Highway and Street Sign Reflectivity: A Review of the History, Science and Standard
Street signs, whether they be in your local neighborhood or on the interstate, are important components of vehicular and pedestrian safety. The design, application, and technology behind these signs have changed significantly through the years. The first record of signs being used to guide travelers was on the Appian Way in the ancient Roman empire, where intersections indicated the direction of, and distance to Rome. In the late 17th century King Peter II of Portugal demanded that signs be erected in the narrow streets of Lisbon to indicate which types of traffic had the right-away. And cinema lovers can attest to the many Hollywood movies set centuries ago where a single sign post contained multiple arrow shape signs with the name and distance of a variety of cities.
As time marched on, road signs became more prevalent around the globe. But before Thomas Edison patented the first incandescent light bulb in 1879, and the eventual introduction of headlights made vehicles safer to travel in the dark, the road sign had no need to be reflective. Twenty years after Edison’s patent, the newly formed Automobile Association of America (AAA) began placing signs to prevent drivers from getting lost. Eventually, these directional signs would start popping up around the nation, and it would be more than a decade later when the first stop sign was installed at an intersection in Detroit.
As America moved into the roaring twenties, vehicle traffic grew exponentially and the need for standardization of road signs became a national focus. Through the 1920s and 1930s, the various design of signs for warnings, directions, and traffic control purposes began to take shape. It was during this period that the original reflectivity guidelines were published. In 1935 the first iteration of the Manual on Uniform Traffic Control Devices (MUTCD) was formed. This set of guidelines continued to evolve and it remains the national standard today, nearly a century later.
Developments in Retroreflectivity
The earliest ingenuity related to street and highway sign reflectivity is credited to Harry Heltzer, a 3M employee who was charged with making the white and yellow line paint on the roads more visible for Minnesota drivers. Heltzer first used small glass beads in the paint to improve reflectiveness. Over the next 50 years the technology of road and sign reflectivity continued to improve until in 1989, glass was replaced with prismatic reflector particles, dramatically improving the brightness of the reflection. Also, in the 1980’s the Center for Auto Safety (CAS) requested that the Federal Highway Administration (FHWA) develop standards related to the minimum reflective ability of road signs. These new requirements were implemented to all publicly accessible roads in 1992.
The science of street and road signs now includes the term “retroreflectivity,” which simply stated means the ability of a sign to return light back to the source. This return of the light is what makes the sign visible for passing drivers. The science of retroreflectivity enables the reflective material to be designed so that the returned light is directionally controlled, enabling the largest possible amount of the light to be reflected in the opposite direction of the light source.
The light returns to the source in a conical shape that is referred to as the cone of reflectivity. The brightness of the returned light is not only impacted by the quality of the reflective material in the sign, but also by the position of the driver in the cone. As the driver draws nearer to the sign, or the center of the cone, the brighter the returned light appears. The aspect angle of the driver in the cone also impacts the brightness measurement. If the passing vehicle is on the edge of the cone, rather than in the center, the brightness is diminished. This principle helps determine the proper placement of road signs in relation to the roadway. Signs that are too far off the road will provide less reflectivity than those closer to the roadway. The remarkable advancements in reflective technology and its application to street and highway safety have made traveling safer. Yet, according to the Federal Highway Administration, the nighttime highway travel fatality rate is three times greater than that of daytime travel. While no single factor can be blamed for this disparity, clearly visible street signs with high reflectivity are an important component of nighttime roadway safety.
Standards and Regulations for Street and Roadway Sign Reflectivity
Below is a list night visibility terms and concepts that you should be familiar with before you invest in new street signs for your neighborhood or city. You can also find value information and MUTCD updates concerning sign reflectivity at the U.S. Department of Transportation Federal Highway Administration website.
If you are looking for new street signs for a newly build neighborhood or park, or of you are in need of replacement signs due to environmental degradation or vandalization, click here to learn more about the variety of sign options from Brandon Industries.
Sign reflectivity terms
- Color – As the MUTCD guidelines evolved, the necessity for color standards were implemented. Signs with black letters on a white background are used for traffic control, e.g. speed and direction. Yellow is commonly used to warn drivers of a potentially unsafe road condition and red is used to indicate the driver must stop or turn around because they are driving in the wrong direction. Brown backgrounds often indicate an attraction or place of interest. Green with white letters indicates the designation of the highway or street.
- Luminance – this measurement indicates how much light is received by the driver from the reflection of the sign.
- Headlamp aiming procedure – an inspection method whereby a vehicle using low-beam headlamps passes by a road sign at night in the travel lane and the reflectivity of the sign is measured.
- Contrast – while related to luminance, contrast measures how clearly defined the content of the sign is against the background sheeting. In this way, the contrast is more important than the overall luminance of the sign.
- Engineer grade reflective sheeting – typically only used on street and road signs that are not considered critical. These may include way-finding signs, public park signs or parking lot signs.
- High Intensity prismatic sheeting – this highly reflective sheeting is known for its rugged coating which protects it from damage, making it ideal for construction area signs, barricades and hazardous environments.
- Diamond grade sheeting – Visible from up to 1,600 feet, signs that use this sheeting make the sign more visible in any light condition. Because it returns up to 60% of the light it receives, it is required in school zone signs as well as work zone signs.
- Conspicuity – the likelihood that a driver will notice the road or street sign. Luminance and position are the primary contributing factors.
- Legibility – closely related to contrast, this refers to the ability of the driver to read or understand the message on the sign. Color combinations between sheeting and letters/symbols can impact the legibility of a sign.