The limit to the eye's acuity is actually related to diffraction by the pupil. What is the angle between two just‑resolvable points of light for a 2.75 mm diameter pupil, assuming the average wavelength of 539 nm ? angle between two points of light:

Take the result to be the practical limit for the eye. What is the greatest possible distance a car can be from a person if he or she can resolve its two headlights, given they are 1.60 m apart?
greatest distance at which headlights can be distinguished:
What is the distance between two just‑resolvable points held at an arm's length (0.650 m) from a person's eye?
distance between two points 0.650 m from a person's eye: