Nighttime Visibility of Traffic Signs: Chapter 2 - Night Retroreflectivity Demonstration

CHAPTER 2 - NIGHT RETROREFLECTIVITY DEMONSTRATION

One of the key activities of the workshop was a nighttime demonstration of sign retroreflectivity. The nighttime demonstrations of sign retroreflectivity were conducted to familiarize the workshop participants with the nighttime visual appearance of signs at various levels of retroreflectivity. This was an important element of the workshops because few of the participants had an understanding of the differences in visual appearance between signs of different retroreflectivity levels. Specific purposes of the nighttime demonstration included:

• Demonstrating various factors that influence nighttime sign visibility,
• Illustrating the difference in appearance of the same sign at different retroreflectivity levels, and
• Helping participants relate the visual appearance of signs at various retroreflectivity levels to the appearance of signs in their own jurisdictions.

By taking part in the demonstration, participants were able to associate the visual appearance of the signs in their respective jurisdictions with the signs used in the nighttime demonstration, some of which were near research recommendations for minimum retroreflectivity levels. This allowed them to make more informed comments about the potential impacts of minimum retroreflectivity levels during the workshop discussion period on the second day.

DEMONSTRATION DESCRIPTION

The demonstration took place on the evening of the first day of the workshop. The demonstration varied from one workshop to another, but essentially consisted of the workshop participants driving through a course with signs at various retroreflectivity levels. As they drove through the course, the participants rated each sign as acceptable or unacceptable.

Site Descriptions

For the convenience of the participants, the nighttime demonstrations were conducted in the general proximity of the workshop locations. As a result, there were unique differences among the four demonstration sites. This presented numerous challenges in setting up and conducting the demonstrations. It also made it difficult to compare the results of the demonstrations from each of the sites. The major attributes of each demonstration location are described below.

• Lakewood, Colorado - The nighttime demonstration took place on the street network of the Denver Federal Center. This facility is similar to what might be expected in a small college/university or business park with numerous buildings and a small road network. It could also be considered as representative of a typical urban environment except for the lower speed limits on the road network. The signs used in the demonstration were installed so that they covered the existing traffic signs. During the demonstration, access to the Federal Center was limited to federal employees, who had to pass through a security checkpoint. As a result, there was virtually no traffic on the demonstration course while the demonstration was underway. There was street lighting throughout the demonstration course, providing some ambient lighting that would be representative of some urban environments. Typical travel speeds through the demonstration course were approximately 25 mph.
• Hudson, Wisconsin - The nighttime demonstration took place on a dead end frontage road located in Minnesota about 10 miles from the workshop location. The signs used in the demonstration were installed on temporary sign supports. While the roadway was open to public travel during the demonstration, there was very little traffic on the frontage road, due to the fact that it was a dead end roadway. There was little ambient light except the headlamps from the adjacent freeway. Typical travel speeds through the demonstration course were 35-40 mph.
• College Station, Texas - The nighttime demonstration took place at the TTI Riverside Campus Retroreflectivity Demonstration Complex. This facility was formerly an Air Force Base that is now a part of Texas A&M University. The demonstration was conducted on several former runways and taxiways of the old airfield. Signs were installed on signposts at the edge of the runways and taxiways. The course was closed to traffic throughout the demonstration, so there were no conflicts with other vehicles. There was no ambient lighting on the demonstration course. Typical travel speeds through the demonstration course were 35-40 mph.
• Hanover, Maryland - The nighttime demonstration took place on the road network of an industrial/business park located adjacent to the workshop site. The signs used in the demonstration were installed on temporary sign supports. The road network was open to public travel during the demonstrations, but traffic volumes were relatively low while the demonstration was underway. There was more traffic at this site than at the other three. There was some roadway lighting as well as some commercial and parking lighting from the properties adjacent to the demonstration course. These lighting sources provided higher levels of ambient lighting than was present at any of the other demonstration locations. Typical travel speeds through the demonstration course were 25 mph.

Demonstration Signs

[GH6] The signs used in the four demonstrations were intended to represent a range of retroreflectivity levels from new to minimum levels and below. All of the signs were new, obtained specifically for the workshop demonstrations, with some being artificially degraded to lower levels of retroreflectivity. Retroreflectivity was degraded by spraying a sign with a clear polyurethane finish. Additional degradation was achieved by using multiple coats of the polyurethane finish. In general, the degradation process worked well. However, there may have been some small level of non-uniformity in the appearance of some signs. The process was not exact and retroreflectivity could be reduced only to general levels and not specific values.

There were two sets of signs used for the demonstrations. The first set was only used for the first demonstration in Lakewood, Colorado. This demonstration included numerous types of signs. However, participants' comments indicated that the retroreflectivity levels of these signs were not sufficiently degraded to represent signs at minimum levels. Participants also commented that there were so many different signs that it made it difficult to compare the retroreflectivity of the signs. Therefore, the researchers created a second set of degraded signs for use in the remaining workshops. Three types of signs were used (Stop, Curve, or Divided Highway Ends text). For each type, there were four individual signs, each with a different level of retroreflectivity - new Type III (high intensity), new Type I (engineering grade), partially degraded Type I, and highly degraded Type I. Table 4 describes the demonstration signs that were used in the second through fourth workshops. The signs were chosen to represent a white on red iconic sign that conveys its message primarily through shape and color, a bold symbol sign that should be easy to recognize, and a fine symbol sign that requires legibility as opposed to recognition. The objective of having four levels of retroreflectivity for each sign type was to evaluate the overall acceptance at various levels. For instance, new beaded high-intensity signs were thought to be adequate for all conditions and that most, if not all, workshop participants would pass these signs. On the other end of the scale, it was believed that most workshop participants would fail engineering grade signs that were degraded to a relatively low retroreflectivity level. The middle grouping of retroreflectivity levels was established near the proposed minimum levels.

Sign Evaluation Process

Before sending the participants onto the demonstration course to view the signs, the workshop facilitators met with the participants to provide instructions and guidance on conducting the demonstration. The general instructions that were presented to the participants are shown in Figure 7. Once the pre-demonstration briefing was finished, the workshop facilitators led the participants to the location and started the demonstration. Before beginning the course, the two participants in each vehicle were given an evaluation form (Figure 8 presents a typical evaluation form). They would rate each sign on the course and indicate the rating (acceptable or unacceptable) on the evaluation form. After completing the course, the participants were provided with a second evaluation form that indicated the measured retroreflectivity level for each sign. They were encouraged to drive through the course a second time so that they could associate the visual appearance of the signs with the measured retroreflectivity levels. They were instructed not to change their initial ratings after learning of the sign retroreflectivity levels. Participants were also encouraged to drive the course in different vehicles to observe the impact that different types of headlamps have on sign visibility. Before leaving the demonstration area, the workshop facilitators collected the evaluation forms. These were tabulated later that evening for presentation the next morning.

DEMONSTRATION RESULTS

The second day of the workshop began with a review of the nighttime demonstration and the presentation of the demonstration results. This presentation included both the results from the night before and the results from previous workshops. This was followed by a brief general discussion of the workshop results among the participants.

A total of 71 workshop participants rated the 12 signs that were used in the last three demonstrations. This resulted in a total of 851 observations. Table 5 lists the common signs presented in the three demonstrations, each sign's retroreflectivity level, and the evaluation results. Figure 9 illustrates the relationship between the passing ratings and the retroreflectivity levels for each of these signs.

Figure 9. Nighttime Demonstration Evaluation Results

These results provide some degree of confidence in the proposed minimum retroreflectivity levels. Overall, 49 percent of the participants would have passed the three signs at the proposed minimum levels. However, subjective visual inspections such as those performed and reported herein, will produce relatively large levels of variability and therefore it is difficult, if not impossible, to speculate the most appropriate percent passing level. Furthermore, there are numerous limitations associated with using the results to represent minimum retroreflectivity levels.  The limitations are discussed in the next section.

The night demonstration was not intended to define the minimum level of retroreflectivity needed by road users. In order to complete the demonstration within a reasonable time period, numerous compromises were made. Examples of these compromises include: variable conditions between demonstration sites; differences in the order and placement of the demonstration signs; variations in the illumination provided by individual vehicles (vehicle headlamps); variations in ambient lighting at each location; differences in individual evaluation thresholds; variations in vehicle speeds; and limiting the impact of driving tasks on the participants. One of the key differences is that the workshop participants were generally younger than the criteria used to establish the proposed minimum levels. Furthermore, the workshop participants may not have been bias-free or representative of the visibility needs of typical drivers. This may have impacted their evaluations of the demonstration signs, leading them to pass signs that would be rejected by other drivers. Due to these and other compromises in the procedural aspects of the nighttime demonstration, the results should not be viewed as a scientific evaluation of the minimum level of retroreflectivity needed for adequate sign visibility.