Improperly aimed headlights can affect safety on the road. Researchers studied how misaimed headlights develop and ways to prevent it. They looked at headlamp conditions in states with and without vehicle inspections. Misaiming headlights can affect how well drivers see objects on the road and can cause glare for other drivers. Upward misaiming increases visibility to front targets but causes glare to oncoming drivers. Downward misaiming reduces glare to oncoming drivers but also reduces visibility distance. Both upward and downward misaiming with high beams decrease visibility distance. Misaiming can be caused by changes within the lamp, vehicle loading, and headlamp inspection. The most serious cause is changing the vehicle's loading. Data can help predict visibility with different headlamp systems.