3.0 Signing, Markings and Channelization

3.6.5 Recommended Guidelines

CMS Design Issues

• After years of evaluating the performance of CMS using fiber optic technology versus LED technology, LED has evolved as the preferred technology, largely because of the rapid advancement in LEDs that are used to comprise the sign messages. Fiber optic signs typically use a halogen light source that needs frequent replacement, even with recent technology advancements. A catwalk should be installed in conjunction with the CMS to accommodate access for maintenance without the need for lane closures. Alternatively, a bucket truck or lift platform can be used for CMS maintenance after closing the lane below.
• Visibility under all ambient lighting and weather conditions supports the use a light based technology such as LED and fiber optic over a mechanical technology such as prism and flip disk.
• Light based technology signs have significantly higher capital and operating expenses relative to mechanical technology signs.
• To achieve maximum visibility for approaching traffic, the CMS must be tilted toward the plaza pavement at the angle and height recommended by the manufacturer. The CMS can be manufactured with the front face at the recommended angle and having a vertical backplane to reduce installation complexity. The color of the characters should be selected to comply with the MUTCD, while achieving maximum visibility. The height of the characters must be selected to be visible at a distance at least equal to the length of the queue zone. The CMS needs to be centered over the toll lane.
• CMS employing a light based technology should be capable of multiple brightness levels that are selected based on data inputs from multiple photocells attached to the top of the CMS.
• The canopy structure must be structurally sound to support the weight of the CMS, catwalk and two maintenance technicians.

CMS Guideline Development:

• The advantages of CMS using a light based LED technology are now significant and should be deployed for all new installations. Design of the catwalk for maintenance service must assure sign visibility is not diminished by the presence of the catwalk as the user approaches the toll lane. The finish on the CMS cabinet (i.e., sides and back) needs to minimize the absorption of heat to reduce the cooling load on the CMS ventilation system and to prevent the cabinet surface temperature from exceeding a safe threshold for performing maintenance. The catwalk should be fabricated from low maintenance, non-corrosive materials. Grounding and lightning protection needs to be provided for the housing cabinet and copper power and communication lines.
• If the manufacturer’s recommended tilt is not fabricated into the design of the cabinet, CMS support brackets must be securely attached to the canopy framing to achieve the recommended angle. The CMS should be visible for a distance at least equal to the depth of the queue zone plus an additional 40 - 50% approach factor. The minimum height of the CMS letters when the speed limit is less than 55 mph should be 10.6 inches, and where the speed limit is 55 mph or greater, the minimum letter height should be 18 inches.
• CMS employing a light based technology should be capable of a minimum of three levels of brightness that are based on data inputs from a minimum of three photocells.

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