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Publication Number:      Date:  Autumn 1993
Issue No: Vol. 57 No. 2
Date: Autumn 1993


Changeable Message Signs

Avoiding Design and Procurement Pitfalls

by Pamela P. Marston


Traffic control devices, particularly signs, located along the roadway are the backbone of the traveler information system. The use of changeable message signs (CMS), which display real-time information to motorists, has assisted in efforts to improve roadway operations and safety of existing facilities. Even though the use of CMS is common on today's roadways, there are problems that prevent the optimal use of changeable message equipment. Some of the difficulties that currently exist with the procurement and implementation of CMS are due to several major issues including: inadequate specifications, insufficient definition of protocols, low bid acceptance practices, and no industry standards. This article describes the various CMS technologies and their uses, explains some of the challenges inherent with their use, and recommends solutions to the problems.

Sign Technology

Some of the more common uses of CMS include traffic warning, regulation, routing, and management. The real-time information displays assist in modifying the behavior of the driver and thereby improve the traffic flow for all motorists. CMS, as well as other types of real-time motorist information sources, can be used to address traffic developments in five operational categories: (1)

Traffic problems resulting from the above conditions can be significantly reduced with the use of real-time information displays like CMS. Depending upon the needs of the public agency, CMS can be either permanently located or transported to various locations.

Although there are many types of CMS on the market, they can be categorized into three different groups: light-reflecting, light-emitting, and hybrid. Briefly, light-reflecting signs reflect light received from an external source such as headlights, the sun, or illuminators. Light-emitting signs create light, either on or behind the viewing surface, with lamps located within the sign housing. Some sign suppliers have combined characteristics of both reflective and emitting technologies to create hybrid signs. (1)

The most common types of CMS use fiber optics, light-emitting diodes, reflective disks, or a combination of these technologies. The fiber-optic (F-O) sign uses light radiated from a halogen lamp located within the sign casing. The light is sent through polished glass fibers and directed to the sign face. On the sign face, each F-O light guide appears as a point of light. These points of light are arranged in a matrix structure and are adjusted in size. Color filters can created almost any color sign. Messages are either flashing or static, but they are displayed only when the internal light source is turned on. Messages on the fixed-grid matrix can be either symbols -- within limits -- or words and can be changed almost instantaneously. (1) An example of a fixed-grid matrix sign is a sign that indicates whether a lane or a toll booth is open or closed to traffic. F-O signs can also be made with tiny shutters that cover the ends of the light guides. In this sign configuration, F-O light guides are arranged in a pixel. These pixels, which are normally one inch square and contain two or three F-O light guides, are typically grouped in a 5 by 7 array creating a character module. The character modules display the letters that constitute messages to the drivers. In a typical configuration, if there are three shuttered light guides, two of the shutters are open and lit when needed in a message. The third shuttered light guide serves as a backup and stays closed at all times, unless one of the other two shutters malfunctions and does not open. The pixels, although most commonly found in a 5 by 7 array, can be arranged in matrices of varying heights. Symbols are harder to create with this type of shuttered F-O configuration, but word messages can be created easily and changed immediately. (1)

The light-emitting diode (LED) clustered matrix is a grouping of solid state devices that glow when voltage is applied. Very little power is needed to use an LED. However, because of the limited brightness and small sizes of the LEDs, many must be used to produce an effective message. Since LEDs are low voltage, they need high currents to illuminate them. A problem with the LEDs is that the intensity of light tends to decrease over time because of material deterioration and constant high temperatures. Therefore, LED clusters must be well-ventilated to prevent losing color brightness. (1) LEDs are most commonly found in green, red, and amber hues; white, which contrasts the best with a black background, is not available.

Reflective disk CMS use disks -- circular, rectangular, and three-dimensional -- made of a reflective color, usually yellow, on a dark background. The disks are permanently magnetized, pivoted, and arranged in an array so that when an electromagnet is used to flip the disk, a message is shown. Reflective disks can be internally illuminated -- for example, fluorescent lights within the sign housing -- or externally illuminated by the sun, headlights or bottom-mounted lighting. If the sign is internally lit, then power is necessary whenever a message is displayed. If the sign is not light emitting, then power is only needed when the disks are rotated or flipped.


This is a portable reflective disk CMS illuminated from within by fluorescent bulbs.

The hybrid CMS incorporate either LED or F-O with reflective disks. In this sign technology, the reflective disk, which is black on one side and yellow -- or similar contrasting color -- on the other side, is used as a shutter for a light source, typically LED or F-O. When the shutter is "on," the yellow side of the reflective disk and the light emitter can be seen. When the shutter is "off," the light emitter is covered by the disk, and the black side of the disk is seen. The hybrid signs require less light from the emitter, as they are not attempting to overcome the power of the sun. The reflectors are given more visibility with the incorporation of the LED/F-O, while still reflecting the sun, headlights, or internal illumination.

Issues Concerning Optimal CMS Use


Properly written specifications can be a safeguard for a public agency to ensure quality equipment is being purchased and used. Agencies can assure that they get the proper equipment out in the field by moving away from "cookbook" or generic specifications to a more performance-oriented document which properly reflects the transportation agency's goals and objectives. Many specification documents are concerned with how a product is made and what it looks like; they convey little interest in the product's performance. (2)

Agencies sometimes write general specifications, particularly for electronic equipment, because they lack knowledge of the most recent developments in rapidly developing technologies. Often the state or local agency writes the specification so generally that any CMS product or technology is applicable. If the specification is too generic, the agency may obtain inappropriate equipment that nevertheless meets the specified requirements.

Some public agencies write very general specifications because of concern about unintentional sole sourcing. United States Code Title 23, Section 112(b)(1), requires the states to foster competition in bids when federal money is involved; however, the law does allow sole-source procurement in emergencies and when it is in the best interest of the public.

If the agency obtains "low performance" equipment, who is responsible for the low quality? It is not the contractor who fulfilled all the specified requirements. Inevitably, the transportation agency that used inadequate specifications is responsible.

The problems with CMS specifications can also be evident in the way a prime contractor purchases signs for a project. The contractor's duties include the purchase and installation of the CMS. This is typical of the "all-or-nothing" approach to contract awards. Consequently, a prime contractor, who may not fully understand the public agency's present system and may know even less than the state or local agency about CMS technology, is responsible for finding signs to fulfill the contract. If the agency does not provide good CMS specifications, the contractor alone determines what is required, and the contractor is free to select the least expensive option without regard to quality. As a result, the public agency may end up with low quality equipment.

Since the practice of contractor-purchased CMS is price driven, the public agency needs to remove the incentive for purchasing low-quality equipment. Three suggestions that will help to assure the purchase of high-quality equipment are:

Low bidding

Low-bid acceptance combined with poorly defined specifications contributes to low-quality equipment purchased by transportation agencies. United States Code Title 23, Section 112(b)(1), states that, in projects using federal monies, a public agency shall award contracts through competitive bidding, unless the agency demonstrates a more cost-effective method of bidding or an emergency exists. The law goes on to state that the contract will be awarded to the lowest bidder that successfully meets the bidding criteria established in the bid announcement.

The public agency can use this federal regulation to its advantage by adopting criteria that bring in quality products. One method to accomplish this goal is to accept the lowest adjusted bid. With this strategy, in the bid submitted to the agency, a supplier would include a life-cycle costing analysis that would extend through the life of the project, including estimates for maintenance and operation for the duration of the contract period. All bids must be presented in a similar format so that they can be compared fairly. The agency can incorporate the life-cycle costing without sacrificing competition, as this type of bidding does not exclude anyone from the process.

The most important factor in using life-cycle costing, or any other bid practice, is that everything must be spelled out explicitly in the state or local agency's bid announcement and specifications. This is especially true for the criteria by which all the bidders will be evaluated. Then competitive low bidding can actually help the agency.

Standard Protocols

One of the biggest problems associated with the use of CMS is the need for standard protocols for communications and software. In this case, protocol refers to "a set of conventions, or rules, on the format and content of messages transmitted between devices." (3) When a CMS is purchased by a state or local agency, it often comes with its own communications package. At one time, the need for a total package of sign, controller, and communications may have been necessary, but with the advances in electronics, standard communications and software equipment has been developed to make CMS systems much more efficient.

CMS suppliers often dictate the communications aspect of CMS systems to the public agency. Frequently, when an agency buys a sign, it must change its communications network to fit the sign and controller. This works to the advantage of the supplier because the agency now has a system that will only communicate with that supplier's brand of sign.

The transportation agency should specify the communications protocols and have the sign suppliers adhere to the agency's requirements, instead of the other way around. It is very important for the agency to be able to communicate with any sign, or groups of signs, at any given time. If signs from different suppliers cannot communicate readily with each other or the agency, then a serious problem exists.

To address this problem, the public agency should include in the CMS specification package a standard or open protocol. There are two ways to create a functional system. The first way is to accept multiple proprietary sign-driver software that communicates with the agency on standard communication equipment. The other way is to implement a central software package to be used by the agency and understood by all signs within the system regardless of the supplier.

If the public agency is willing to maintain control over the individual proprietary software packages needed to drive the signs, the key is to have a standard interface. For example, a standard interface could be modems communicating over voice-grade lines using industry standard protocols. The controller of the CMS would receive the data from the traffic management center (TMC) and transfer it to the sign. If the TMC should purchase another CMS from any supplier, the TMC could just plug into the new sign's controller and send data with the same type of modem and phone lines. However, should the number of controlling software packages get large, the agency may need a significant amount of time and equipment to sustain the CMS network.

If the system is to use central software designated by the purchasing agency, then the issue must be addressed explicitly in the specification. For a company to be able to bid on this type of system, the public agency must provide the format in which the data must be transmitted within the communications system. The development of a central software package could be time-consuming and expensive, but the ability to control the CMS network regardless of the number of signs or suppliers could be a significant benefit to the agency.

It must be noted that using a central software package and requiring the sign suppliers to adhere to it may prevent the suppliers from adding extras -- such as failure detection, multiple colors, or animation--to the sign system.

If a state or local agency has standard equipment--voice-grade lines, modems, central computing facilities -- the sign supplier would be responsible for making the sign tie into the existing system. Having this standard equipment, the agency could concentrate on the sign's ability to accept the data on the communications system, make a visual display of it, and test its reliability. Having open protocols will allow the agency to use various suppliers and technologies within its system because integration and installation will be uncomplicated.

Another concern may be that a transportation agency does not have the expertise -- hardware, software or computer engineers -- to control the protocols or communication systems. The agency needs to include a communications specialist on staff or to have a private firm develop the communications network. The consultant may also administer the network under an open-ended maintenance agreement and/or teach the public agency employees the system.

Cost should not be a factor for either the public agency or the supplier in adopting more advanced computer technology. Electronic equipment is decreasing in cost, while increasing in capability. Also, adopting standards would save money for the agency because CMS installation would become routine and not every job would need to be customized. (4)


One concern with any CMS equipment is maintenance on the controller and sign. The supplier can provide warranties or guarantees for its product and services over time as specified in the contract. Although some suppliers may not like this arrangement, providing guarantees helps a companies in the long run by supporting the perception that the company is reliable and confident in its quality. The use of guarantees is very common in Europe, particularly in roadway design and construction.

Although a guarantee seems to be a quick and easy solution, it can be complicated. Most importantly, the limits of the guarantee must be explicitly clear to all parties involved. Defining the criteria and exactly what is covered in the guarantee -- for instance, system hardware, software, performance, sign access, and traffic control responsibilities -- is vital to protect the interests of both the public agency and the supplier. Another issue is the question of proprietary information if the supplier is not available to work on a piece of failed equipment and someone else must perform the maintenance.

Lane Shift Ahead

This is an example of a portable reflective CMS for traffic control.

Independent testing, bonds, performance references, and just-in-time delivery

There are several other methods to ensure that quality products are being used on the American roadways. Some of these techniques include independent laboratory testing, multiyear maintenance contracts with bonds and/or liquidated damages, and performance references.

In several European counries, products are tested in government-approved laboratory facilities. For example, a potential CMS supplier to the French government must provide a typical sign. After rigorous testing by a French laboratory, a certificate of quality is issued to the supplier. Only companies with certificates are allowed to bid on projects offered by the French government. Needless to say, this pre-qualification system protects the French government from receiving low quality equipment that is unfamiliar. (5)

A system of independent lab testing may work in the United States if groups of states could agree on the specifics. For example, states in regions with similar characteristics, such as weather and topography, could jointly fund tests to make them cost-effective to all the agencies involved. Quality testing is not new to this country or even to traffic control devices; testing has been successfully done for pavement markings and signing. However, the testing facilities and procedures must be flexible so that newly developed more advanced products are not excluded.

CMS combining a static guide sign and reflective technology (top), reflective flip disk CMS (bottom)This CMS combines a static guide sign with reflective technology. (top) / This shows a reflective flip disk CMS with full graphic capabilities

Combining multiyear maintenance contracts with surety bonds is a way that public agencies can make sure that the signs in the field are serviced for the duration of contract. Surety bonds are normally issued for the installation of the signs, but surety bonds could also be used to ensure that signs will be maintained for a specified length of time. Liquidated damages can be assessed against the CMS supplier if the maintenance agreement is not fulfilled. If maintenance payments made on a yearly or as-needed basis, the penalties can be deductions from payments. If the maintenance portion has been pre-paid, then liquidated damages backed by a surety bond provides an easy means of collection. Once again, all the details concerning the duration of the maintenance contract and the surety bond must be clearly stated in the bid documents, specifications, and contract. The bonding company must be kept advised of any contract changes or concerns to ensure coverage.

Performance references can be essentially defined as a supplier's resume. These types of references are provided by the supplier and can include information such as: where its CMS have been deployed, how long the signs have been in use, and to whom at the organization using them questions should be directed. One significant issue concerning past performance requirements is that small or new firms may find it very hard to break into an existing market and bid on projects since they may not have any references for deployed equipment.

"Just-in-time delivery" means that the supplier provides the sign just prior to installation. This method, especially for large purchases, has several advantages. First, the transportation agency can save considerable costs because it is not necessary to provide storage facilities for signs waiting to be deployed. Second, the signs will not be damaged by transport or storage while waiting to be used. Third, quality control is better maintained because the supplier is not pressured to produce many signs at once.

Arizona example

The Arizona Department of Transportation provides a good example of the way that a transportation agency can approach CMS. In 1992, ADOT bid for 19 CMS to be installed and tied into the Freeway Management System. The bid requested light-emitting F-O, LED, or F-O enhanced flip disk (hybrid) technologies which were able to run through a telecommunications system. ADOT also required from every potential supplier a declaration of guaranteed parts and an independent laboratory certification of acceptability of color output, environmental condition adequacy, line transients and of rated lives for character modules, halogen lamps, LEDs, sign control units, and communications units. ADOT also requested that the proposal include at least three customer references of installed, outdoor CMS meeting specific criteria.

ADOT requested that the potential suppliers provide a guaranteed life-cycle cost on the equipment for the 10-year life of the project. The life-cycle costing would include the cost of the signs, a yearly guaranteed electrical cost, and an annual guaranteed maximum repair cost.

The electrical and repair guarantee was a unique way to control ADOT's cost of ownership during the project. For the electrical guarantee, the potential supplier had to estimate the total cost of operating one sign for eight hours a day, on various settings, at a fixed cost per kilowatt-hour. This quantity would then be multiplied by 365 to estimate the yearly cost of operation. When ADOT receives the signs, it will randomly select signs to be similarly tested, and similar calculations for yearly cost will be done. If there is a discrepancy between the ADOT estimate of total operating cost and the supplier's estimate, then the difference will be multiplied by 10 (10-year project life). This amount will then be subtracted from the amount paid to the supplier for each sign. If the ADOT estimate for electrical cost should come in under that of the supplier, then no monies will be withheld, but none will be awarded either. For guaranteed yearly maintenance, if the cost of maintenance, including traffic control costs, should exceed the maximum, then the supplier is responsible for paying the excess regardless of who performs the maintenance -- ADOT, supplier, or private firm. (6)

Ongoing Research

There are several ongoing research projects concerning CMS. The Texas Transportation Institute is completing a synthesis of practice as related to CMS. This project of the National Cooperative Highway Research Program includes information on technical standards, procurement and operational practices, maintenance experiences, and measures of performance.

The Turner-Fairbank Highway Research Center and The Last Resource have combined efforts to work on CMS visibility and photometry. The research includes field studies of inservice CMS and laboratory studies on luminance, visibility, color, contrast, fonts, and other variables.

The Virginia Transportation Research Council is assessing the uses of CMS on the roadways of Virginia. They are considering flash rates, information limitations, utility versus traffic conditions, and operational parameters.

Conclusions and Recommendations

The key to producing a specification that addresses the needs of the public agency without compromising any one company or technology lies in creating a performance-based specifications considering the agency's goals and objectives. This type of document concentrates on performance and life-cycle costs and frees the public agency to spend more time developing the evaluation criteria and the final selection process. The competitive bidding process can be maintained even with the development of detailed specifications. The agency can specify one particular technology if there is more than one supplier or if there is justification for one technology because it meets the agency's best interest. The agency is responsible for the justification of the final choice. The agency can use economic engineering principles as stated in the bid document to support the technical selection. By implementing several improvements to the present system, a win-win situation for the supplier and the agency can occur.

Table 1 - Key aspects when specifying changeable message signs

Aspects to Consider
  • Fiber-optic cable
  • Twisted wire pair
  • Cellular
  • Radio
  • Coaxial cable
  • Common interfaces
  • Data rates and formats
  • Frequency of communication
  • Control software
  • Terrain and environment
  • Supplier provided
  • Contractor provided
  • Inhouse
  • Bonding
  • Responsetime
  • Sign & software access
  • Proprietary information
  • Start of maintenance period
  • Functional specifications
  • User interfaces
  • Message failure rates
  • Legibility
  • Message verification
  • Ease of use
  • Graphics and/or text
  • Color and font
  • Inhouse
  • Independent (labs)
  • Certification acceptance
  • Contractor field tests
  • Acceptance criteria
  • Environment criteria
  • Certificates
  • Approved listing
  • Life-cycle
  • Just-in-time delivery
  • Initial capital cost
  • Lifetime electrical cost
  • Lifetime maintenance cost
  • Storage of signs
  • Transportation of signs
Quality Assurance
  • Pre-qualifications
  • Post-qualifications
  • Approved bidders list
  • Warranties
  • Guarantees
  • Past performance
  • Bonding of equipment
  • Deployed signs
  • Company history

Table 1 reflects some aspects that a transportation agency may consider when developing a specification. A specification document is unique and must be written to address the individual needs of an agency. Although specifications from different agencies can be similar, the detail must be specific to the user. Some of the major functional areas that may be considered include communication, maintenance, operations, testing, cost and quality assurance. These areas are fairly broad, and as can be seen by Table 1, there are many different practices and aspects that may be selected. In any event, it is unacceptable for an agency to get a sample protocol from a supplier, change it, and hope that the equipment can be made to work properly. This way of preparing specifications is very unreliable and can leave the agency with a document that can only be satisfied by the supplier who developed the original paper. The agency must take the initiative by providing CMS suppliers with documents explaining specific required functions, thus receiving appropriate and reliable CMS equipment for the field. If possible, it should lso provide for the supplier an opportunity to exceed the specifications.

By adopting standards and common protocols throughout the system, the state or local agency may be able to take advantage of the extras that a supplier may incorporate into CMS.

Standards developed by the CMS manufacturing industry would be applicable to all suppliers regardless of technology. If the sign suppliers take responsibility for producing their own industrywide standards, then each technology can expect accurate representation. Also, if the CMS manufacturing companies work together to develop their own standards, as opposed to waiting for a government developed document, the result would likely be more amenable to the CMS manufacturing community.

It should be the responsibility of the supplier to build the CMS to interface with the TMC; the TMC should not have to change its system just to communicate with one sign. For example, in the personal computer industry, any printer can work with any computer system because of a standard printer interface. Perhaps the CMS industry should follow the lead established by personal computer industry and adopt similar standards and protocols. Open protocols will allow projects to be accessible to more suppliers, and the agency should be confident of receiving quality equipment for field use and be assured that many vendors can supply replacements when needed.

Given the choice of either buying a group of portable or permanent CMS to be given to prime contractors on an as-needed basis or giving the prime contracting company specifications and having it choose the signs, the public agency should purchase its own signs. By purchasing the signs in bulk and giving them to the contractor to be installed as necessary, the agency can be sure of equipment quality and can also motivate the supplier to provide high-quality products if the initial contract is written to provide for the possible purchase of additional signs in the future. More importantly, the agency can be sure that all the signs in the system are compatible. Once the contract is awarded, the agency need only provide the signs and support personnel to the contractor during installation.

Since there is no appropriate test bed for CMS, public agencies, with the assistance of the American Association of State Highway and Transportation Officials or the American Society for Testing and Materials, could jointly establish a location for testing CMS equipment. A national laboratory facility could provide a central site that isalready established for research purposes. CMS sent to this lab would be tested for minimum standards and issued a certificate of acceptance. The certificates could be presented in bid documents if required by an agency. The testing must be impartial; neither the national lab nor any participating organization would endorse any one technology or supplier.

One way a public agency can use prior performance requirements in its specifications package is to either pre-qualify or post-qualify the bidders. If the agency decides to do this, it must be sound in the justification of the final choice.

Pre-qualification refers to the process by which only qualified suppliers are invited to submit bids. The Federal Highway Administration does not have any rules regarding pre-qualification of prime contractors or subcontractors as this has always been at each state and local agency's discretion. If the agency does decide to use pre-qualifications in its project, care must be taken not to exclude any company or technologies without well-founded justification.

For post-qualification, all companies are invited to bid for a project and the company's references are submitted with its bid documents. When the agency begins the final selection process, references will be considered.

There are various types of CMS, each with its own strengths and weaknesses depending upon its application. It is the transportation agency's responsibility to develop the proper specifications so that quality equipment is installed in the system. Guarantees, warranties, performance/liquidated damages bonds, and independent testing and certification can be successful tools in helping the agency acquire and maintain an efficient and safe CMS network, avoiding the design and procurement pitfalls associated with CMS.


(1) Conrad L. Dudek. Guidelines on the Use of Changeable Message Signs, Publication No. FHWA-TS-90-043, Federal Highway Administration, Washington, D.C., July 1991.

(2) Dave Geiger of Office of Engineering, Federal Highway Administration. Personal communication with author on May 10, 1993.

(3) Arthur Godman. Barnes & Noble Thesaurus of Computer Science, Barnes & Noble Books, New York,1984.

(4) Bob Rupert of the Office of Traffic Management and Intelligent Vehicle-Highway Systems, Federal Highway Administration. Personal communication with the author on May 13, 1993.

(5) John Cunningham of Fiberoptic Display Systems, Inc. Personal communication with the author on March 15, 1993.

(6) Alan Hansen of the Arizona Division, Federal Highway Administration. Personal communication with the author on June 22, 1993.

(7) John Mauro of Tele-Spot Systems. Personal communication with the author on May 20, 1993.

(8) Bill Wellman of ADDCO Manufacturing Co. Personal communication with the author on June 2, 1993.

(9) Bruce Zimmerman of Daktronics, Inc. Personal communication with the author on June 10, 1993.

Pamela P. Marston is a highway engineer in the Federal Highway Administration's Traffic Operations Training Program. Ms. Marston received both her bachelor's (1990) and master's (1993) degrees in civil engineering from the University of Virginia.



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