Road Safety in a new light

The percentage of retroreflected light depends on the general optical properties of the sheeting. If the sheeting returns more amount of light to smaller observation angles, it is at the expense of the larger observation angles. If more is returned to larger observation angles, less remains for smaller observation angles. For instance, if a sheeting is 30% efficient, and if this sheeting returns 20% of light to 0.50 or smaller observation angles, the remaining 10% is returned to observation angles larger than 0.50. If the percentage of incident light returned to 0.50 and above is increased to 15%, the portion that is returned to lower observation angles will have to be 15%. The total will be 30% at all times.

Truncated Cube Vs Full Cube Technology

The glass bead sheeting could provide a theoretical optical efficiency of about 28%. As a counter to this low efficiency sheeting, microprismatic sheeting was developed which promised more than double the optical efficiency of about 65%. This was achieved by Truncated Cube Technology. But 3M’s ingenuity did not stop at that. Following the mantra of innovating newer and better products, Full Cube Technology was born out of 3M’s labs in 2005. This technique removes the non-reflecting portion of the truncated cube corner optical elements and makes it most optically efficient, with optically no “dead” corners. This results in the brighter reflective sheeting with optimum performance. Theoretical Optical Efficiency for this sheeting is 100%. When this technology is incorporated into 3M™ Diamond Grade™ (DG3) Reflective sheeting, it nearly doubles the average brightness of road signs.

The only way to ensure adequate return of light to the driver at all relevant angles without a need for additional sign illumination is to increase the overall light-return efficiency. Increasing light-return efficiency in significant scale requires a notable change in the optical design of the sheeting. This is why 3M has designed its new Diamond Grade™ (DG3) sheeting to address as many driving conditions as possible.

Sign Sheeting Comparison

During daytime, drivers have many cues for safe navigation such as guardrail, vegetation, snow banks in the winter, textured shoulders and Traffic Control Devices (TCDs). However at night, only retroreflective TCDs remain. With so few cues remaining at night, the design of TCDs becomes critical.

When first considering a new retroreflective sheeting design, 3M wanted road users (drivers) to help shape the performance of the sheeting by asking the question: “If sign sheeting could reflect additional usable light, where should it be directed?” Full Cube Technology provides additional efficiency in returning light back to the driver, and 3M wanted to know if that light should be used for high “target value” performance (long distance visibility) or for better performance on approaches closer to the sign. Research helped to shape the “response curve” of this new sheeting.

Most drivers see sign brightness in terms of luminance, and luminance can be measured. Luminance is defined as the intensity of a surface in a given direction per unit of the projected area. In other words, it is the physical measure of brightness.

A meaningful discussion of sign sheeting performance requires understanding of luminance curves. Luminance curves act as a comparative tool to show the differences in brightness of the installed signs and help determine if a sign meets the drivers’ needs. A luminance curve provides information in terms of what a driver sees (i.e. level of sign brightness during and through the approach). A complete and informative luminance curve shows the distance involved and the brightness that appears in terms of candelas per meter square.

To meet the needs of drivers at night, there is a general agreement that traffic signs need to at least meet a minimum level of brightness but then, what is the optimum level of performance to meet the drivers’ needs? Needs of a driver are a function of his / her age. Older drivers need more light to see and read a sign. Also, the reaction time is more in case of older drivers. A research on human factors provides an indication of how bright signs need to be. The lighting engineers use these values as a reference.

The standard specification for retroreflective sheeting to be used for traffic control is provided by ASTM International under the designation D 4956. This standard, which was recently revised in 2009, defines the various types and classes of retroreflective sheeting which find their application on traffic control signs, delineators, barricades and other devices.

The earliest reflective sheeting incorporated the use of glass beads for retroreflectivity. Hence, the name – Glass Bead Technology. ASTM D4956 identifies these sheeting as Type I, II & III. The further types employ the Microprism Technology. In microprismatic sheeting, cube-corner optics reflect light back to the driver. For the incoming headlight beam to be reflected back to the driver, the light beam should be reflected from all three faces of the cube corner.

Dr Paul Carlson at Texas Transportation Institute (TTI) studied both – minimum luminance requirements and desired luminance requirements and presented his findings in, “A Proposal for Performance-based Sign Sheeting Criteria”. He found that drivers prefer much higher levels of luminance to the bare minimum. Dr Carlson drew a comparison of the available sheeting products with the equation, specifically ASTM Types VIII, IX and XI. From this chart, it was evident that Type XI comes closest to providing the desired luminance profile. The luminance curve of 3M’s DG3 coincides with the Type XI curve.

DG3 has been a widely accepted and used product – both nationally and internationally. Some of the success stories include installation of DG3 signs by the Departments of Transport (DOT) in the states of Colorado and Wisconsin in the USA as early as 2007; as also DG3 installations in Norway, Sweden, Taiwan, China and Japan. In India, many prestigious projects have been associated with this grade of sheeting viz. PWD (Delhi) and NDMC (Delhi) signing projects, Noida Development Authority, Pune Municipal Corporation, MMRDA (Mumbai), BandraWorli Sea Link, Bangalore city signing, Rajasthan Infrastructure Development Corporation (RIDCOR), and the international airports of Delhi and Hyderabad. Such a wide-scale acceptance testifies the fact that DG3 sign is far superior to the other sheeting types for conspicuity, legibility and information transfer.

Harish Banwari

General Manager

Traffic Safety Systems division

3M India Limited