DYNAMIC MESSAGE SIGNS / VARIABLE MESSAGE SIGNS
The National Electrical Manufacturers Association (NEMA) is the association of electrical equipment and medical imaging manufacturers. NEMA provides a forum for the development of technical standards that are in the best interests of the industry and users, advocacy of industry policies on legislative and regulatory matters, and collection, analysis, and dissemination of industry data.
The NEMA Standards Publication, TS 4, “Hardware Standards for Dynamic Message Signs (DMS), with NTCIP Requirements”, was developed as a design and implementation guide for dynamic traffic messaging equipment that can be safely installed and provided to the end user with operational features based on current technology.
The goal of this standard is to provide the user with safe, dependable, functional, and easily maintained DMS Equipment.
The scope of the standard is to define the minimum hardware and functional characteristics of electronically controlled Dynamic Message Signs used for displaying messages to travelers.
EN12966 is European standard for Dynamic Message Signs. It has been prepared by Technical Committee CEN/TC 226 “Road equipment” under a mandate given to CEN by the European Commission and the European Free Trade Association, and supports essential requirements of EU Directive(s).
The main properties of DMS (such as optical performance, structural performance, etc.) are divided into classes. Based on the requirements of the project, the purchaser chooses different combination of classes.
The standard defines main properties of DMS, DMS testing methods, and factory production control methods. Informative annexes are provided as useful guidance on additional DMS aspects, helping those purchasing signs or signing systems.
Both NEMA TS-4 and EN12966 have the same purpose – they define performance parameters of LED displays (Variable Message Signs, Dynamic Message Signs) in all relevant aspects. NEMA TS-4 focuses on US client perspective, while EN12966 focuses on European client perspective.
Although they are not exactly the same in scope and content, they concur in important parameters like legibility.
EN12966 certification is mandatory in European Union - no LED displays can be installed on the road if not certified.
NEMA TS-4 is not mandatory in USA.
CE label is mandatory for Dynamic Message Signs sold and installed in the European market. The CE label can be affixed to a VMS only if the equipment has passed the rigorous EN12966 compliance testing.
CE certificate is verifying that all mandatory tests have been passed, and that specific quality classes have been approved. The tested class combinations are acknowledged in the CE certificate.
EN12966 test reports are the mandatory part of the accompanying CE-Certification documentation.
NTCIP is a group of communications standards for transmitting data and messages between microcomputer controlled devices used in Intelligent Transportation Systems (ITS).
A set of NTCIP standards define all relevant aspects of Variable Message Signs (Dynamic Message Signs) communication. Including, but not limited to:
NTCIP 1101 – Simple Transportation Management Framework:
- NTCIP 1102 – OER Base Protocol
- NTCIP 1103 – Transportation Management Protocol
- NTCIP 1201 – Global Object Definitions
- NTCIP 1203 – Object Definitions for DMSs
- NTCIP 2101 – PMPP Using RS-232 Subnetwork Profile
- NTCIP 2102 – PMPP Using FSK Subnetwork Profile
- NTCIP 2103 – Point-to-Point Protocol over RS-232 Sub-Network Profile
- NTCIP 2104 – Ethernet Sub-Network Profile
- NTCIP 2201 – Transportation Transport Profile
- NTCIP 2202 – TCP/IP and UDP/IP Transport Profile
- NTCIP 2301 – STMF Application Profile
DMS are electronic LED sign with full matrix or line matrix of pixels, in full color (Red/Green/Blue - RGB) or monochromatic (amber or white).
Matrix is a collection of equally distanced pixels arranged into a fixed number of rows and columns. Full matrix Dynamic Message Sign has the entire visible area covered with matrix of pixels. Full matrix DMS’s are suitable for displaying texts and images, and combination of both. Line matrix Dynamic Message Sign has multiple matrices of pixels so called “line matrices”. Each line matrix is intended to display one line of text. Line matrix Dynamic Message Signs are suitable for displaying text messages only.
VMSs are used on highways, roads and in tunnels to display traffic-related messages to drivers. VMS messages contain multiple lines of text and/or images, ranging from speed limit, to advisory messages.
The term Variable Message Signs is interchangeable with Dynamic Message Sign (DMS) or Changeable Message Sign (CMS).
There are several different versions of DMS:
- Lane Control/Use Signs (LCS/LUS) - usually smaller in size, used to navigate the drivers in each lane,
- Side-mounted Dynamic Message Signs (SDMS) - usually bigger than LCS, used for informing drivers through text message or images
- Walk-in VMS - large signs with line matrices or full matrix, amber or full color, used to transfer information through text or images
LCS are usually smaller in size, mounted above each traffic lane. They usually display traffic symbols used for managing lanes such as green straight arrows for open lane, yellow rotated arrow for drivers to merge out of the lane they are in, red cross for closed lane, and speed limits. Other graphics or text is possible.
SDMS are usually mounted on the side of the highway. They usually display text messages or graphics (i.e. speed limit).
Walk-in DMS is a large DMS with a large, walk-in housing. Enables maintenance personnel to enter inside the housing and perform maintenance activities in secured environment.
It has the same display capabilities as regular VMS/DMS/CMS.
The working environment for DMS is relatively harsh: temperature fluctuations, various types of precipitation (rain, snow, sleet, etc.), high CO2 levels, extensive heat, dust, etc. The equipment is expected to last in this exposed, corrosive environment for a minimum of 10 years. It is essential that all materials and manufacturing processes take this into account.
VMS performance parameters are defined by NEMA TS-4 and EN12966 and are grouped in classes. The parameters are:
• Legibility parameters
- Luminance/Contrast ratio
- Viewing angle / Beam width
- Uniformity of visual performance
- Visible flicker
• Environmental parameters
- Resistances against corrosion,
- Enclosure protection (resistance to water and dust penetration)
• Structural parameters
- Resistance against impacts,
- Resistance against vibration,
- Wind loads
- Dynamic loads
- Temporary deflection by bending
• Electrical parameters
- Electrical performance,
- Electromagnetic compatibility
According to NEMA TS-4, Legibility is the ability to discern the content of a display, while Visibility is the ability to recognize that a display exists.
The elements that influence legibility of a sign are:
• contrast ratio of the display,
• luminance level of the display,
• viewing angle of the display,
• color of the display,
• and uniformity.
Legibility distance is proportionally related to character size.
As the main purpose of a Dynamic Message Signs is delivering visual message to the drivers and travelers, the major Dynamic Message Signs requirement is good legibility and visibility throughout the required viewing range. This performance is defined through the main optical parameters – luminance, luminance ratio (contrast ratio) and viewing angle.
Legibility of Dynamic Message Signs is the most critical during daytime operation. When sun glare directly strikes the sign face, it severely reduces the brightness contrast between the sign display and the sign background, resulting in inability to decipher what is displayed on the sign.
The key parameter for assuring satisfying legibility in such cases is Contrast Ratio.
EXPLANATION: Contrast Ratio is the degree of pixel illumination over the degree of ambient illumination (daylight)
A good Contrast Ratio is achieved by a sign front face, able to absorb ambient light, and not to reflect it. This is achieved with a proper front face paint (deep black matt paint with ultra-low reflection ratio) and special pixel design. Pixels should absorb the ambient light, not to reflect it – this is achieved by placing specially crafted lenses in front of LED.
Poor contrast ratio
Good contrast ratio
IMPORTANCE: Contrast ratio should always be considered in conjunction with Luminance. Contrast ratio assures that LED Dynamic Message Signs display with sufficient Luminance (as per EN12966 or NEMA TS-4) features satisfactory legibility.
Without proper contrast ratio, even with sufficient luminance, LED display will be poorly legible.
On the other hand, Dynamic Message Signs with proper contrast ratio will be legible to the travelers.
USA-Europe COMPARISON: EN12966 defines 3 luminance ratio classes. NEMA TS-4 defines one luminance class which is equal to R2 European class.
Conclusion: EN12966 gives an option to require even more stringent contrast ratios than NEMA TS-4.
EXPLANATION: Luminance is the level of light that human eye perceives. Different colors require different luminance to be equally perceived by humans. Because of this, Luminance is defined for each basic color separately.
IMPORTANCE: Luminance should always be considered in conjunction with contrast ratio. Luminance assures that Dynamic Message Signs with sufficient contrast ratio (as per EN12966 or NEMA TS-4) features satisfactory legibility.
USA-Europe COMPARISON: EN12966 defines 3 luminance classes. NEMA TS-4 defines one luminance class which is equal to the strongest European L3 class (class for highway applications).
Conclusion: NEMA TS-4 and EN12966 require the same luminance performance.
Contrast Ratio (R) and Luminance (L) determine legibility of Dynamic Message Signs.
In order to achieve satisfactory legibility, satisfactory contrast ratio and luminance have to be achieved. If any of these parameters does not achieve required class, the DMS will not be sufficiently legible.
To achieve legibility, either the sign luminance has to be increased (sending more power through the sign to light up the LEDs brighter), or the sign's front face reflection has to be decreased.
As demonstrated in the previous paragraph, contrast ratio is achieved either by increasing sign’s luminance or decreasing front face reflection.
Decreasing front face reflection is more advanced as it results in the following:
- Lower energy consumption because the sign:
- Uses less luminance to achieve desired legibility
- Dissipates less heat due to lower LED currents (less luminance requires less current), meaning that it requires less energy for cooling
- Increased sign durability because it operates with lower LED current (less luminance requires less current), which consequently results in less LED wear and tear and prolongs LED life cycle
Beam Width (viewing angle, cone of vision) is the third parameter that determines legibility of the sign.
Beam Width is the visual region in which Dynamic Message Sign is still legible to the driver.
It is a set of three viewing angles from the center (horizontal left, horizontal right, vertical down), at which the luminance is at least 50% of the minimum required on-axis (L3, 0º horizontal, 0º vertical) luminance. No point in the viewing angle shall be less than 50% of the minimum luminance of the center point (0º horizontal, 0º vertical).
Both NEMA TS-4 and EN12966 define the same values and conditions for viewing angles. The only exception is B5 angle which does not exist in NEMA TS-4.
On most of today’s arterial and similar multi-lane roads the satisfactory cone of vision is class B6, or NEMA class.
The following is a simulation of B6 beam width on the road:
Over specifying cone of vision (for example B7/class f instead of B6/class e) can quadruple Dynamic Message Signs energy consumption without proper need.
Both NEMA and EN12966 define color coordinates of all primary colors (white, amber, red, green, blue) to make it compliant with traffic regulations. Today, most of LED manufacturers deliver LED’s which fulfill those requirements.
Light pollution is excessive, misdirected, or obtrusive artificial light. Over-illumination, as one form of light pollution, occurs in traffic. Over-illumination means the excessive use of light. It happens mostly due to the incorrect choice and design of LED fixtures in traffic signs. This is the result of not directing light only to the areas needed, but letting light dissipate and, thus, not providing optimal light pattern.
Light pollution by Dynamic Message Signs can be caused by:
• too high intensity
• non-optimal light pattern
• too wide B (beam width)
Consequences of light pollution are numerous, but the most critical are waste of energy and disruption of eco-system.
Traditional Variable Message Signs waste a large portion of light projected toward the road because they have symmetrically shaped beam of light.
Well-designed optical system shapes the light in such a way that it's projected exactly to the road with no light is wasted.
Beam width (cone of vision):
The satisfying cone of vision on most highways, arterial and similar multi-lane roads, is defined by class B6 or NEMA class e. It assures legibility of the sign mounted over the lanes and on shoulders.
Luminance and Luminance Ratio:
Taking into consideration variance in drivers' vision, and unfavorable environmental conditions, the safe choice may be to select classes L3 and R3 (see tables above).
Selecting the correct parameters for a given application has a direct impact on VMS energy consumption. Insisting on cone of vision that is too wide adds to the operation costs, wastes energy and creates light pollution.
LED is abbreviation for light-emitting diode. A diode is an electrical component with two terminals which conduct the electricity only in one direction. You can imagine LED as a tiny light bulb that fit easily into an electrical circuit. They are illuminated solely by the movement of electrons in a semiconductor material. In an incandescent bulb the filament is heated to produce light. So, unlike ordinary incandescent bulb, LEDs don't have a filament that will burn out. That makes them highly efficient and durable light source.
Different types of LED’s are used in today’s LED displays. One common thing for all of them is that they emit light in rounded cones. The angle of the cones is different (30 degrees, 60 degrees, 120 degrees).
The most important fact that determines the level of efficiency between different displays is that emitted light is not focused to NEMA/EN12966 bean widths (cone of vision) but to all directions.
An optical lens is a device which transmits and refracts light, converging or diverging the beam. The purpose of the lens is to focus scattered LED light into a shape that corresponds to standard’s beam width. The lens increases efficiency of the LED display because it maximizes use of LED light for intended purpose. It doubles, even triples efficiency of the sign and that way reduces power consumption.
Graphically presented, it turns this:
The lens creates optimal light pattern
It is obvious from the images that most of the LED light is wasted without the lens.
Optical system is composition of LED, LED lens and signs front face (primarily front face paint). Those three elements determine efficiency of LED sign in terms of optical performance and energy consumption.
Pixel or also known as a picture element is a physical point in a raster image, or the smallest addressable element in a display device. It is the smallest controllable element of a picture represented on the screen. By controlling the pixels various characters and pictures can be displayed on the screen. They can consist of a single LED or multiple LEDs.
The working environment for VMS is relatively harsh and equipment that is deemed "fit for purpose" is expected to last in this exposed, corrosive environment for a minimum of 10 years.
A lifetime performance of LED display can be tested through ISO 11997 (Corrosion) and ISO 4892 (accelerated weathering) test procedures. A successful performance in these tests would prove that a LED display is designed for such exposed and corrosive environment for a minimum of 10 years lifetime.
Resistance to environmental impacts of water and dust would be secured if the display would be hermetically sealed. It would assure stable environment for LEDs and driving electronics through entire display’s lifetime.
Structural performance is proved through calculations based on either US (AASHTO - American Association of State Highway and Transportation Officials) or European (EN12899) standards. The standards take into account geographic location of the project and provide reference performance values that have to be proven with calculation.
Simulations and calculations made according to AASHTO and EN12899 are done for construction elements of a sign as well as for the entire sign.
Both standards proscribe conditions under which simulations and calculations of structural elements or the whole sign will be calculated or simulated.
Simulation (calculation) is done for wind gusts and static loads.
Wind gusts are the critical loads for the sign because sign is a big surface exposed to wind strikes of different strength depending on geographic location. Both standards proscribe calculations that need to be done and factors that have to be considered.
Static loads are other loads that signs have to withhold. Those are snow loads, sand loads, snow or sand drifts caused by road cleaning or storms, etc.
Reliability and durability of Dynamic Message Sign can be assured by:
a) choice of verified components from verified suppliers (LED’s and LED module components)
b) operating most critical components (LED, power supplies) under low stress
c) Assuring safe environmental conditions for the components of LED module and LEDs
d) Assuring long-term durability of components exposed to environmental influences
e) Using manufacturing procedure that assures proven and certified level of quality
Cost of ownership of variable message signs is made of initial cost (equipment purchase cost) and operation and maintenance cost (power consumption cost, preventive maintenance cost and repair maintenance cost)
The figure graphically illustrates these costs on an example of standard amber matrix sign widely available on the markets.
USDOT ITS costs database states that operation and maintenance (O&M) cost of installed DMS annually reaches up to 5% of its capital cost
During purchasing stage, operation and maintenance costs are often neglected even though they create significant amount of overall cost, during sign’s entire lifetime.
On the other hand, capital cost of equipment is usually considered as the only relevant factor during choice of suppliers, although it creates only smaller part of overall cost at the end.
The statistics say that operation and maintenance cost can reach initial purchase cost of LED display during its lifetime, and even surpass it.
Operation cost is usually related to the cost of electrical energy used to operate displays.
Maintenance cost is composed of regular (preventive) maintenance and repair maintenance costs.
Optimally designed, reliable, easy to maintain, and energy efficient sign may reduce operation and maintenance costs by more than 60% compared to today’s standards.
The choice of right technology is determined by multiple important factors such as Standards compliance, Lifetime cost structure, Energy efficiency, Work safety, O&M complexity, Environmental performance, Structural parameters, Reliability, Weight, Aesthetics, etc.
NextGen technology has been designed to accomplish and improve performance characteristics of traditional VMS and DMS technologies in terms of reliability and efficiency.