Maybe the bees just don't like traffic and decide not to come to your particular establishment because there are easier places to get to?
On a slightly more serious tack, traffic causes not only air pollution, but air currents are also changed. Noise is also significantly higher. I have no idea about a bee's hearing, just expanding highway traffic's effects beyond air pollution. We saw USGS seismographs showing the decrease in vibrations with the pandemic's decrease in traffic. Humans have an impact on the environment in so many ways that go unacknowledged/not understood.
I think there's a very good chance you're right. But!
Unless you know where the bees are coming from another very likely scenario is the colony just died. A newly established wild hive has only like a 20% chance to survive its first winter so if that's the first summer you saw them that likely explains it.
If you're near two highways it's likely you're in a fairly dense area and feeding a nearby beekeeper's hives. But a lot of beekeepers are hobbyists and like all hobbies, people give up on beekeeping all the time. Even if it was a managed hive a likely explanation is that the colony simply isn't there anymore.
Because its hard to predict the wacky weather here in the UK, and not knowing if the next time we cut the lawn might precede a period of hot weather, we have taken to cutting the lawn on its highest cut to preserve the grass during summer months as it traps more moisture at soil level thus preventing it from going brown and dying and we only cut it when there is rain forcast. So the grass can grow to be several inches in height like a meadow, obviously more flowers/weeds pop up, but its easier pulling the weeds out and leaving the flowers that pop up for the insects.
So fast forward to this year, and besides seeing an increase in insects, butterflies, and a wider range of birds, year on year I saw 6 goldfinches descend on the bird bath yesterday, never seen that before, and we are also seeing loads and loads of grasshoppers.
Havent seen this many grasshoppers since the 70's and 80's which is nice to see. I could probably walk out and catch 5-10 from the lawn in 10minutes. I'd be lucky if I saw one grasshopper a day in previous years.
We have been trying to make the garden more insect friendly since Covid and I think its beginning to pay off.
Ozone mentioned in this study will always be found in built up areas, the human health impact is it blocks UVB to a degree which means humans get less UVB light and thus less vit D3. So even though circa 100% of the UVB will reflect off man made surfaces, the ozone stops alot of it from reaching the ground meaning you get less vit D3 in built up areas.
In the countryside the vegetation absorbs alot more UVB (circa 25%), so gardeners in the countryside have higher D3 levels simply from the absence of ozone.
And Bee's like other insects use UV light reflecting off flowers to choose the different flowers to bumble between in their search for pollen. Reduce the ozone to get more UV light to the ground and the bees can see the flowers better.
It would be interesting to see of more bees got to flowers where some UV lighting is placed over the flowers.
I'd imagine also a highway with traffic would act as sort of air barrier when there is traffic zooming thru it whole day making noise and wind, even if we ignore pollution.
There are also not much bees anymore. There are studies on this as well. Didn’t you notice you no longer need to clean your front glass of your car as much as you used to?
Global flying insect population by biomass remains stable while bees decrease and flies/mosquitos increase rapidly.
If you’re talking about honeybees, they are livestock. And they travel something like 2 or 3 miles from their homes, not more. if i decided to move next year, everyone in my neighborhood would see and alarming 75% drop in bee visits. And im just a hobbyist, keeping about 3 colonies.
But, yeah, I'm inclined to believe this. I live a few hundred meters (yards in Freedom Units<tm>) from two major highways.
Last year, at the tail end of the pandemic-induced lull in traffic, my flowers would attract groups of over 20 bees, easily, every day.
This year, with similar weather, pretty much the same flowers, and definitely the same location, it's a good day if I see 4 individual bees.
So, the answer 'air pollution' to the question 'what changed' seems at least likely?