They do, they just look different. At least that's how it is in France.
Missing link? In NZ, and presumably other countries, personal letterboxes in rural areas can be used to send too, which I'm guessing is the same as what you're referring to.
Honestly, as a driver, when I see a bicycle, I make sure to provide as much room around them as possible. When I see a motorcycle from behind, I have no idea what the thought processes of the rider will be and just continue on in a straight line and allow them to make a decision. A bicycle in my rearview mirror poses no threat as they cannot keep up. A motorcycle can easily overtake in way ordinary cars cannot (between lanes, etc). They are much more difficult to predict. (not even taking into consideration the tendancy of motorcycle drivers being less risk averse).
So, if you want to teach the AI that a bicycle and a motorcycle are the same thing, then yes, you might inadvertantly lead to an AI making the wrong decision leading to a death. I wouldn't necessarily blame you for picking the wrong image in a captcha, but I'd definitely blame the devs for not sanitizing user input.
The machine doesn't know the answer so it doesn't really matter. You need to emulate the crowd and also realize you are contributing to the crowd. So you can actually pollute it with wrong answers to delay the robot uprising. Or at least cripple it when they keep running into fire hydrants they didn't see.
Hilton.com is a big annoyance.
I know why they force me though, I use content blockers on macOS and iOS to block tracking and ads, but that is not a good reason.
I also hate sites that force you to do so... eg. twitter, where it decides it won't let you register without a phone number (making a new twitter account for a project), then won't allow you to use the phone number, because you already used it for your primary account.
The issue is that it may flag your account as more suspicious. After three days I had my account flagged. I couldn't view tweets or use the account in any way. On log in it would redirect me and ask for my phone number for "verification". Funny how they can verify something they never received. It took about ten days for support to correct but my account is active again and they still don't know my phone number.
I mean, maybe I can see/accept an argument for that. But when I have a recent order? When I'm logging in from the link you just emailed me?
I fully understand this isn't going to have any effect. Maybe if a million of us were doing it, it would. But it still makes me feel better.
I remember far worse...
It would just be simply people refusing to work on captchas more than a certain amount. Basically, if you make me hunt for bikes and there's more than 3 of them, then I'm not doing it. More akin to a deliberate labor slowdown than a DDoS, or even a strike.
I mentioned DDOS because every time you fail to satisfy captcha criteria website requests a new batch of captcha for you; in that way you would be constantly requesting a new captcha if you would be deliberately failing your current one or as you described imposing a "hunt limit" upon yourself.
In a quite a few instances, I find it flattering, because it implies "we haven't sucked enough data on you already to know if you are a human, so we need to be sure."
If it says "select a sign" and I see there's also a sign far away in the distance only a few pixels in size, I select it because it is a sign. I think most people don't because I frequently get many tests after, because I'm not human enough for them.
My surprise if the sites that do those things turned out to be using humans for free labeling of images would be an exact, unsigned zero.
In all fairness, i usually do that.