Lighting levels for typical roadways may be designed by calculating either the illuminance levels, luminance levels, or both.

Illuminance in roadway lighting is a measurement of the amount of light that hits the pavement surface. Illuminance is measured in foot-candles (US customary units) or lux (SI units). TxDOT projects are typically calculated in foot-candles. Illuminance layouts may be designed with lighting design software or with illuminance templates. The illuminance at any point will be the sum of illuminance from one or several contributing sources.

Luminance in roadway lighting is a measurement of the reflected light from the pavement surface that is visible to the motorist's eye. Luminance is measured in candela per square meter (cd/m2).The reflectance characteristics of the pavement must be known to calculate luminance, and lighting design software is required for roadway luminance calculations.

The level and uniformity of illuminance or luminance along a highway depends on several factors, including the lumen output of the light source, luminaire distribution, mounting height, luminaire position, pavement reflectance, and pole spacing and arrangement. The same average level can be obtained by different installation arrangements, such as a few high-output light sources or a greater number of low-output sources.

The proper light loss factor (LLF) should be used in all lighting calculations. LLF is a depreciation factor applied to the calculated initial average illuminance or luminance. Each TxDOT LED luminaire has a unique LLF that is shown with that fixture on the Material Producer’s List (MPL) for Roadway Illumination and Electrical Supplies. TxDOT HPS luminaires use a LLF of 0.65 for all models.