An autonomous driving system is either good enough to drive on our roads without supervision — or it's not. If it is good enough, it shouldn't matter whether others know if the driver isn't human. Any unsupervised driving system is likely to be around two orders of magnitude safer than a human driver.[0] So effectively they're special lights to tell others that a car is extremely safe.

This will backfire horribly. While there is some residual benefit to law enforcement, these lights will signal to human drivers and pedestrians that this vehicle can be bullied with impunity, and with no risk of angry retaliation. Human drivers will cut in front of them mercilessly. Pedestrians will step onto the road in front of them. Maybe not in classy affluent suburbs, but definitely in rough neighborhoods.

--

[0] I think it's obvious that anything less than two orders of magnitude (100X safer) is socially unsustainable. Think about it. 40 thousand people die on roads in the USA each year. Replace all drivers with an AI that's "only" 100X safer and we'll initially marvel at the 39k lives saved every year. But that will quickly be forgotten. What's left is the brutal carnage of 400 people mercilessly killed by robot cars annually...