As soon as you see that there is oncoming traffic.
Turn the situation around.
When would you want oncoming traffic to dim their lights?
150 meters
100 metres?
EMP is not 'built', it happens spontaneously as the result of certain nuclear explosions.
your local area will have a version of a Highway Traffic Act, it will outline the regulations about lighting, in my area you need to have working headlights that are visible for 150 meters and give visibility for 150 meters. Look up you ministry of transportation, and usually under equipment standards/lights.
Yes most life does exist within a few meters of the earth.
It's hard to say. Lights and electronics aren't switched off by electromagnetic pulses, and without a lot of serious experimentation, it's hard to know exactly what it would take.
500 ft. (150 meters)
It has to be within 50 meters
100 meters
100 meters
10 metres
Kilo. Kilometer or KM is l000 Meters. This is used for distance and speed, 100 Kilometers equals about 60 Miles. No metric traffic signs, please.