You don't really have to know which squares are actual crosswalks or what a crosswalk really is, reCAPTCHA doesn't care and it thinks that any street with white striped lines is a crosswalk even if the lines are for indicating something else (which is also annoying because it'll ask you to verify you are human again if you don't check those non-crosswalks). So do as I do and select any street that has white striped lines (same goes for its more rare bicycle/motorbike, small truck/car or truck/bus mix-ups).