Evaluation of Dynamic Speed Feedback Signs on Curves: A National Demonstration Project

EXECUTIVE SUMMARY

BACKGROUND

Lane departure crashes are a significant safety concern. The majority of these crashes occur in rural areas, mostly on two-lane roadways. A disproportionate number of them occur on horizontal curves. Curve-related crashes involve a number of roadway and driver causative factors.

The frequency and severity of curve-related crashes correlate to a number of geometric factors, such as curve radius, degree of curve, length of curve, type of curve transition, lane and shoulder width, and preceding tangent length.

The primary driver factor is speeding, given a large number of run-off-road fatal crashes on curves are speeding related. The amount of speed reduction from the tangent speed to the speed required to traverse a curve also has an impact on the frequency and severity of crashes on curves.

Dynamic speed feedback sign (DSFS) systems are one method to reduce vehicle speeds and, consequently, crashes on curves. DSFS systems show promise, but they have not been fully evaluated.

The Center for Transportation Research and Education (CTRE) at Iowa State University undertook this project to evaluate the effectiveness of DSFS systems in reducing speed and crashes on curves on rural two-lane roadways. The project was sponsored by the Federal Highway Administration (FHWA), the Midwest Transportation Consortium, the Iowa Department of Transportation (Iowa DOT), the Iowa Highway Research Board, and the Texas Department of Transportation. This report summarizes the results of the study.

PROJECT OVERVIEW

The project included the following tasks:

• Select a sample of geographically representative States to participate in the study.
• Select high-crash curve sites within those States to serve as treatment and control sites.
• Select candidate DSFS systems for evaluation.
• Collect speed and volume data before and at regular periods after installation of the signs at treatment sites.
• Assess the effectiveness of the signs in reducing speeds on curves.
• Conduct a statistical analysis to compare the impact of the signs on reducing crashes at the treatment and control sites.
• Report the results.

BRIEF DESCRIPTION OF SITE AND SIGN SELECTION

The objective of this project was to conduct a national evaluation of the effectiveness of DSFS systems. The team made every effort to obtain geographic diversity in selecting States where the signs would be tested. The team selected seven States during the Request for Proposals stage of the project or after the project started.

In most cases, an initial list of sites for an individual State was provided by a local or State agency. In two States, the team had access to crash data, so they selected initial sites and then discussed those sites with the corresponding agency.

After reviewing the initial sites and selecting those that met the criteria (defined in chapter 2 of this report), the team made visits to each State and surveyed the potential sites.

During the site visits, the team conducted a brief speed study using a radar gun to ensure that a speeding problem existed. After the site visits, the team selected final sites based on the number and type of crashes, whether a speeding problem existed, and other factors. A total of 51 viable sites across the 7 study States resulted. The researchers randomly selected 22 treatment sites, and the remaining 29 sites served as control sites for the crash analysis.

Two different types of dynamic speed-activated signs were selected for evaluation in the study. (Chapter 2 describes the methodology for selecting the signs.)

Figure 1 shows one type of sign displaying vehicle speed (referred to as the speed display sign), and figure 2 shows the other displaying a curve warning sign (referred to as the curve display sign).

Figure 1. Photo. Speed display sign used in study.

Figure 2. Photo. Curve warning display sign used in study.

Both signs activate when drivers exceed the 50th percentile speed. The speed sign displays the vehicle's actual speed, up to a certain threshold, and then the speed indication is replaced by the actual posted speed limit. The threshold was selected to avoid drivers using the sign to test their speeds above the threshold.

Sign installation is described in chapter 4 of this report. It should be noted that, in all cases, the signs were considered supplementary traffic control devices and did not replace existing traffic control.

BRIEF DESCRIPTION OF DATA COLLECTION

Speeds collected after installation of the signs were compared with speeds collected before installation. Data were collected only at treatment sites. There were not sufficient project resources to collect data at 29 additional sites, so the control sites were used only in the crash analysis.

Prior to installation of the signs, speed and volume data were collected at each site using pneumatic road tubes and counters. Data were collected at 1, 12, and 24 months after installation of the signs. Data were collected using Trax I automatic traffic recorders (also called pneumatic road tubes) manufactured by JAMAR Technologies, Inc. These units can collect individual speeds, headways, vehicle class, and volume.

Speed and volume data were collected at three points: approximately 0.5 miles upstream from the curve, at the point of curvature (PC) where the sign was installed, and at the center of curve (CC). Speed and volume data were collected 0.5 miles upstream of the signs to provide some indication whether speeds had changed independent of the signs given drivers at the upstream location had not yet encountered the speed feedback signs. This site served as a comparison location.

Chapter 5 of this report describes the data collection methodology. Chapter 6 presents results for each site. The final evaluation assesses sign performance over a 24-month after period.

Chapter 6 reports results of the speed analyses. A summary of the speed analysis is discussed in the next section.

SUMMARY OF SPEED ANALYSES

Mean and 85th percentile speeds were calculated for each curve for each data collection location before installation of the signs and at 1, 12, and 24 months after installation. The fraction of vehicles traveling at or above the posted speed limit or advisory speed by a certain threshold amount was also calculated. If an advisory speed was present, the fraction of vehicles traveling 5, 10, 15, and 20 miles per hour (mph) or more above the advisory speed was calculated. If no advisory speed was present, the fraction of vehicles traveling a certain threshold over the posted speed limit was calculated.

This metric provides a measure of the number of vehicles traveling at high speeds. In many cases, agencies are more concerned with reducing the number of drivers traveling at excessive speeds than with simply reducing average speeds.

Results are presented for traffic traveling in the direction of the DSFS system. Data for each after period were compared with the before period. Speed metrics for the before period were subtracted from the after period, so a negative change indicates that speeds were reduced from the before to the after period.

The changes in mean speeds from the before to the after periods were evaluated using a t-test, and the changes in the fraction of vehicles traveling over the posted or advisory speed by a certain threshold were compared using a test of proportions. Unless indicated otherwise, differences in means and percents over the posted or advisory speeds were statistically significant at the 95-percent level of significance.

The changes in the fraction of vehicles traveling a certain threshold over the posted or advisory speeds are presented showing the percent change from the before to the after period. Percent change was calculated by subtracting the fraction of vehicles exceeding a particular threshold in the before period from the fraction exceeding the threshold in the after period and dividing this by the fraction in the before period.

For example, if the fraction of vehicles traveling 5 mph or more over the posted speed limit was 0.413 for the before period and the fraction of vehicles traveling 5 mph or more over at 1 month after installation was 0.083, the percent change is (0.083 - 0.413) ÷ 0.413 = -0.799. Therefore, 79.9 percent fewer vehicles exceeded the posted or advisory speed by 5 mph or more after the sign had been in place for 1 month.

During the course of the study, maintenance and vandalism issues occurred with some signs, and data could not be collected for a particular interval. As a result, data for 21 sites were available for the 1-month after period, data for all 22 sites were available for the 12-month after period, and data for 18 sites were available for the 24-month after period.

Change in Mean and 85th Percentile Speed at the Point of Curvature

Table 1 provides the change in speed metrics averaged over all treatment sites for data collected at the PC. Table 2 through table 11 summarize overall results by individual curve. The speed data shown are the difference, in mph, between the before period speed and the specific after period speed.

These tables provide the curve identification number, sign type, road name, and posted speed limit for each curve. An "S" for Sign Type indicates a speed display sign, and a "C" designates a curve display sign. When an advisory curve speed was displayed, the advisory speed is shown as well.

Table 1 shows the average change in speed over all sites by analysis period. As shown, the change in mean speed at the 1-month after period was a decrease of 1.8 mph. The average decrease in mean speed at the 12-month after period was even greater than the 1-month after period with a decrease of 2.6 mph. The average speed decrease of about 2.0 mph for the 24-month after period was similar to the 1-month period. The change in 85th percentile speed was a decrease of about 2.2 mph for the 1- and 24-month after periods while the average decrease was 2.9 mph for the 12-month after period.

Also shown in table 1, sites on average had a decrease of 12 percent in the fraction of vehicles traveling at 5 mph or more over the posted or advisory speed limit for the 1-month after period. The fraction of vehicles traveling 10 mph or more over the posted or advisory speed decreased by an average of 30 percent for the 1-month after period and by 36 percent for the fraction traveling 15 mph or more over the posted or advisory speed. Similarly, the average decrease in vehicles traveling 20 mph or more over the posted or advisory speed was 29 percent.

Results for the 12-month after period were somewhat higher, with average decreases of 19, 34, 36, and 50 percent for the fraction of vehicles traveling 5, 10, 15, and 20 mph or more over, respectively.

Results for the 24-month after period were similar to the 1-month after period.

Data were also tabulated and compared by sign type. In general, larger decreases were noted for the speed signs than for the curve signs, although differences were not statistically significant. A closer examination of results by sign type is provided in Comparison of Mean and 85th Percentile Speed Changes Over Time.

Table 2 through table 4 show changes in the speed metrics at the PC for data collected about 1 month after installation of the signs. Changes in mean speed range from a decrease of 5.6 mph at site AZ-6 to an increase of 3.3 mph at site FL-32. The changes in 85th percentile speeds at the PC 1 month after installation ranged from a decrease of 8 mph to an increase of 4 mph.

Table 1. Average change across sites at the PC.

Table 2. Summary of results for individual sites at the PC 1 month after sign installation (part 1).

C = Curve display sign
S = Speed display sign

Table 3. Summary of results for individual sites at the PC 1 month after sign installation (part 2).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 4. Summary of results for individual sites at the PC 1 month after sign installation (part 3).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 5 through table 7 provide changes in the speed metrics at the PC for data collected about 12 months after installation of the signs for individual sites. Decreases in mean speeds ranged from 6.5 mph to an increase of 0.6 mph. Decreases in 85th percentile speeds range from a decrease of 8 mph to an increase of 1 mph. Signs were functioning for all 22 sites for the 12-month after period.

Table 5. Summary of results for individual sites at the PC 12 months after sign installation (part 1).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 6. Summary of results for individual sites at the PC 12 months after sign installation (part 2).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 7. Summary of results for individual sites at the PC 12 months after sign installation (part 3).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 8 through table 10 show changes in speed metrics at the PC for the 24-month after period. Signs at two sites in Oregon and two sites in Texas had various issues between the 12- and 24-month after periods. Given a number of other signs had already been repaired, it was determined there were not sufficient project resources to make additional trips to perform maintenance at those sites.

As shown, in the 24-month after period, decreases in mean speeds ranged from 0.8 to 5.7 mph, with one site experiencing an increase in mean speed of 0.5 mph. Decreases in 85th percentile speeds ranged from 1 to 6 mph, with one site having an increase of 1 mph.

Table 8. Summary of results for individual sites at the PC 24 months after sign installation (part 1).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 9. Summary of results for individual sites at the PC 24 months after sign installation (part 2).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 10. Summary of results for individual sites at the PC 24 months after sign installation (part 3).

C = Curve display sign
S = Speed display sign

Decreases in mean and 85th percentile speeds were plotted to show the distribution of change. Figure 3 shows the percent of sites experiencing decreases in mean speeds of a certain magnitude at 1, 12, and 24 months. Figure 4 provides changes in 85th percentile speeds of a certain magnitude.

Figure 3. Chart. Percent of sites experiencing a change in mean speed of a certain magnitude at the PC.

Figure 4. Chart. Percent of sites experiencing a change in 85th percentile speed of a certain magnitude at the PC.

As shown, at 1 month, 2 of the 21 sites had increases in mean speeds between 1 and 4 mph, 9 of the sites (43 percent) experienced virtually no changes in mean speeds, 5 sites (24 percent) experienced decreases of 1 to 4 mph, and 5 sites had decreases between 4 and 7 mph.

As illustrated in figure 4, two sites (10 percent) experienced increases between 1 and 7 mph, four sites (19 percent) had little change in 85th percentile speeds, seven sites (33 percent) had decreases between 1 and 4 mph, six sites (29 percent) had decreases of 4 to 7 mph, and two sites (10 percent) had decreases of more than 7 mph.

Also, as shown in figure 3, no sites experienced significant increases in mean speeds at 12 months, 3 of the 22 sites (14 percent) had little change, 15 sites (68 percent) had decreases of 1 to 4 mph, and 4 sites had decreases from 4 to 7 mph.

As shown in figure 4, no sites had increases in 85th percentile speeds while 3 of the 22 sites (18 percent) had little change, 10 sites (45 percent) had decreases of 1 to 4 mph, 6 sites (27 percent) had decreases of 4 to 7 mph, and 2 sites (9 percent) had decreases of 7 mph or more.

Figure 3 and figure 4 also show results for 24 months after installation of the signs. Data were available for 18 sites. (As indicated in chapter 6, issues had occurred with several other signs, so 24-month after data were not available for all sites.)

As shown in figure 3, at 24 months, no signs had significant increases in mean speeds while three sites had little change. The majority (13 sites or 72 percent) had decreases of 1 to 4 mph while 2 sites (11 percent) had decreases between 4 and 7 mph. As shown in figure 4, at the 24-month after period, 1 site (6 percent) had no relevant change in 85th percentile speed, 14 sites (83 percent) had decreases of 1 to 4 mph, and 2 sites (11 percent) had decreases of 4 mph or more.

Percent of Vehicles Exceeding the Posted or Advisory Speed, by Speed Bin at the Point of Curvature

Figure 5 through figure 8 show changes in the percent of vehicles traveling 5 mph or more, 10 mph or more, 15 mph or more, and 20 mph or more, respectively, over the posted speed or advisory speed at the PC. Researchers used advisory speed if present; if not present, the posted speed limit was used. Figure 5 shows the percent of sites with a change at a particular magnitude. For instance, the first interval is the percent of sites that had decreases of 70 percent or more in the number of vehicles traveling 5 mph or more over the posted or advisory speed.

Figure 5. Chart. Changes in percent of vehicles traveling 5 or more over posted limit or advisory speed at the PC.

Figure 6. Chart. Changes in percent of vehicles traveling 10 or more over posted limit or advisory speed at the PC.

Figure 7. Chart. Changes in percent of vehicles traveling 15 or more over posted limit or advisory speed at the PC.

Figure 8. Chart. Changes in percent of vehicles traveling 20 or more over posted limit or advisory speed at the PC.

Data for vehicles traveling 5 mph or more over the posted or advisory speed are fairly consistent over the 1-, 12-, and 24-month after periods. As shown, 5 to 10 percent of sites had reductions of 70 percent or more, and about 15 percent of sites had decreases from 35 to 70 percent. The majority of sites for all time periods had decreases up to 35 percent. A small number of sites had little change, with up to 10 percent having increases up to 25 percent and about 5 percent with increases of more than 25 percent.

Figure 6 also shows data for the proportion of vehicles traveling 10 mph or more over the posted speed limit or advisory speed. The majority of sites (41 to 55 percent, depending on time period) had decreases up to 35 percent in the fraction of vehicles traveling 10 mph or more over the posted or advisory speed while 23 to 35 percent had decreases between 35 and 70 percent.

About 10 percent of sites for 1 month, 14 percent for 12 months, and 6 percent for 24 months had decreases of 70 percent or more. At 1 month, 14 percent of sites, and at 12 months, 5 percent of sites, had increases up to 25 percent in the fraction of vehicles traveling 10 mph or more over the posted or advisory speed. Six percent of vehicles had increases of more than 25 percent for the 24-month after period. Up to 12 percent of sites had no change.

Figure 7 also shows changes in the fraction of vehicles traveling 15 mph or more over the posted or advisory speed. Less than 6 percent of sites for any analysis after period had increases or no change in the fraction of vehicles traveling 15 mph or more over the posted or advisory speed. Twenty-four percent of vehicles, 18 percent at 12 months, and 41 percent at 24 months had decreases up to 35 percent in the percent of vehicles traveling 15 mph or more over the posted or advisory speed. The majority of sites, 43 and 64 percent for 1 and 12 months after, respectively, and 29 percent for 24 months after had decreases between 35 and 70 percent. Up to 19 percent of sites had decreases of more than 70 percent.

Figure 8 also shows results for changes in the percent of vehicles traveling 20 mph or more over the posted or advisory speed. The majority of sites (35 percent) at 24 months had decreases up to 35 percent in the fraction of vehicles traveling 20 mph or more over the posted or advisory speed.

About 9 and 14 percent of sites experienced decreases in that range for the 1- and 12-month after periods, respectively. The majority of sites, 38 percent for 1 month after and 41 percent for 12 months after, had decreases of 35 to 70 percent in vehicles traveling 20 mph or more over the posted or advisory speed. Between 19 and 32 percent of sites had decreases in the fraction of vehicles traveling 20 mph or more over the posted or advisory speed.

A few sites (14 percent for 1 month after and about 5 percent for 12 and 24 months after) had increases of more than 25 percent. About 5 percent had increases up to 25 percent and about 6 to 9 percent had no change. About 20 percent of sites at 1 and 24 months after and about 30 percent at 12 months after had decreases in the percent of vehicles traveling 20 mph or more over the posted or advisory speed.

As noted, significant reductions in the number of vehicles traveling over the posted or advisory speed occurred for all of the after periods at the PC. In most cases, the majority of sites had reductions between 35 and 70 percent in the fraction of vehicles exceeding the posted or advisory speed. This was the case for all of the speed thresholds (5, 10, 15, and 20 mph or more over). In addition, reductions of greater than 70 percent were noted for all time periods and thresholds except for one. This indicates the signs were effective in reducing high-end speeds, as well as average and 85th percentile speeds.

Change in Mean and 85th Percentile Speed at the Center of Curve

Table 11 provides the change in speed metrics averaged for all sites for data collected at the CC. Table 12 through table 20 summarize overall results by curve. The speed data shown are the difference in mph between the before period speed and the specific after period speed. These tables provide the curve identification number, sign type, road name, and posted speed limit for each curve. An "S" for Sign Type indicates a speed display sign, and a "C" designates a curve display sign. When an advisory curve speed was present, the advisory speed is also shown.

Table 11 provides the average change in speed overall for all sites by after analysis period. As shown, the change in mean speed at the 1-month after period was a decrease of 2.1 mph. The average decreases, 1.7 and 1.8 mph, in mean speeds at the 12-month and 24-month after periods, respectively, were smaller than the average decrease at 1 month.

The average changes in 85th percentile speeds were a decrease of 2.5 mph for 1 month, 1.6 mph for 12 months, and 1.9 mph for 24 months. Results are also presented by sign type. A more in-depth discussion of results by sign type is presented in Comparison of Mean and 85th Percentile Speed Changes Over Time.

Table 11. Average change across sites at the CC.

Table 12 through table 14 show results at the CC for individual sites at the 1-month after period. Speed reductions were generally larger at the CC than at the PC. Changes in mean speeds ranged from a decrease of 10.9 mph to an increase of 2.8 mph. Changes in 85th percentile speeds ranged from a decrease of 12 mph to an increase of 6 mph.

Data are presented for 21 of the 22 sites. At the 1-month after period, a sign had been knocked down at one of the Texas sites and had not been repaired when data were collected.

As noted, most sites had significant decreases in the fraction of vehicles traveling 5, 10, 15, or 20 mph or more over the posted or advisory speed. Reductions up to almost 100 percent were reported for the fraction traveling 5, 10, or 15 mph or more over the posted or advisory speed. One site had a 211-percent reduction, and another site had a 161-percent reduction in the fraction of vehicles traveling 20 mph or more over the posted or advisory speed limit.

Table 12. Summary of results for individual sites at the CC 1 month after sign installation (part 1).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 13. Summary of results for individual sites at the CC 1 month after sign installation (part 2).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 14. Summary of results for individual sites at the CC 1 month after sign installation (part 3).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 15 through table 17 provide results for the CC for 12 months after installation of the signs. Changes in mean speeds ranged from a decrease of 7.9 mph to an increase of 3.7 mph. The changes in 85th percentile speeds ranged from a decrease of 9 mph to an increase of 3 mph.

Table 15. Summary of results for individual sites at the CC 12 months after sign installation (part 1).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 16. Summary of results for individual sites at the CC 12 months after sign installation (part 2).

C = Curve display sign
S = Speed display sign

Table 17. Summary of results for individual sites at the CC 12 months after sign installation (part 3).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 18 through table 20 provide changes in speed metrics for the 24-month after period. Data are presented for 18 sites, given 4 sites were no longer functioning at the 24-month after period. One site had an increase in mean speed of 2.0 mph, while the remaining sites had decreases from 0.8 to 7.0 mph. Two sites experienced increases in 85th percentile speeds (1 and 2 mph), and two sites had no change. The remaining sites had decreases in 85th percentile speeds from 1 to 8 mph.

Table 18. Summary of results for individual sites at the CC 24 months after sign installation (part 1).

*Not statistically significant at 95-percent level of significance
C = Curve display sign
S = Speed display sign

Table 19. Summary of results for individual sites at the CC 24 months after sign installation (part 2).

C = Curve display sign
S = Speed display sign

Table 20. Summary of results for individual sites at the CC 24 months after sign installation (part 3).

The following figures show the distribution of results. Figure 9 shows the percent of sites experiencing changes in mean speeds of a certain magnitude. As indicated, the majority of sites had decreases in mean speeds from 1 to 4 mph.

Figure 9. Chart. Percent of sites experiencing a change in mean speed of a certain magnitude at the CC.

At 1 month, 2 of the 21 sites (10 percent) had increases in mean speeds that were greater than 1 to 4 mph, 5 of the sites (24 percent) experienced virtually no change in mean speeds, and 11 sites (52 percent) experienced decreases of 1 to 4 mph. Finally, three sites (15 percent) had decreases of 4 mph or more.

Similar results occurred for the 12-month and 24-month after periods. At 12 and 24 months, about 14 percent of sites had increases between 1 and 4 mph. Eighteen percent of sites at 12 months and 6 percent at 24 months had little change.

The majority of sites (55 and 72 percent) had decreases in mean speeds from 1 to 4 mph. At 12 months, 9 percent and 6 percent of sites had decreases between 4 and 7 mph, respectively. About 5 percent of sites for both the 12- and 24-month after periods had decreases of more than 7 mph.

Figure 10 shows the changes in 85th percentile speeds at the CC at 1, 12, and 24 months. Overall, as indicated, the majority of sites for all after periods had decreases in mean speeds from 1 to 4 mph.

Figure 10. Chart. Percent of sites experiencing a change in 85th percentile speed of a certain magnitude at the CC.

At 1 and 24 months, two sites (about 10 percent) had increases that were more than 1 mph while four sites (18 percent) saw an increase at 12 months. Between 6 and 14 percent of sites experienced little change in 85th percentile speed depending on the after period.

Thirteen sites (62 percent) at 1 month, 10 sites (45 percent) at 12 months, and 10 sites (56 percent) at 24 months had decreases from 1 to 4 mph. Two sites (10 percent) at 1 month, four sites (18 percent) at 12 months, and three sites (17 percent) at 24 months had decreases from 4 to 7 mph. One site (about 5 percent) for each after period experienced a decrease from 7 to 10 mph, and one site (5 percent) had an 85th percentile speed decrease that was more than 10 mph at 1 month after.

Percent of Vehicles Exceeding the Posted or Advisory Speed, by Speed Bin at the Center of Curve

Figure 11 through figure 14 show changes in the percent of vehicles traveling at 5, 10, 15, and 20 mph or more over the posted speed limit or advisory speed at the CC. Each figure shows the percent of sites that experienced a change within a particular range. Data are fairly consistent over the 1-, 12-, and 24-month after periods.

Figure 11. Chart. Changes in percent of vehicles traveling 5 or more mph over posted limit or advisory speed at the CC.

Figure 12. Chart. Changes in percent of vehicles traveling 10 or more mph over posted limit or advisory speed at the CC.

Figure 13. Chart. Changes in percent of vehicles traveling 15 or more mph over posted limit or advisory speed at the CC.

Figure 14. Chart. Changes in percent of vehicles traveling 20 or more mph over posted limit or advisory speed at the CC.

As shown, 18 and 6 percent of sites had increases in the fraction of vehicles traveling 5 mph or more over the posted or advisory speed at the 12- and 24-month after periods, respectively, and 5 percent had no change at the 1-month after period.

The majority of sites (67 percent for 1 month, 59 percent for 12 months, and 65 percent for 24 months) had decreases up to 35 percent in the fraction of vehicles traveling 5 mph or more over the posted or advisory speed. About 24 percent of sites had reductions between 35 and 70 percent, and about 5 percent had reductions of 70 percent or more.

Figure 12 provides results for the percent of vehicles traveling at 10 mph or more over the posted or advisory speed at the CC. Five to 9 percent of sites at 1 and 12 months, respectively, had increases up to 25 percent and 10 percent, and 5 percent of sites had no change for the 1- and 12-month after periods, respectively.

Most sites had reductions in the fraction of vehicles traveling 10 mph or more over the posted or advisory speed that were up to 35 percent or between 35 and 70 percent. About 29 percent of sites at 1 month and 41 percent at 12 and 24 months experienced reductions in the fraction of vehicles traveling 10 mph or more over the posted or advisory speed. Thirty-six to 41 percent of sites had reductions between 35 and 70 percent. Finally, about 20 percent of sites at 1 month, 9 percent at 12 months, and 12 percent at 24 months had reductions in the fraction of vehicles traveling 10 mph or more over the posted or advisory speed.

Figure 13 shows changes in the fraction of vehicles traveling 15 mph or more over the posted or advisory speed. Five percent of sites at 12 months had increases of more than 25 percent, and 5 percent had no change for that same time period. Between 6 and 14 percent of sites had increases of up to 25 percent. Five percent of sites at 1 month, 18 percent at 12 months, and 35 percent at 24 months had reductions up to 35 percent in the percent of vehicles traveling 15 mph or more over the posted or advisory speed.

The majority of sites (48 percent for 1 month and 41 percent for the 12- and 24-month after periods) had decreases from 35 to 70 percent. Eighteen to 33 percent had reductions of more than 70 percent in the fraction of vehicles traveling 15 mph or more over the posted or advisory speed.

Figure 14 shows results for the percent of vehicles exceeding the posted or advisory speed by 20 mph or more. One or two sites showed increases in the percent of vehicles exceeding the speed limit by 20 mph or more for the 1- and 12-month after periods, respectively. Two sites for the 24-month after period had increases from more than 1 to 10 percent. Five to nine sites (23 to 53 percent) had decreases from 35 to 70 percent.

As noted, large reductions in the number of vehicles traveling over the posted or advisory speed occurred for all of the after periods at the CC. The majority of sites had reductions up to 35 percent in the fraction of vehicles traveling 5 mph or more over the posted or advisory speed. The majority of sites had decreases up 70 percent in the fraction of vehicles traveling 10 mph or more over the posted or advisory speed. Most sites had reductions of 35 percent or more in the fraction of vehicles traveling 15 or 20 mph or more over the posted or advisory speed. These results indicate the signs were effective in reducing high-end speeds as well as average and 85th percentile speeds.

Comparison of Mean and 85th Percentile Speed Changes Over Time

Data were collected over a period of 2 years to assess whether regular drivers become habituated to the signs, which might lessen their effectiveness. Table 1 showed the average change in mean and 85th percentile speeds at the PC. The average change in mean speed at 1 month after was -1.82 mph, and the average changes (-2.57 and -1.97 mph) at 12 and 24 months were greater than at the 1-month after period. The average changes (-2.19 and -2.17 mph) in 85th percentile speeds at the PC were similar for the 1- and 24-month after periods, and the decrease (-2.86 mph) was greater at 12 months than at 1 month. These data anecdotally suggest that the signs remained effective over time.

To test that assumption, a Wilcoxon-signed rank test was used to test differences among the 1-, 12-, and 24-month after periods. The Wilcoxon-signed rank test is a non-parametric test and was used given the data were not normally distributed. The test compares the absolute value of the differences between observations, which are ranked from smallest to largest.

The individual changes in mean and 85th percentile speeds for sites at the PC were compared using the Wilcoxon-signed rank test. Results of the analysis indicated no statistically significant differences among changes in mean speeds at the PC for any of the time periods. The following shows the test statistics:

• 1 and 12 months (p = 0.29).
• 1 and 24 months (p = 0.43).
• 12 and 24 months (p = 0.43).

Similarly, no statistically significant differences in the change in 85th percentile speeds at the PC were noted with the following test statistics:

• 1 and 12 months (p = 0.45).
• 1 and 24 months (p = 0.60).
• 12 and 24 months (p = 0.36).

As shown in table 11, the average change in mean speed at 1, 12, and 24 months at the CC was -2.08, -1.65, and -1.76 mph, respectively. The average change in 85th percentile speed at the CC was -2.52, -1.55, and -1.89 mph for the 1-, 12-, and 24-month after periods, respectively. In both instances, the average decrease in speeds at the CC at 1 month after was slightly greater than for the 12- or 24-month after period.

To test whether the differences were statistically significant, the individual changes in mean and 85th percentile speeds for sites at the CC were compared using the Wilcoxon-signed rank test. As shown by the following test statistics, there were no statistically significant differences among changes in mean speeds across sites over the three after periods:

• 1 and 12 months (p = 0.87).
• 1 and 24 months (p = 0.99).
• 12 and 24 months (p = 0.88).

Results were similar for changes in 85th percentile speeds at the CC. As the following statistics show, there were no statistically significant differences in changes in 85th percentile speeds across sites:

• 1 and 12 months (p = 0.53).
• 1 and 24 months (p = 0.50).
• 12 and 24 months (p = 0.98).

As indicated, changes in mean and 85th percentile speeds appeared to be consistent across the three after periods. This suggests the signs may have a long-term impact on speed.

Summary of Results by Sign Type

Drivers may respond differently to different sign messages. In addition, different signs may be more effective in different situations. However, given only 22 sites were included in this project, testing a range of signs with different driver messages was beyond the project scope.

In addition, given the project intent was not to compare different sign types, the experiment was not designed for comparison. However, there was some value in evaluating the data by sign type to assess whether there was evidence of differences by sign type, which may lead to further research. As a result, data were disaggregated by sign type, and general comparisons were conducted.

As indicated, the experiment was not set up to test differences by sign type, and the sample size is low. Consequently, caution should be used in interpreting the results.

Figure 15 through figure 17 show the percent of sites that showed a change in average speed of a certain magnitude for each after period at the PC. Results are presented by type of sign (curve advisory versus speed sign).

At 1 month, about 10 percent of sites with both sign types had an increase of more than 1 mph in average speed. The majority of sites with curve signs (58 percent) had little change in average speed while 22 percent of sites with speed signs had little change. Seventeen percent of sites with curve signs had decreases of 1 to 4 mph, and another 17 percent had decreases of more than 4 percent, while 33 percent of sites with speed signs had decreases of 1 to 4 mph, and 33 percent had decreases of more than 4 mph.

At 12 months, 23 percent of sites with curve signs had little change in mean speed while 62 percent of sites with curve signs, and 78 percent of sites with speed signs had a decrease in average speeds between 1 and 4 mph. Fifteen percent of sites with curve signs and 22 percent of sites with speed signs had decreases of 4 mph or more.

Results for the 24-month after period are very similar to those for the 12-month after period.

Figure 15. Chart. Changes in mean speed at the PC by sign type about 1 month after sign installation.

Figure 16. Chart. Changes in mean speed at the PC by sign type about 12 months after sign installation.

Figure 17. Chart. Changes in mean speed at the PC by sign type about 24 months after sign installation.

Figure 18 through figure 20 illustrate the percent of sites with changes in 85th percentile speed of a certain magnitude at each after period by sign type at the PC. Results are presented by sign type.

About 10 percent of sites for both sign types experienced an increase in 85th percentile speed of more than 1 mph. Approximately one-third of sites with curve signs experienced no change, while one-third of sites with both sign types had decreases between 1 and 4 mph. One-quarter of sites with curve signs and more than 50 percent of sites with speed signs had decreases of more than 4 mph.

About 31 percent of sites with a curve sign showed little change in 85th percentile speeds. The majority of sites with both types of signs (38 percent of sites with curve signs and 56 percent of sites with speed signs) had decreases in 85th percentile speeds between 1 and 4 mph. Thirty-one percent of sites with curve signs and 44 percent of sites with speed signs experienced decreases in 85th percentile speeds of 4 mph or more.

Figure 18. Chart. Changes in 85th percentile speed at the PC by sign type about 1 month after sign installation.

Figure 19. Chart. Changes in 85th percentile speed at the PC by sign type about 12 months after sign installation.

Figure 20. Chart. Changes in 85th percentile speed at the PC by sign type about 24 months after sign installation.

At 24 months, 50 percent of curve signs had little change in 85th percentile speeds while 13 percent of speed signs had no change. Forty percent of curve signs had decreases in 85th percentile speeds that were 1 up to 4 mph, and 75 percent of speed signs saw the same decrease. Ten percent of curve signs and 13 percent of speed signs had decreases of 4 mph or more.

Sites with speed signs appeared to be slightly more effective based on the data shown in figure 9 and figure 10. To test that assumption, the researchers conducted a statistical test to evaluate differences between sign types for both average speed differences and differences in 85th percentile speeds.

The data were not normally distributed, so the Wilcoxon-signed rank test, a non-parametric test, was used. Results at 1 month showed no statistically significant difference in either average speed (p = 0.39) or 85th percentile speed (p = 0.22).

Similarly, results for 12 months showed no statistically significant difference in either average speed (p = 0.20) or 85th percentile speed (p = 0.15). Results for the 24-month after period were similar, with no statistical difference in mean (p = 0.66) or 85th percentile speeds (0.29). Consequently, at the PC, there was no evidence to suggest that one sign type was more effective than the other was.

Figure 21 through figure 23 show information for mean speeds at the CC by sign type for 1 month, 12 months, and 24 months after installation of the signs.

Figure 21. Chart. Changes in mean speed at the CC by sign type about 1 month after sign installation.

Figure 22. Chart. Changes in mean speed at the CC by sign type about 12 months after sign installation.

Figure 23. Chart. Changes in mean speed at the CC by sign type about 24 months after sign installation.

At 1 month, about 10 percent of both sites with curve and speed signs had increases of 1 mph or more while 17 percent of sites with curve signs and 33 percent of sites with speed signs had little change in mean speed (defined as changes between -1 and 1 mph). The majority of sites with curve signs (67 percent) and 33 percent of sites with speed signs had decreases from 1 to 4 mph. A small number of sites with curve signs (8 percent) and 22 percent of sites with speed signs had decreases in mean speed of 4 mph or more.

At 12 months, 20 percent of sites with speed signs had an increase in mean speed of 1 mph or more. Almost one-third of sites with curve signs and 10 percent of sites with speed signs had little change in mean speeds. The majority of sites for both the curve signs (50 percent) and speed signs (60 percent) experienced decreases in mean speeds between 1 and 4 mph while 17 percent of sites with curve signs and 10 percent of sites with speed signs had decreases of 4 mph or more.

As stated, figure 24 through figure 26 also show changes in mean speed by sign type for the 24-month after period. Both the curve and speed signs had increases of more than 1 mph in mean speeds while 10 percent of curve signs had little change in mean speeds. Eighty percent of curve signs and 63 percent of speed signs had decreases between 1 and 4 mph. One-quarter of the speed signs had decreases of 4 mph or more at the 24-month after period.

Differences in 85th percentile speeds between sign types for data collected at the CC are shown in figure 24 through figure 26 for 1, 12, and 24 months after installation of the signs.

Figure 24. Chart. Changes in 85th percentile speed at the CC by sign type about 1 month after sign installation.

Figure 25. Chart. Changes in 85th percentile speed at the CC by sign type about 12 months after sign installation.

Figure 26. Chart. Changes in 85th percentile speed at the CC by sign type about 24 months after sign installation.

Seventeen percent of sites with curve signs and 11 percent of sites with speed signs at 1 month after had increases in 85th percentile speeds that were more than 1 mph. Another 11 percent of sites with speed signs had no change in 85th percentile speeds (defined as a change between -1 and 1 mph).

The majority of sites for both sign types (75 percent for curve signs and 44 percent for speed signs) had decreases in 85th percentile speeds between 1 and 4 mph. A small number of sites with curve signs (8 percent) and 33 percent of sites with speed signs had decreases that were 4 mph or more.

Similarly, at 12 months, a similar number of sites (17 percent for curve sign sites and 20 percent for speed sign sites) had increases in 85th percentile speeds that were more than 1 mph while 17 percent of sites with curve signs and 10 percent of sites with speed signs experienced little change. The majority of sites (50 percent of curve signs and 40 percent of speed signs) also experienced decreases between 1 and 4 mph. Seventeen percent of sites with curve signs and 30 percent of sites with speed signs had decreases in 85th percentile speeds that were 4 mph or more.

Figure 26 also shows changes in 85th percentile speeds for the 24-month after period. About 10 percent of sites for both curve and speed signs had increases of more than 1 mph and no change. Eight percent of curve signs and 63 percent of speed signs had decreases of 1 to 4 mph, and 10 percent of curve signs and 38 percent of speed signs had decreases of more than 4 mph.

Similar to results at the PC, sites with speed signs appeared to be slightly more effective based on the information provided in figure 24 through figure 26. A Wilcoxon-signed rank test was also used to test differences between sign types for both average speed differences and differences in 85th percentile speeds.

Results for 1 month after showed no statistically significant difference in either average speed(p = 0.64) or 85th percentile speed (p = 0.11) by sign type. Similarly, results for 12 months after showed no statistically significant difference in either average speed (p = 0.63) or 85th percentile speed (p = 0.35). At 24 months, results showed no statistically significant difference (p = 0.69) in mean or 85th percentile speed (p = 0.92).

Results suggest there is no evidence of a difference in effectiveness between sign types at the CC. However, results should be used with caution given the small sample size.

SUMMARY OF CRASH ANALYSES

A crash analysis was conducted in addition to the speed analysis. Crash data were collected for up to 4 years before and up to 3 years after installation of the signs. To select treatment and control sites in early stages of the project, crash data were requested for 3 years before installation of the signs from the corresponding State or county agency for all sites except Iowa. The team had access to the Iowa crash data and was able to extract all of the necessary crash variables.

Once the signs had been installed for at least 2 years in States other than Iowa, the team contacted the corresponding State or county agency again and requested crash data for the intervening period from the original data request up to 2 years after installation of the signs. In some cases, more than 2 years had elapsed, and the agency provided more than 2 years of after data.

Data were evaluated for several different scenarios, including the following:

• Total crashes for both directions of travel.
• Total crashes by direction.
• Single-vehicle (SV) crashes for both directions of travel.
• SV crashes by direction.

Crashes were modeled in the direction of the DSFS sign given it was thought that the sign was most likely to reduce crashes for vehicles traveling in that direction. However, crashes for both directions were also modeled, given slowing vehicles in one direction may have some impact on vehicles in the opposite direction, particularly multivehicle crashes involving vehicles from both directions. Total crashes were modeled as well as single-vehicle crashes. Not all States had the same variables in their crash records so lane departure crashes could not be identified consistently. As a result, SV crashes were modeled, given they are overwhelmingly lane departures.

Crashes were modeled by quarter rather than by year because the after period was limited to about 2 years and use of quarters allowed the quarter in which installation occurred to be excluded from the analysis without having to exclude the entire installation year. In addition, the signs were not functioning at several sites for various periods, so the quarter in which the signs were nonfunctional could also be excluded from the analysis without discarding the entire year.

Two different analyses were conducted. A simple descriptive statistical analysis compared reductions in crashes from the before to after period for treatment versus control sites. Before and after data are compared for each site in chapter 7.

A summary of data aggregated by treatment and control site is provided in table 21. As shown, total crashes in both directions decreased by 0.08 crashes per quarter for the control site, while crashes per quarter at the treatment sites decreased by 0.22 (a 17-percent versus 40-percent reduction). SV crashes for both directions decreased by 0.07 crashes per quarter at the control site and by 0.21 at the treatment sites (a 19-percent versus 47-percent reduction). Decreases at treatment sites were 2.75 and 3.0 times greater than at control sites.

Total crashes in the direction of the outside of the curve increased by 0.02 crashes per quarter for control sites and decreased by 0.12 crashes per quarter in the direction of the sign for the treatment sites (an increase of 9 percent compared with a decrease of 35 percent). Similarly, SV crashes decreased by 0.01 crashes per quarter at the control sites compared with a decrease of 0.14 at treatment sites (a decrease of 4 percent compared with a decrease of 49 percent). Decreases at treatments sites were 6 to 14 times greater than at control sites.

Table 21. Decrease in crashes using simple descriptive statistics.

SV = Single vehicle

Descriptive statistics are provided to indicate overall trends. Caution should be used in applying the results, given data were not normalized by season and more quarters of a particular season may have been present in the before period than the after period. However, results show a trend that a much greater decrease in crashes per quarter occurred for treatment sites compared with control sites.

A before-and-after analysis was also conducted using a Full Bayes Model to develop crash modification factors (CMF). Predictive models were developed using data from control sites for all periods and before data for treatment sites. The models accounted for season, repeated measures at the same site, and differences in the length of sites. The models were then used to calculate the number of crashes for the after period for treatment sites that would have been expected had no treatment been applied. CMFs were calculated by dividing the observed crashes by the predicted values. Table 22 shows results and 95-percent confidence intervals (CI).

Table 22. Results for calculation of crash modification factors for DSFS.

CMF = Crash modification factor
STDE = Standard error
CI = Confidence interval

For total crashes in both directions, the CMF was 0.95. In other words, total crashes for both directions are expected to decrease by 5 percent with installation of the DSFS system, and the difference is statistically significant. Total crashes in the direction of the DSFS system are expected to decrease by 7 percent (CMF = 0.93), and the result is statistically significant.

SV crashes in both directions are expected to decrease by 5 percent, and SV crashes in the direction of the sign are also expected to decrease by 5 percent. Both changes are statistically significant. Results of both statistical analyses indicate that the DSFS systems are reasonably effective in reducing crashes.