There's some security researchers that have done this before as well, and some "grey hats" that reportedly used this technique to get Google to route traffic away from them during their commute by spinning up a whole bunch of phones in their car like this.
Do you recall any of the names? I'd love to read those papers and see how effective it was. Depending on the city and the route, people might have no choice but to go over a bridge like that regardless of traffic reports.
While I don't remember his name, I remember there was a Darknet Diaries episode about the researcher who first investigated the problem. The episode was very thorough, I liked it a lot. I also don't remember the name of the episode, so I guess this comment is kinda useless
Even with gps off, they can get rudimentary location data based on pings to cell towers. It's not pin point obviously, but it roughly tells how many people are in a given area with a phone on them, smart or dumb.