I've driven extensively in both, and London's hands-down more challenging°. Boston drivers are significantly more aggressive, though, so Waymo might find the reverse. I dunno.
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°Which makes it more fun: I love driving in London, but I'm a weirdo. My biggest regret about my last / first visit to Delhi is that I let my wife talk me out of driving. It may indeed have been a prudent choice, but I think I was there long enough to "get" enough to manage next time.
This is also important in the US and is a capability the Waymo Driver has.
> it's still far more challenging than any American city.
How would you rate the difficult of London vs. Boston?