Just try to not shine the lights at driver's eyes...keep the beams toward the ground.
I ride with two Nite Rider 750's when mountain biking and with just one I have enough light to ride down the trail but I have noticed that the beam is quite strong so aiming it correctly makes a big difference. Those little lights really do put out a strong beam and you don't want to blind a driver coming at you.
+1 It's more about the angle of the light. I looked up car head lights, they are
700 lumens on low beam. So the NiteRider 750 (my light of choice too) is no brighter than a car; but if it is angled into someone's eyes - it will be too bright. I see no reason for my headlight to be pointed more than 25 yards in front of my bike, light the pavement, and that should never blind a driver.