Example of a halogen low beam plot viewed from the driver's seatshowing light with a center hotspot and the way light spreads out around it.  The line directly above the blue hot spot (0.0 on the y-axis) is the horizon and is the base line for measuring light and glare in oncoming traffic's field of vision.  Lights are designed to have a certain amount of the output above this line for reflection off of street signs and other objects.  Having too much above this is disctracting to other drivers (potentially caused when incorrect or high wattage bulbs are used).  

Example of a halogen low beam on-road beam pattern viewed from above.  This shows how headlights and reflectors are designed to spread light across the road but mainly down the right side to detect objects like pedestrians.  High Performance lights like SilverStar and XtraVision are designed to enhance the blue section in the image below.  This enhancement is greater luminance for XtraVision or whiter light and greater luminance for SilverStar.  This improves visibility by allowing drivers to see objects downroad and in the peripheral vision sooner.