[djtango]

Doesn't it really depend? Like I recall Oxford Street in London at one point was notoriously bad because it was a bottleneck for a lot of slow moving traffic so that one length of road was especially bad. 10x bad I don't know... But it's not hard to imagine some of the quieter roads filtering off like Berwick St or Dean St would be considerably better

[labcomputer]

Slow moving traffic at one intersection and free flowing traffic at another could easily account for a 10x ratio of particulate pollution, especially in European capitals where diesels are prevalent.

But a 10x ratio on the same road is also plausible if the Google car is following a large truck on one pass and then driving by itself on the second.