(This article is reproduced, with permission, from the Fall 2000 Issue (Issue 68) of WST2: Washington State Technology Transfer, a technical newsletter of the Highways & Local Programs Service Center- -WSDOT and the Local Technical Assistance Program.)

By: Patrick Hasson, Safety Engineer, FHWA Midwestern Resource Center, Ernie Huckaby, Rudy Umbs

Background

The risk of dying in a crash at night is nearly three times the risk of dying in daylight hours. In 1998, about 27,000 people died in nighttime traffic crashes in the United States, even though only about 25 percent of travel is at night. One of the reasons that nighttime driving risk is so much higher is because in the daylight the road is filled with more visual cues that help to guide drivers and keep them on the road. Though a single causal factor cannot be assigned to nighttime crashes, it is clear that an individual driver's night vision characteristics and a lack of adequate visual guidance information are significant factors. In either case, if cues that are essential for safe driving are inadequate at night, the potential for a driving or judgmental error to result in a serious crash are considerably increased. The situation is only made worse when other factors--i.e. fatigue, intoxication, inclement weather, higher speeds of travel on some roadways, etc.--combine with inadequate traffic control devices to make nighttime driving less safe.

The issue of visibility on rural roads--i.e. the greatest distance under given weather conditions to which it is possible to see--is of special interest when one considers that there are many reasons that the risk of dying on a rural road is higher, including: differences in operating speeds; road geometry; functionality; and other factors. It is these factors that create the situation in which nearly 80 percent of all fatal rural road crashes are either run-off--the-road, intersection or head-on collisions. The possibility for any of these crash types is heightened at night, and for each, visibility is a key factor.

The Case for Retroreflectivity Visibility can be improved through a variety of means such as retroreflectivity, roadway lighting, and automobile headlights. Though retroreflectivity in the form of pavement markings and sign sheeting does not resolve all of the problems, especially in wet or other adverse weather conditions, its relative low cost and versatility makes it a preferred alternative for most applications today.

At night, with many of the visual cues missing, the driver relies on the added retroreflective elements of signs and markings such as edge lines and post-mounted delineators for curve preview and center lines for guidance in the curve. It is very possible that these will be the major visible elements to a driver on a road at night. The retroreflectivity of signs and 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 signs and markings is a critical ingredient in creating a much safer road environment.

As an example of the safety value added by retroreflectivity, consider a sharp curve on a rural road. In the daytime, there are many visual cues such as a line of trees or a guardrail that can alert the driver to the sharpness of the turn in time for a driver to alter [his/her] speed accordingly. Road engineers also use pavement markings to reinforce these cues. A retroreflective edge line in this situation will provide the driver with a long-distance preview of the curve while the center line will provide other useful guidance through the turn.

Retroreflective materials are subject to deterioration brought on by the natural elements and the ability for a sign, delineator or pavement marking to continue to provide quality information or guidance to a driver decreases over time. If some minimum retroreflectivity is not maintained, the sign, delineator or marking will not accomplish the job it was intended to perform. While the Manual on Uniform Traffic Control Devices (MUTCD) has required since 1954 that signs and pavement markings shall be reflectorized or illuminated, the MUTCD contains no minimum in-service retroreflective requirements for signs or markings. (Note: ASTM D4956-89 Standard Specifications for retroreflective sheeting purchase specification used by the States is not to be confused with in-service minimum levels of retroreflectivity.) This fact coupled with the recognized importance of retroreflectivity to highway safety motivated the U.S. Congress to pass a law in 1993 that required the Federal Highway Administration (FHWA) to establish minimum maintained levels of retroreflectivity for signs and pavement markings. What Does This Mean to You?

The minimum maintained levels of retroreflectivity that are accepted will have many potential impacts. First, it is likely that the guidelines will have the greatest impact on the maintenance of signs on the National Highway System (NHS). Beyond the NHS the impacts are less clear. Certainly, some States may require that all local road agencies adopt the minimum guidelines. However, other states may not go that far. But, the mere existence of minimum guidelines could create a situation in which local agencies are compelled for one reason or another--i.e. liability issues, etc.--to begin applying the minimum guidelines in their regular practice. As well, from strictly a safety point of view, minimum guidelines will provide a valuable tool for road engineers to use on roads that have high traffic volumes or for high hazard locations. For all of these reasons, it is essential that road managers and engineers stay abreast of the development of the guidelines and consider how they will have an impact in their future road programs.

In another vein, the costs associated with implementing minimum maintained levels of retroreflectivity for signs have been examined. Based on the average condition of road signs in 1994, the FHWA estimated in 1998 that 5 percent of signs under the State jurisdictions and about 8 percent of those under local jurisdictions would not meet the proposed minimum maintained levels of retroreflectivity and would therefore need to be replaced. This translated to a cost of about $32 million for the State agencies combined, and about $144 million for the local agencies combined. These are costs associated with replacing all signs at one time. The report concluded, however, that on a practical level, sign replacement rates would probably not be accelerated above current levels and many agencies would not likely feel any impact of implementing the minimum maintained levels of retroreflectivity. The report went further to state that the development of a sign inventory program that includes retroreflectivity measurements would lead to making investments in a planned manner that, in the long run, are likely to reduce the overall maintenance and replacement rates of traffic signs in the future.

What's Being Done?

The FHWA was performing retroreflectivity research to improve nighttime driving safety since the early 1980's. Some of the areas covered in this research included studies on the service life of sign[s], sign and pavement marking management systems, and traffic sign and pavement marking retroreflectometers. Following the Congressional requirement in 1993, the FHWA also completed research on what levels of retroreflectivity are needed to safely guide drivers at night, and analyzed the economic impacts to the public if minimum retroreflectivity values are established. The overall goal of all of these studies was to obtain information necessary to establish minimum maintained levels of retroreflectivity and to develop management programs and measurement devices which will be needed by the States and others to maintain traffic control devices at an adequate level. Thirty-two States were actively involved with the FHWA in this research.

Recognizing that there are already many different models of hand-held retroreflectivity measuring devices available today, the FHWA began to develop mobile units capable of measuring the retroreflectivity of signs and markings while driving at highway speeds. A van capable of measuring the retroreflective qualities of pavement markings was introduced and demonstrated a few years ago. Private industry is now manufacturing and selling these units as well as providing contractual support for their operation and maintenance. Similar vans for measuring the retroreflective qualities of signs are now in development by the FHWA and will be available for demonstration purposes in 2000. In relation to the FHWA efforts, an AASHTO task force is actively reviewing completed research on this subject and intends to make a recommendation to FHWA on minimum maintained levels of retroreflectivity. The FHWA will consider this recommendation and other information before it issues a notice of proposed rulemaking (NPRM). After analyzing the comments to the NPRM, a Final Rule could be issued in 2001 for signs and 2002 for pavement markings. The rules will address plans to implement minimum maintained levels of retroreflectivity for each. The Federal Register notice invites the widest possible review and comment by the public.

Other Steps to Improve Visibility

Recognizing that retroreflective devices have their limitations in some circumstances, the FHWA is also examining a host of other possibilities for making roads safer at night.

For instance, there has been preliminary research on the use of ultraviolet headlights in automobiles. These headlights allow drivers to use their low beam level yet see fluorescent traffic control devices as if they had their high beams on. This allows drivers to see better at night but does not create the glare problems associated with standard headlights. The FHWA, in cooperation with Volvo and others, will be performing an extensive demonstration project with UV headlights and fluorescent signs and pavement markers next year.

Another area of interest is phosphorescent materials that could be incorporated into traffic control devices so that they will glow at night. This is an area that is developing rapidly and leading to new materials that glow for longer and longer periods. It is likely that in the next 5 years or less, there will be materials that can glow all night or for days at a time. These products may help to overcome some of the limitations of retroreflective materials. Finally, there have been tremendous advances in the development of LED lights for use in augmenting pavement markings, and several products are available today. Generally, the LED lights are small solar- powered markers that are installed in the pavement. Though most of the current applications of these lights have been outside of the United States, it is recognized that there may be value in using these devices in some hazardous locations. Currently, there is a proposal to install LED lights in a heavy fog area in California.

Conclusion