Fun fact: in Europe they have the same deciduous trees as in the U.S.: oaks, maples, etc. But their leaves don’t turn those amazing neon orangey red colors that they do here in the northeastern states- they just turn a pale yellow.
The reason going from memory, without bothering to look it up right now, has something to do with the way mountain ranges in the U.S. affect insect migration. Trees here evolved ‘insecticide’ chemicals that cause the orangey red colors in the Fall.
Seems to be a working theory, anyway.
Leaves turn the most spectacular colors when they are not doing well. The trees along the roads are less healthy due to the salt on the roads, that’s why they tend to turn earlier and more vibrant. They’re mostly the same species as those on the hillsides.
This may be true, but it also varies by species of tree. Here in SE Michigan sugar maple leaves turn the most amazing glowing orangey red colors. Silver maple leaves, on the other hand, turn a dull grayish purple, and take forever to fall off the tree, meaning I’m still dealing with leaves on Thanksgiving weekend. 
That’s also true, especially with non-native species. Swamp maples, which are the same maple trees that grow on the hillsides, have a reputation for the most spectacular color because those trees are under stress. The same species in a different location may not produce the same color. Drought and weather will also contribute to whether we get a good or bad foliage season.
I can’t say for sure, but I doubt people planted trees along roads for the foliage. They get planted for longevity, shade, and aesthetics the majority of the year. Foliage is a nice bonus.
I guess I’ll just chime in as being from a place where the trees look so pretty that I found the idea of leaf peeping weird. Sure, you couldn’t see it as much in cities, but I assumed everyone had something like Scenic Highway 7.
Like @Chronos, I was thinking it was some sort of critter, though in my case some sort of insect that looked like a leaf.
I sort of did too, except that I knew better. I’ve encountered the phrase before, but I still find it really weird.
If only the question had been: Where does “leaf peepers” come from?
I don’t know, I have lived my whole life in the Sauerland, a mountainous region covered with forests, in Westphalia, Germany, and I see all kinds of colors on leaves of several shades of brown, yellow and red every fall. In fact, I can look out my window at this moment and see trees covered with red leaves. I googled pictures with the term “Herbst im Sauerland”, “fall in the Sauerland”, and get a bunch of pictures like these which capture exactly like nature looks here in the fall.
https://image.geo.de/30155954/t/Ur/v4/w960/r0/-/sauerland-herbst-s-730691134-jpg--89012-.jpg
I grew up in PA, but have lived in MA for the past 30 years. As close as they are, New England leaves are far more spectacular than the mid-atlantic area. I don’t know if it’s the mountains, or the colder temps, but the colors are brighter and have a more vivid mix than in PA.
Those certainly are some nice oranges and reds. I don’t know, maybe trees in some regions of Europe did evolve in a similar fashion to trees in the northeastern U.S. Or maybe I had bad information. Ignorance fought!
That is a depressingly rational-sounding explanation. The only reference I could find on the subject says that stress makes trees turn color earlier, not brighter. And on that particular day I was there, the trees in the park had also turned, but yellows and browns rather than oranges and reds. I didn’t attempt to see whether they were the same species as the trees along the road.
Here in the NC Piedmont the predominant leaf colors might not be quite as vibrant, but I think the more important factor is that “leaf season” is spread out over several months. Some species are already bare, some haven’t even turned yet, and some manage to spread out the coloring and leaf dropping over at least a couple of months all by them selves.
Growing up in SoCal, there’s largely no such thing as leaves changing at all. Lots of deciduous trees look substantially the same year-round. With some allowance for wet versus dry season. Yes, a few species get pale yellow or brownish or just look a little ragged/wilty. But that’s about it.
Ditto now in Florida. As our local joke goes: “Florida does too have Fall colors; you can tell it’s Fall when the license plates change color.” (i.e. to mostly match northeastern states and Canadian provinces as that horde migrates south for the winter yet again).
Our Minnesota fall foliage is very pretty too. Some maples are so brightly yellow that it looks like a light is on. There is a maple behind our house that has both bright yellow leaves and bright red leaves on it. I also love the popple trees. Their round yellow leaves remind me of coins.
I happen to be in Detroit this afternoon. Where it’s gray, dim, windy, spitting rain, and 43F at 2pm.
There are bright red maple (?) trees everywhere. Not a huge percentage of all trees, but they really stand out. I took some pix; even in the dim light they’re impressively vibrant.
@Chronos, I was expecting that, too.
Here in MinneSnowta, the leaves turn all kinds of glorious colors. I was very surprised to see that in England, there wasn’t a lot of color, because they don’t have the amazing reds and oranges we do. But it doesn’t get as cold there either. The first time my ex was here for the fall colors, he was gobsmacked by the reds, purples and oranges.
I didn’t know how lucky I was to see these colors every single year.
I thought it was about leaf hoppers. Never heard of people leaf peepers. There are some exceptionally bright red leaves in the Cleveland area this year.
I’ve always lived somewhere with deciduous trees. So while I appreciate the color change - especially if I’m somewhere where the tree to house ratio is large enough to make a nice scene - it would never occur to me that someone would TRAVEL to see it.
The first time I ever heard of this was 30+ years ago, when we needed to go to somewhere in upstate New York for a memorial service. The closest hotel we could get - and it was rather expensive - was Pittsfield, Massachusetts (most of an hour away from the service location). The hotel staff suggested that this was due to people coming for the fall foliage,
To get to the event, I took a train up from Manhattan to Albany. I will admit, the trip up the Hudson was quite lovely. But not enough that I’d imagine people travelling just for that.
Speaking of, northern Illinois used to have a lot of bright golden yellow and purple colors from ash trees in fall foliage. Until the emerald ash borer invaded and killed them all 
And, yeah, I have two silver maples in my yard and swear they don’t finish dropping their leaves until February.
Yeah, those little bastards. A lot of fine trees died. Had a ton of ash for firewood for awhile- that stuff burns great. A really dense wood that burns hot for a long time. Small consolation for the trees lost, but you make the best of it. My sister and her then husband got a great deal on new ash flooring for their house about 10-12 years ago because there was so much of it. It looked pretty good, but still had those squiggle marks in spots from the ash borers.