I just had another thought. The Boston area has been rolling out bus lanes and they're pretty fantastic. However, a crowded rush hour bus may have a perverse effect on their traffic jam calculations. Where normal cars will be at a stand still, Google will also see ~50 users absolutely flying down the road in comparison. What happens then?
I am being serious when I say this: they probably do. I get that comes off as “haha tongue in cheek we’re all being spied on amirite!?” but I imagine google can and does figure that stuff out very, very quickly. Plus most people aren’t just entering the google ecosystem for the first time with their new phone.
A pattern would be, for example: is he ever seen moving more than 5mph? Does he use driving directions? Is he only seen moving faster along bus routes? If he is only seen along bus routes does his pattern of moving and stopping align with the bus stops? Is there usually traffic jams at the same time of the week on different days, etc.
Just build up a database of patterns and check a 1 week old account with that to see what it fits best.
Perhaps in an area with no predictable traffic and no bus routes, that a wagon of phone matches the pattern of a traffic jam caused by a blockage on a road. It's less likely to be a bus, perhaps?