Properly-aimed, the typical low-beam headlight illuminates 150 to 200 feet of roadway, and the typical high-beam illuminates 250 to 350 feet. Other drivers can see your headlights from nearly a mile away on a bright summer day, and even farther at night. Clearly, the more you can see at night, the more aware you are of your surroundings and able you are to react appropriately.
Unfortunately, dirty headlights can severely impact your visibility, not only to other drivers, but your own view of the road. Depending on the severity, dirty headlights can reduce light output by up to 95%, reducing projection distance by up to 90%. Driving with dirty headlights could mean you are only able to see 10% of what’s really in front of your vehicle. At highway speeds, it would be impossible to react to an animal crossing, stopped vehicle, or vehicle with no lights. It would be nearly impossible to see anything beyond a few feet in front of the vehicle and perhaps the reflective lane markings on the road.
Source : https://www.thoughtco.com/how-to-clean-headlights-4147344