Variable Message Sign (VMS)

Light Emitting Diodes (LEDs) have found a permanent home in traffic VMS (Variable Message Sign) applications. After a somewhat auspicious start in the 1980's, LEDs have become the preferred VMS light source for the new millenium. Virtually maintenance free, LEDs offer numerous advantages over other VMS options available today.

The first commercially usable LEDs were developed in the 1960's. LEDs first appeared in variable message signs in the late 1970's. These early variable message signs were primarily used for commercial advertising. Their popularity grew quickly because of their reliable, maintenance free operation, relatively good resolution and excellent readability.

LED variable message signs were originally limited to indoor applications because of limited brightness. LED technology, although almost 15 years old by this time, had still not reached a level where it was easily visible in high ambient light conditions. LEDs began to move outdoors in the early to mid 1980's. Technological advances improved the brightness of LEDs to the point where messages were readable outdoors under most conditions.

Red LEDs

Red was the first LED color to achieve a brightness level that could be effectively used outdoors. The development of GaAlAs (Gallium, Aluminum, Arsenide) LED technology brought LED brightness to a new level. Initial outdoor applications were commercial in nature, primarily time and temperature displays in front of a bank or advertising on a marquis. It wasn't long, however, before red leds found their way into traffic oriented variable message signs.

Original attempts to make variable message signs for traffic applications with red leds met with mixed results. First, there is a significant segment of the population which is colorblind. Unfortunately, red, due to it's longer wavelength at the upper end of the visible light spectrum is typically the first color to be lost to colorblindness. Additionally, a segment of the population finds it difficult to focus on red due to a phenomenon called chromatic aberration. This condition results in the characters displayed having a fuzzy, out of focus appearance. To remedy these issues some early LED VMS pioneers mixed red and green leds in clusters. These colors, when mixed in the proper proportion will yield the yellow leds / amber leds color that we see in signs today. This attempt, as well, had its share of problems. Green LED technology had not kept up with the red leds in brightness and consequently had to be run at higher currents to yield suitable brightness. The red leds, as well, were sometimes run at higher than advisable currents to maximize brightness. This overdriving of the LEDs caused premature brightness degradation and failures. The results of that failure were often signs of virtually every shade between red and green.

Although GaAlAs was a major breakthrough in LED technology, there were also other significant drawbacks to GaAlAs material. First, it was only available in a red 660nm wavelength. Second, the light output degradation of GaAlAs is greater than that of standard technology. It has long been a misconception with LEDs that light output will decrease by 50% after 100,000 hours of operation. In fact, some GaAlAs LEDs may decrease by 50% after only 50,000 to 70,000 hours of operation. This is especially true in high temperature and/or high humidity environments.

Current LED Technology Today's LED VMS, both large fixed installations and portable work zone signs, utilize InGaAlP (Indium Gallium Aluminum Phosphide) LED technology. Derived from solid state visible laser diode technology, InGaAlP LEDs were first introduced in 1990 and since then have managed to address virtually all of the shortcomings previously associated with LEDs in traffic VMS applications.

Traffic VMS Systems

LEDs have become the technology of choice for today's VMS As our highways become more and more congested we can expect to see a proliferation of VMS systems as an alternative to the prohibitive cost and environmental impact of constructing new roads in congested areas. It has become imperative that we efficiently utilize the highway infrastructure that we have in place. Traffic VMS systems will be used extensively to guide motorists through a maze of road maintenance and repair detours, rush hour bottlenecks, weather concerns and delays caused by traffic incidents. In order to meet this challenge the VMS units deployed must be clearly legible under all weather conditions, cost effective and as reliable as possible.

Today's InGaAlP LEDs are ideally suited for this challenge. With the VMS unit subject to all possible weather conditions, continuous vibration, and the effects of dirt, dust, and road salt, LEDs have become the obvious technology choice. The reliable, solid state design of the LED makes it highly resistant to the effects of weather as well as vibration and mechanical shock. LEDs are proven entities in both hot and cold climates as is evidenced by their acceptance for use in the demanding automotive industry for both interior and exterior lighting applications.

LEDs equal and even exceed the light output of incandescent and halogen lamps. They are not, however, subject to the maintenance requirements associated with conventional lamp and shutter technologies. LEDs have a MTBF (mean time between failures) in excess of one million hours. This level of service reliability completely eliminates the need to replace burned out lamps or repair malfunctioning shutters. This ensures that the intended message remains legible and clearly reaches the motorist. A burned out lamp or stuck shutter can cause a driver to divert their attention from driving to interpreting the message. The legibility of a LED VMS is even further enhanced through the use of numerous brightness levels. As solid state devices, LEDs are easily controlled with respect to brightness. It is not unusual for an LED VMS to have well over a hundred brightness levels, automatically controlled, to provide optimum legibility under virtually all ambient light and weather conditions.

LEDs, Cost Effective Solution

In addition to all of the technological advantages, LEDs offer a cost effective solution as well. Cost savings are realized in several ways, first, LED technology is extremely energy efficient. The LED work zone VMS of today are virtually all solar powered eliminating the need for diesel generators and the related support and maintenance costs. Larger, fixed LED VMS also realize a significant savings in power costs. In addition, ongoing maintenance for both large fixed and portable LED VMS units is reduced to cleaning and the servicing of the electronics which is required of all types of units. Finally, an LED VMS unit can be expected to provide many years of this low cost service. InGaAlP LED life (hours of operation until the LED reaches one half of its original brightness) expectations are in excess of 100,000 hours of operating time.

In GaAlP LEDs offer a reliable, cost effective solution to the increasing demands that our highly mobile society has placed upon variable message signs. Not subject to the failures expected of alternative technologies, LED VMS provide dependable, long term service, low power consumption and excellent legibility for all VMS applications from portable work zone signs to large, fixed overhead installations.