WHAT IS RETROREFLECTIVITY?
Retroreflectivity, in scientific terms, describes the ability of a surface to return light back to its source. Diffuse reflection occurs on rough surfaces such as a road or clothing, while specular reflection only occurs when light strikes a smooth surface such as a mirror or calm body of water. See Figure 1. Retroreflectivity is one technology that is being used to enhance the readability and perception of information displayed in low-light and nighttime conditions. Some common applications of retroreflectivity can be seen on bicycles, running shoes, clothing, and traffic control devices (traffic signs and pavement markings).
RETROREFLECTIVITY IN THE TRANSPORTATION INDUSTRY
For many years vehicle operators and the transportation industry have realized the importance of legible traffic signs and quality pavement markings for the purpose of highway safety and economical traffic flow, especially in low-light conditions. Light from a vehicle's headlights striking retroreflective traffic control devices, bounces back to the driver's eyes 
allowing them to more easily see the road. Therefore, most traffic control devices are covered with retroreflective materials. These include transparent decals with embedded microprismatic reflectors for traffic signs and paint that has small sunken glass beads in the pavement markings. See Figure 2. Images of the various sheeting patterns can be seen on the Federal Highway Administration (FHWA) website (L1). Unfortunately, these retroreflective materials degrade over time and become less effective. The FHWA realized this problem and has recently updated the minimum levels recommended for a traffic sign's retroreflectivity. (L2) provides a complete description.
One of the most important factors in determining the retroreflectivity of a sign is knowing the angle at which the driver views the sign (observation angle). The observation angle is defined as the angle created between the driver's line of sight to the reflective material (traffic control device) and the line from the reflective material to the light source (headlights).1 See Figure 3. The entrance angle is of secondary importance. This is defined as the angular displacement of the incident 
light relative to the normal vector from the traffic control device. See Figure 4. As a vehicle gets closer to a traffic sign, the observation and entrance angles increase. Conversely, when a vehicle is further away from a traffic sign, observation and entrance angles decrease. As the entrance angles and observation angles increase, the retroreflectivity of traffic signs 
decrease accordingly. See Figure 5. For a more in-depth look into the physics of retroreflectivity, visit (L3) and (L4). Retroreflectivity has become an increasingly important topic of conversation in the transportation industry for several of the following reasons.
MINIMUM RETROREFLECTIVITY LEVELS
The FHWA issued The Manual on Uniform Traffic Control Devices (MUTCD) in order to establish “the guiding principles for the use of traffic control devices.” 2 The MUTCD “is intended to promote national uniformity of traffic control devices.” 2 Before the MUTCD was modified in 1993, it stated that traffic signs had to be reflectorized or illuminated, but it did not provide any guidelines. The standard minimum levels were revised in 1998 and further updated in 2003. The most recent MUTCD includes research recommendations for minimum maintained levels of sign retroreflectivity and proposes minimum allowable values for the retroreflectivity of traffic control devices. Additionally, the most recent MUTCD addresses sign visibility through factors such as design, placement, maintenance, operation, and uniformity. This will ensure that signs are designed, placed, and maintained in a way that will allow traffic control devices to be seen by drivers traveling at a reasonable speed down the road. Ultimately, the vehicle operator will be able to safely and efficiently navigate the roadway. The FHWA estimates that up to half of the 58 million traffic signs in the U.S. are beyond their useful lifespan (estimated at 10 years) from a reflectivity standpoint.11 This is one major reason that approximately 42,000 people have been killed on the nation's highways each of the past 8 years.3 To view the most recent version of the MUTCD, follow (L5).
WHY IS RETROREFLECTIVITY IMPORTANT?
Retroreflectivity is used to allow drivers to more easily see vital traffic control devices in nighttime and low-light conditions. See Figure 6. Traffic control devices encourage orderly movement of drivers on streets and highways and ultimately promote highway safety and efficiency.
According to the Road Management and Engineering 
Journal, “The retroreflectivity of [traffic] signs and [pavement] markings can serve to provide positive visual guidance that helps drivers keep their cars in their lanes or on the road. They also offer the possibility to share critical warning, timely location and other information to drivers. The retroreflectivity of [traffic] signs and [pavement] markings is a critical ingredient in creating a much safer road environment.” 4 Making retroreflectivity even more crucial is the fact that 77 percent of America's highways are two-lane roads, which means that when a vehicle does vier out of its lane, the possibility of serious injury is increased.
Nighttime/Low-light Driving Conditions
During daylight hours, the driver of a vehicle has many visual cues to help them stay on the road. Some of these important cues that are visible in daylight conditions but are difficult to reference in dark driving conditions are vegetation (trees, bushes, shrubs) and guardrails. “While only 25 percent of travel occurs at night, more than half of traffic fatalities occur during nighttime hours. Inadequate and poorly maintained [traffic] signs are cited often as a contributing factor to crashes and fatalities.” 3
Additional information from the Traffic Safety Facts for 2000, published by the National Highway Traffic Safety Administration (NHTSA), shows that the fatal crash rate for nighttime driving is three to four times higher than daytime driving.5 Even when alcohol-related crashes and fatalities are taken out of the equation, the statistics still show that the nighttime fatal crash rate is nearly twice as high as daytime fatalities. Not surprisingly, the FHWA reports that there are a higher number of injuries and more property damage resulting from night crashes. It is clear that retroreflectivity plays a vital role in the safety of nighttime driving conditions. An additional area where improving the often times substandard retroreflective levels could greatly increase safety, is on America's rural roadways.
Safety of Rural Roads
The main area where the retroreflectivity of traffic control devices fail is on rural roads. The Road Management and Engineering Journal states that vehicle operators traveling on rural roads do not have the proper amount of time or visual cues to allow them to safely and effectively navigate the roadways. Nearly 80% of all fatal rural road crashes are run-off-the-road, intersection collisions or head on collisions. 4 These types of crashes are usually the result of poor traffic control device visibility.
A similar article, written by the FHWA's Public Roads Magazine, shows that, “... it is apparent that at the international level as much as 75 percent of all crashes on rural roads fall into three categories: single-vehicle crashes (especially running off the road), head-on collisions and collisions at intersections. This number is completely consistent with figures for the United States, which indicate that these three crash types account for as much as 79 percent of rural crashes.” 6
In all three categories, poor road infrastructure is consistently listed as a possible cause for the accidents. The FHWA realizes this problem and suggests upgrading edge lines and centerlines, and installing reflective pavement markers and roadside delineation as necessary improvements for making rural roads safer. In addition to the important role that proper retroreflectivity levels play on rural roads, they also have shown to be of significant help to an aging U.S. Population.
Vision of Older Drivers
According to the FHWA, “Maintaining minimum levels of sign retroreflectivity on the nation's roads is becoming increasingly important as the driving population ages. Older drivers have diminished visual capabilities that are most apparent under dark conditions. Currently, 26.2 million drivers are 65 or older and by 2010 an estimated 33.7 million drivers will be 65 or older. Traffic signs that are easier to see and read can help this growing segment of drivers as well as provide benefits to all drivers (not just the elderly) at night. The FHWA believes that the benefits associated with this rule-making will far outweight the costs.” 7
In support, FHWA goes on to say that, “Studies show that starting at age 20, the amount of light needed to see doubles every 13 years.” This means that a 59 year-old driver needs 8 times more light than a 20 year-old to see the same object while driving. See Figure 7. “By 2020, about one-fifth of the U.S. population will be 65 years of age or older. In general, older individuals have declining vision and slower reaction times. Signs that are easier to see and read can help older drivers retain their freedom of mobility, remain independent, and reduce their likelihood of being involved in traffic crashes.” 8
Economic Impacts
The final reason that retroreflectivity is important on our roadways is that substantial economic costs are inherent with poor retroreflectivity. An article by Public Roads Magazine entitled Rural Road Safety: A Global Challenge attests that, “[the] socio-economic costs of rural road fatalities are approximately $120 billion [worldwide] per year. It is likely that personal injuries in rural road crashes are equally staggering in their number and cost.” 6 On top of that, the World Bank Group estimates that, “Road 
crashes cost approximately 1 to 3 percent of a country's annual Gross National Product (GNP). These are resources that no country can afford to lose, especially those with developing economies.” 9
With the implementation of the updated minimum retroreflectivity guidelines proposed by the MUTCD and innovative methods of monitoring traffic control devices such as Facet Technology Corporation's RetroView™ and RetroCurve®
, traffic safety will be greatly improved. See (L4) or Facet Technology Corporation's Signs and Pavement website. Nighttime navigation can be aided by better visibility of traffic control devices. This will help the navigation of not only the vision-impaired elderly but those who frequently travel on the very hazardous rural roads as well. See Figure 8.
WHY DO WE NEED TO MONITOR RETROREFLECTIVITY?
Unfortunately, the effectiveness of the retroreflective materials tends to deteriorate over time; they lose some of their color, legibility, and visibility. The deterioration is a result of several environmental forces including corrosion caused by man-made contaminants (exhaust emissions and industrial pollution) and erosion caused by natural elements (sun exposure, dirt, ice and wind). See Figure 9. Other factors that contribute to a sign's deterioration are the sign's placement with respect to the roadway as well as the direction it is facing.10
Although deterioration plays a role in the visibility of traffic control devices during daylight driving, the effects of weathered traffic control devices are more noticeable during nighttime or low-light driving conditions. See Figure 10. Currently, there are several assessment and management methods that are being used by agencies to collect and monitor the retroreflectivity of traffic control devices. When compared to Facet Technology Corporation's RetroView™ , current monitoring methods fall short in several vital respects and some of them even seem primitive. See (L4) or Facet Technology Corporation's Signs and Pavement website.
