We had a somewhat unusual bee experience a few years back. We returned home one day to find a really large number of bees in the air all over the yard. I eventually noticed there was a clump of them assembled on one branch of a Norway Maple just outside my office window. My assumption was they were in the process of building a nest there. Close enough that I decided that just wasn't going to be acceptable.
After a bit of googling to confirm my suspicions I learned that often you can get them removed for free by a beekeeper. Unless they are inside in which case apparently nobody removes them for free. So I called up a nearby beekeeper and he arrived in short order with a hive.
I don't recall all the details but I think he relocated the queen into the hive and then left it on the ground under the tree until nightfall, at which time he said the rest of the loose bees would gather in there with her and he would return to claim them.
Any chance this is what was going on on Colorado Ave yesterday?
That's called swarming, and happens when a queen bee is looking for a new place. [I have an online beekeeper friend who goes and captures swarms like that (near Dallas TX, so no help in WY).]
I can't say for sure it wasn't a swarm. And I didn't get stung, which would be one indication of a swarm. But it's very late in the season for bees to be swarming, and it doesn't make sense given my impression was there were several bee hives in the pickup.
I decided not to re-visit the area today. What if another pickup arrived overnight, parking on the opposite side of the street, with more bees and beehives? Better to not tempt fate rather than testing the notion that fortune favors the bold.
I think the suggestion is that the bees weren't coming out of the hives, but rather going in. Although maybe it's the "several" part that gives you pause. And yeah, it would be unlikely in September.