Compendium of student papers: 2011 undergraduate transportation scholars program. Page: 25
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Late Night Roadway Visibility
Figure 1. Horizontal or Overhead
Illuminance. Figure 2. Vertical or Forward Illuminance.
Some segments of highways have luminaires that contribute greatly to the amount of overhead
and forward illuminance on the roadway. Luminaires come in different types, sizes, and spacing.
Conventional luminaires can be single arm or double arm. Double arm luminaires, also known as
butterflies, are mounted along the median. Conventional luminaires are less than 50 ft tall and
only have one or two light bulbs per pole (2). High mast lighting refers to luminaires that are
over 100 ft tall with a group of lights at the top of the pole (2).
On a typical highway, the four main sources of illuminance come from roadway luminaires from
above, commercial lighting on the sides, other vehicle headlights on the front and back, and
environmental factors such as cloud cover or moonshine from above. The illuminance at any
point will be the sum of illuminance from one or several sources. The research involves isolating
the individual contributions of illuminance from different sources as well as determining how
those contributions change with respect to time.
The illuminance from the moon will be insignificant compared to the illuminance from
luminaires so the effects of the moon can be ignored. However, the lighting from the moon is
more significant on empty stretches of rural roads that have no lighting installed. Moonlight will
make a greater difference in visibility when it is the only source of illumination.
Illuminance depends on the distance the measurement is taken from the light source. If a surface
is closer to a light source, it will take in more light and thus have a greater illuminance value.
Uniformity is a design criterion specified in the AASHTO Roadway Lighting Guide that
measures how evenly light is distributed to different areas. For example, a 1:1 uniformity ratio
would mean that the roadway is equally bright in all areas; there would be no dark spots or bright
spots. The uniformity ratio is defined as the ratio of the average or maximum illuminance to the
minimum illuminance (6).
The amount of lighting on the highway at night is affected by established design guides. The
TxDOT Highway Illumination Manual decides whether roadway lighting is required for certain
parts of highways and governs how roadway lighting will be installed based on factors such as
traffic volume, existing lighting, road type, etc. These guidelines establish details such as the
types of luminaires installed, the spacing of the luminaires, and the mounting height of the
luminaires all of which affect nighttime lighting. For example, TxDOT typically does not install
lights for frontage roads so this will result in less illumination coming from the sides of the
highway (2). Also affecting lighting are light distribution patterns that are specified by theRuoxin Jiang
Page 25
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Desorcie, Felicia; Jian, Ruoxin; Kelly, Pete; Larson, Shawn; Rasulo, Marcus; Rivera, Joshua et al. Compendium of student papers: 2011 undergraduate transportation scholars program., report, May 2012; College Station, Texas. (https://texashistory.unt.edu/ark:/67531/metapth281729/m1/33/: accessed April 24, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.