That sounds perfectly natural to me. Something changed in the environment without regard for the effect it’d have on crab development, and the crabs evolved in response to that environmental change. It’s not like some human said “I want all the crabs in this lake to look like samurai faces”, and made a deliberate effort to breed them in that direction.
Natural selection is operating every time animals reproduce. It certainly can operate over timescales of 100 years to produce physical changes in organisms. You’re not going to get a mammal evolving from a fish in 100 years, but changes can happen very quickly when the environment changes quickly.
I should have made this distinction in the OP. I wasn’t referring to “learned” behavior … except possibly in the case of an individual animal observing another animal’s unfortunate behavior, and possibly learning from it. But I was referring to the normal behavioral differences already existing within a given species … and individuals with, say, slower reflexes or less agility dropping out of the gene pool.
But I don’t know to what extent these factors are more relevant now than they used to be, when the animal “merely” had to contend with natural predators.
If you think about it, the natural predators are going to be a much, much bigger factor-- an overwhelming factor. They are trying to kill the squirrels. We usually try not to, but even when we’re oblivious, we’re huge and noisy.
squirrels have evolved the instinct to traverse open areas to less visible places with complete disregard for predators (or cars). until squirrels overcome this builtin function i would be very hesitant to say they are made for meaningful urban life.
after thinking long and hard about the cruel fate of squirrels in my suburb, i have decided that the best evolutionary trend that minimizes becoming roadkill is the avoidance of places where cars roam. but since we are killing their habitats, maybe they should just go home and die.
They have???
before the advent of cars
to clarify, the purpose of this is so they don’t stand in the middle of an open area so predators can see them and pounce on them.
So how is that “complete disregard for predators”? Sounds like quite a bit of regard for predators.
just observe a squirrel running through streets with speeding cars
I believe what telecommunications is saying is that when a squirrel decides to cross an open space (my backyard, a field, the street, etc), it does so as quickly as possible and not in a ‘fits and starts, looking around for hawks’ kind of fashion.
Hereabouts they frequently (and obliviously) cross roads at night.
*Oh, it’s roadkill
Drivin’ around in my automobile
For roadkill
Climb out of the cab and hand me a drill
Roadkill
Ain’t gonna pay their hospital bills
Roadkill
Kill a little critter just to get my thrill
Are you gonna run 'em down right now?
Yeah, I’m gonna run 'em down right now*
- The Dickies
Personally, I would limit “artificial selection” to deliberate selection by humans, rather than indirect selection by some incidental change humans have made to the environment. I would probably count your example of the crabs as artificial selection, even though the change was inadvertent. I would count the effect of cars as natural selection.
I would agree with the main points expressed so far;
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The population has not been subject to selection for long enough to make a change in a fairly complex behavior. The main problem is that cars approach far more rapidly than other predators, and it’s tough for animals to adjust their perceptions and behavior to cope.
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The selection pressure may not be that strong compared to other sources of selection. While we notice roadkill, the number of animals that die may be much less than those taken by predators or that die from disease.
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As a corollary to the above, as John Mace suggests, the proportion of the population subject to selection may not be that great. In rural areas, for small animals only those living in the immediate area of the road will be subject to selection. If the population living away from the road is much larger, any effect of selection on the population living near the road will be swamped by interbreeding with non-selected populations.
FL too.
Why did the Florida chicken cross the road?
To show the possum it could be done! 
(I’ve heard they tell the same joke in Texas, substituting the armadillo.)
When I lived in eastern Washington (state, that is), the most common roadkill was some kind of rodent that everyone calls ground squirrels.[sup]1[/sup] When on the road and seeing a car, these guys would always run the long way across the road to avoid being hit. Usually this meant that they ran into danger rather than away from danger. In fact if they didn’t run at all, they usually wouldn’t be in much danger since they were usually on the edge of the road already.
Now if there were a subset of these ground squirrels that always ran to the near side of the road and that trait was genetically influenced to at least some extent, then there would be pressure to select for that trait. Yes, it would be largely swamped by rodents that didn’t live near roads, but it would still be there. So eventually you’ll get squirrels that avoid becoming roadkill.
But the key word is eventually. Rural roads have only been mostly paved in that part of the country for maybe 50 or 60 years. Because of the swamping effect, it’s going to take a long time for any run-to-the-near-side trait to dominate over the run-to-the-far-side trait, assuming that latter is found in most squirrels.[sup]2[/sup] No doubt by time that happens, humans will have switched to air cars.[sup]3[/sup]
[sup]1[/sup] Eastern Washington is largely a treeless area except in the northern section. So the squirrels don’t have trees to live in.
[sup]2[/sup] Observation would seem to indicate that run-to-the-far-side currently dominates, but that may be a selection effect. After all, drivers wouldn’t notice the run-to-the-near-side squirrels because they wouldn’t see them on the road.
[sup]3[/sup] By the 22nd century for sure…