For many people, the flashing light will be significantly more dazzling for both physical and neurological reasons.

Physically, the eye adjusts to ambient lighting. With a steady light, that means the iris stops down a bit and stays that way until you've passed the bright light. With a flashing light, the light hits the eye with the iris wide open, then the iris stops down in time for the light to be off, trying to see in the dark with an eye adjusted for brighter light. So the iris opens again, then the light hits again, over and over again. That means the flashing light hits the retina with a higher intensity, on average, than a steady light of the same intensity.

Neurologically, the brain is programmed to pay attention to flashing lights. The flashing light is therefore more distracting than a steady light of the same intensity.

Now, both responses vary considerably among individuals -- some people will have no adverse reaction to lights that will leave other riders so disoriented they have to stop. The same is true with car lighting -- headlight regulations are designed to maintain the safety of the majority of drivers, even if many drivers could tolerate much more glare in their eyes.