The importance of sign retroreflectivity
Safety Discipline Leader
Federal Lands Highway Division
Federal Highway Administration
In the United States, almost one-half of all traffic fatalities occur during the dark hours of evening, night, and early morning. However, only one-fourth of all travel takes place during those same hours—a startling and grave statistic.
There are a number of reasons for this disparity, such as intoxicated and fatigued drivers. It is also known that drivers receive much less guidance information at night than during the day. For example, during daylight hours, drivers have a number of visual cues, such as signs, pavement markings, roadside vegetation, guardrails, and textured shoulders, making driving relatively simple. This bounty of visual cues allows drivers to focus less on their proper position on the roadway.
At night, however, this changes drastically. On a dark and unlit road, nearly all cues disappear except those that are retroreflective (signs and markings). Those few remaining cues become critical to driving, and should they become so worn that they are no longer easily visible, the chance to miss the information becomes greater, resulting in a greater chance of driver error, and subsequently, a potential crash. Urbanized areas present a different environment related to retroreflectivity of signs. Other light sources, such as advertising signs and business lighting, create a situation where the traffic signs are competing for the driver's attention. In many of these situations, the retroreflectivity of traffic signs must be even greater to stand out among the competing lights.
Retroreflectivity is the property of a material that returns light to the source. In the case of roadways at night, the retroreflective materials may be traffic signs and pavement markings and the source is usually the headlights of a vehicle. Because a driver's eyes are close to a vehicle's headlights, some of the light returned from retroreflective materials reaches the driver's eyes. The amount of light from an object reaching the driver's eyes will have a great impact on how bright that object appears to the driver. Therefore, retroreflective materials that are efficient in returning light to a driver's eyes may appear brighter to the driver than those that are not.
Unfortunately, the retroreflective characteristics of traffic control devices gradually deteriorate over time. As a result, it is important to replace traffic control devices prior to the time when they no longer meet the needs of the nighttime driver. The major question is not whether the devices should be replaced, but when? How do we know when the device no longer meets the needs of the driver?
To address the issue of retroreflective deterioration, the national Manual on Uniform Traffic Control Devices (MUTCD 2003) states, "Regulatory, warning, and guide signs shall be retroreflective or illuminated to show the same shape and similar color by both day and night..." That standard has remained essentially unchanged for 45 years. The MUTCD 2003 also states, "To assure adequate maintenance, a schedule for inspecting (both day and night), cleaning, and replacing signs should be established."
Although retroreflectometers are an excellent tool for evaluating the efficiency of retroreflective materials, they are not the only resource available to judge the nighttime visibility of traffic control devices. Nighttime visual inspections of signs and markings can be one of the best methods/tools. Establishing a process to evaluate your jurisdiction's signs and markings for nighttime visibility, and maintaining those devices appropriately, can be a great service to the public and possibly assist your agency in court cases involving visibility of traffic control devices.
The FHWA is currently developing guidance for public agencies to determine the appropriate level of sign retroreflectivity required by nighttime drivers. In 2004, the FHWA issued a Notice of Proposed Amendment to the MUTCD that would include a standard for maintaining minimum sign retroreflectivity. The comment period has since closed and the FHWA expects to issue a follow-up rule in the near future. Based on the current requirements in the MUTCD, and the expectation that standards will be produced, some agencies have initiated nighttime inspection processes to evaluate the visibility of their traffic control devices. A systematic process to replace worn-out devices can then be implemented to ensure that limited budgets are used efficiently to meet the needs of the nighttime driver.
For additional information, please visit the FHWA retroreflectivity website at www.fhwa.dot.gov/retro.
Greg Schertz can be reached at (720) 963-3764 or firstname.lastname@example.org.