There have been a series of recent national activities that have increased the awareness of performance of traffic signs at night. Also, transportation agencies now have more choices than ever in the selection of retroreflective sign sheeting materials for traffic signs. Accompanying these changes is an increased ability to model the photometric performance of sign materials and predict the luminance that associated with a specific set of conditions. The headlamps may be the one factor that is the most idealized parameter in the photometric modeling process. This paper reviews work on traffic sign luminance modeling and describes how headlamp photometric data are used in these models. It also describes illuminance measurements made of a sample of 46 U.S. passenger cars and light trucks with clean and dirty headlamps. Subsequent modeling of headlamp performance, including an assessment of the impact of dirt on sign luminance is described. Recommendations are presented that can be used to adjust headlamp luminous intensity matrices to yield candela levels at typical sign geometries representing measured candelas from a pseudo random selection of vehicles.