I think the best countermeasure is to know your own personal limitations and warning signs.

Ferinstance, I know that when I'm driving at night and lights start to *hurt* my eyes I'm getting dangerous. It starts with oncoming headlights, then streetlights, traffic lights. When the reflection of my own headlights off of signs starts to hurt...

Also, when that little voice says "Just close your eyes for one little second, you can open them again, just for a second, its ok..."