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Section 9: Illumination

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Overview

This section discusses the aesthetic considerations associated with roadway lighting. Lighting design standards are covered in the Roadway Design Manual.

The aesthetic considerations of highway lighting design include:

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Lighting for Visual Effect

In addition to lighting the traveled way for minimum visibility, designers should consider the illumination of significant elements such as bridges and walls to provide clarity and interest at night. Although urban areas are usually the most appropriate for the addition of highlighting, significant rural interchanges may also benefit from such increased contrast and visibility. Illuminated, backlighted, or directly lighted structures or design elements along the highway can do a lot to improve the overall perception of a highway corridor.

Here are just a few examples of features that can benefit from aesthetic lighting:

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Aesthetic Lighting Guidelines

Aesthetic lighting shall be designed and installed in a way that will not distract drivers or create a safety hazard according to the following guidelines:

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  • The feature may not contain or be illuminated by flashing, intermittent, or moving lights, including any type of screen using animated or scrolling displays;
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  • Luminaires must be shielded, directed, and positioned to prevent beams or rays of light from being directed at any portion of the traveled ways of a regulated highway;
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  • The lighting may not be of such intensity or brilliance as to cause vision impairment of a driver of any motor vehicle on a regulated highway, or otherwise interfere with the driver's operation of a motor vehicle;
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  • Illuminance levels shall conform to the recommendations in Chapter 26, Lighting for Exteriors, from the Illuminating Engineering Society (IES) Lighting handbook, 10th Edition;
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  • Lighting features must be located outside the roadway clear zone, or protected by a TxDOT approved crash barrier;
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  • The lighting may not simulate, resemble, obscure, or interfere with the effectiveness of an official traffic sign, device, or signal;
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  • When possible, cutoff luminaires should be used to minimize light pollution as called for in Texas Health and Safety Code Chapter 425, Regulation of Certain Outdoor Lighting;
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  • For direct view sources like LED or neon, the aesthetic lighting should not exceed 0.3 footcandles over ambient lighting levels when measured according to the International Sign Association's Recommended Night-time Brightness Levels for On-Premise Electronic Message Centers (EMC's);
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  • For lighting designed to change appearance over time, the minimum length of time between changes of scene shall be either:
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    • At least eight seconds; or
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    • Calculated by the formula: Sight distance to lit area (ft) / Speed limit (ft/sec) = Minimum display duration (sec);
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  • Lighting systems should meet the applicable requirements of AASHTO, IES, the National Electrical Code, the TxDOT Highway Illumination Manual, or other standards as deemed appropriate by the TxDOT District Engineer.
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Aesthetic Character of Luminaires and Poles

Luminaires and poles are a highly visible component of roadway design. Designers should consider how they fit with the aesthetics of their surroundings, particularly with regards to architectural character, pole spacing, and color (see Figure 4-67).

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  • The form of the luminaire should fit with the character of its surroundings. For example, contemporary buildings usually suggest luminaires with clean lines that fit the lines of those structures. The Texas Historical Commission should be consulted for guidance on choosing light fixtures for historic districts.
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  • Widely spaced poles are read as single objects in the landscape and should be carefully fitted to the surrounding landscape. Conversely, closely spaced poles form a visual element that tends to define the corridor. In this case, generally use colors that blend well with the background so that the combined effect of multiple elements is unobtrusive.
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  • Choose colors that complement the immediate surroundings and the materials used on the highway.
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Lighting Sources

Until recently, high intensity discharge (HID) lighting has been used for most transportation applications. HID includes lighting with mercury vapor, high pressure sodium, and metal halide lamps. However, light emitting diode (LED) lighting has now replaced HID as TxDOT’s recommended source for most roadway and aesthetic lighting.

The main advantage of LED over HID lighting is long life. HID luminaires use replaceable lamps with a typical useful life of 2-4 years, depending on type. As lamps burn out, gaps in the lighting or diminished illumination can reduce the quality of the lighting from the original design. HID luminaires require lamp changes every few years to maintain the original appearance of the installation.

LED luminaires do not use replaceable lamps, but have a projected useful life of at least 15 years. The long life of LED helps to ensure that the original lighting installation will perform as designed for several years before needing to be revisited for maintenance. LED luminaires also use less energy than their equivalent HID luminaires, which saves on operating costs.

Color temperature should be specified when using white LED for aesthetic lighting. Typical color temperatures range from 2700K for warm white, to 4000K for neutral white, to 6000K for cool white. Lower color temperatures contain more red in their spectrum, and higher color temperatures have more blue. This can affect the appearance of illuminated objects, so the appropriate color temperature should be evaluated for the aesthetic lighting at each location. TxDOT’s standard color temperature for roadway lighting is 4000K, however slightly warmer color temperature (2700K – 3000K) may be more appropriate for landscape lighting.

LED lighting is also available in single colors or with programmable RGB (Red-Green-Blue) color changing arrays. Color changing installations require the luminaires to be connected to a controller, which may be either hard-wired or wireless. When specifying a lighting controller, it is important to specify strong security features to prevent unauthorized operation of the lights.

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Lighting and Plant Materials

The placement of plant materials must be done with knowledge of the illumination plans for the highway. In general, high mast lighting will not be significantly affected by plant material placement. Because of the height of the light source shadows will be cast almost vertically and shadows or dark spots will not be a problem.

Where standard, pole mounted fixtures are used, placement of plant materials becomes more critical. Trees must be placed so future growth will not interfere with the light source. As a rule trees should not be placed forward of any light standard.

Lighting can also be used to highlight trees in a landscape design.

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