individual people may be comfortable about being flexible in following traffic laws, having that behavior encoded or permitted by software is basically a declaration of broad intent to violate the law made by a company.