Rear-Facing School Speed Limit Beacons

H Gene Jr Hawkins
Institute of Transportation Engineers. ITE Journal

May 31, 2007 20:00 EDT

INTRODUCTION

School traffic safety is an important consideration, and encouraging drivers to comply with school speed limits can be a challenge. School zones present many situations that require drivers to devote more attention to the driving task and to watch for young children who may not have sufficient respect for the hazards associated with traveling near moving traffic.

When active, school zones tend to generate greater traffic; have higher proportions of turning and stopping traffic; and expose traffic to children walking or biking in or near the roadway. These distractions can divert driver attention from maintaining an appropriate speed and can lead to violations of school speed limits.

Another possible reason some drivers do not comply with school speed limits is that they may forget they are in the school speed limit zone. This can occur when the school speed zone is excessively long or contains a stop-controlled or signalized intersection. The stop-condition case is particularly significant because drivers may accelerate to the normal (non-school zone) speed before realizing they are still subject to the school speed limit.

Several studies have evaluated the effectiveness of various traffic control devices on vehicle speeds in school areas, including various applications of forward-facing beacons, special pavement markings, standard school speed limit signs and dynamic speed signs. Among the findings of previous research are that drivers' lack of compliance with school speed limits is a direct result of minimal or no police enforcement5 and that drivers are more reluctant to follow school speed limit signs if the school speed limit reduction is unreasonably low, regardless of the signing used.6,7

The flashing beacon often used with school speed limit signs is intended to inform drivers approaching a school speed limit zone that the school speed limit is in effect. However, once they enter the zone, there are no active means of reminding drivers that they are still in the school speed limit zone. Placing a rear-facing beacon at the end of the zone (with the beacon facing traffic traveling within the zone) provides a means of reminding drivers that they are in a school zone and that the reduced speed limit is in effect.

Rear-facing beacons have been used by some agencies to provide a dynamic indication of the end of the school speed limit zone but, more important, to serve as a reminder that the reduced speed limit is in effect. Unfortunately, there have been no published evaluations on the effectiveness of rear-facing school beacons.

To assess their effectiveness, the author conducted before-and-after speed studies of rear-facing beacons at several study sites. The first evaluation was conducted in cooperation with the City of College Station, TX, USA, as part of an unfunded research effort.8 The second evaluation was conducted as part of a research study sponsored by the Texas Department of Transportation and conducted by the Texas Transportation Institute.9

EXPERIMENTAL TREATMENT

For the evaluations conducted in this study, a flashing amber beacon was mounted on the rear of the existing school speed limit sign assembly that already had flashing beacons for traffic approaching the beginning of the school speed limit zone. Because it was mounted on the back of the existing assembly, the rear-facing beacon was located on the left side of the street relative to traffic approaching the end of the school speed zone. An End School Zone sign (S5-2) was mounted below the rear-facing beacon.

Figure 1 shows front, rear and side views of a typical rear-facing beacon installation. Because power already was provided to the assembly, the only additional cost of the treatment was associated with the installation of the additional beacon and the End School Zone sign located below the rear-facing beacon. Rear-facing beacons were not installed on the right side of the roadway at any of the sites.

STUDY DESIGN

The study design for this effort was a classic before-and-after study at four sites. At each study site, researchers collected the "before" speed data when the school speed limit was in effect and during a period before or after the school speed limit was in effect. After collecting the data, the researchers worked with the responsible jurisdiction to install a rear-facing beacon. Two to three weeks after installation, the researchers returned to the study site to collect the "after" data for comparison to the pre-treatment condition.

Data Collection Procedures

To test the effectiveness of the rear-facing flashing beacon, researchers measured vehicle speeds as vehicles traversed the last 500 feet (150 meters) of the school zone at each site. Speed data were collected in only one direction at each site during four different time intervals (active school zone morning, active school zone afternoon, non-active school zone morning and non-active school zone afternoon) both before the treatment was installed and after installation of the rear-facing beacon and End School Zone sign.

The non-active data (school speed limit not in effect-beacons not flashing) were used as the control data. Data were collected only in dry weather conditions. The local police were informed prior to each data collection event and did not patrol the study site while data collection was performed. The data collector used a handheld light detection and ranging (LIDAR) device to continuously measure speeds and distances of individual vehicles as they traversed the study site.

The data collector also noted the lane position (right or left) of each vehicle for which speeds were measured and used the following criteria tor determining which vehicles to include in the measurement sample:

* passenger vehicle (cars, pickups, vans and sports utility vehicles) not towing trailers

* uninfluenced by other vehicles

* traversing the entire study site

* non-erratic behavior (lane changing)

* greater than a 6-second headway

Site Descriptions

Of the four study sites, one included a signalized intersection within the zone; one included a four-way stop-controlled intersection; one represented a long school zone; and one represented a typical length school zone with no intermediate stops. Table 1 summarizes the key characteristics of the four sites. Two of the sites were on highways and the other two were city streets. The difference between the school speed limit and the normal speed limit was either 15 or 20 miles per hour (24 or 32 kilometers per hour). The four sites were located in three different cities.

Data Reduction and Analysis

Data collection with the LIDAR unit produced numerous spot speeds for each individual vehicle over the last 500 feet (150 meters) of the speed zone. The individual speed data were typically recorded at intervals of 15 to 20 feet (4.6 to 6.1 meters). These data points were plotted over the length of the data collection zone. Then the researchers used linear interpolation to determine the individual vehicle speeds at 100-foot (30-meter) increments for each sample vehicle. Vehicles were deleted from the sample if spot speeds could not be interpolated for more than one of the incremental distances. The aggregate speed data at the intervals then were calculated from the individual vehicle speeds at each interval.

Dependent variables (i.e., measures of effectiveness) were selected to reflect the speed-related characteristics that researchers hypothesized would be affected by the rear-facing beacon. The independent variable was the presence or absence of the rear-facing beacon. Dependent variables included the following:

* mean vehicular speeds interpolated at 100-foot (30-meter) intervals through the study site;

* 85th-percentile speeds at 100-foot (30-meter) intervals through the study site;

* percentage of drivers exceeding the school speed limit and specific speed thresholds above the school speed limit; and

* speed standard deviation at each selected distance.

The mean speed data at the interval distances were compared using independent sample t-tests. The z-test was used to determine if there were statistically significant differences in the percentage of vehicles exceeding the speed limit after the treatment was installed. F-tests were used to identify significant changes in speed variance.

RESULTS

Researchers first analyzed the control data to determine if the before and after traffic conditions were comparable. If the control conditions were comparable, the researchers analyzed the before and after treatment data to determine whether the rear-facing beacon had any effect on traffic speeds.

Control Data Analysis

The true benefit of the treatment could be assessed only if the traffic conditions were similar in the before and after periods. To determine this, the researchers collected control data at each site when the school speed limit beacons were off and the school speed limit was not in effect. The researchers used a t-test to determine if the control speeds in the before and after periods could be considered equivalent for analysis purposes.

The analysis of the control data found no significant differences in the before and after speeds at all sites except one. At the Thorndale site, there was a difference in the before and after speeds at one of the measurement distances (indicated by an "X" in the pertinent cells in Tables 2-5). Therefore, this distance was excluded from the treatment analysis.

For the remaining distances at all the sites, the similarity in the before and after control speed data indicated that any differences in before and after school zone speeds for the treatment condition could be attributed to the rear-facing beacons.

Treatment Analysis

Table 2 provides a summary of the results at the four sites for the mean and 85thpercentile speeds. Table 3 indicates the reduction in mean and 85th-percentile speeds that was realized at each distance. Table 3 also presents the realized speed reduction as a percentage of the school speed limit.

Results of ANOVA testing for the school zone speed data showed that treatment (absence vs. presence of rear-facing flashing beacon), vehicle lane positioning (right vs. left) and time period (a.m. vs. p.m.) all had significant and consistent effects on mean speeds throughout the site, with no interactions found between the variables. In other words, the rear-facing beacon had a similar effect on school zone speeds, regardless of lane positioning or time period. Therefore, data were collapsed between lanes and time periods for reporting.

Table 2 shows that, overall, installation of the flashing beacon at the end of the school zone produced statistically significant reductions in mean speeds throughout the final 400 feet (120 meters) of the school zone at three of the four sites. The one site where the before and after difference was not significant was the site that represented a typical school zone, indicating that the rearfacing beacon has the greatest benefit at locations where drivers may forget they are in the school zone.

As indicated in Table 3, mean and 85th-percentile speeds were reduced by the rear-facing beacon, but by less than 4 miles per hour (6 kilometers per hour). The rear-facing beacon appeared to have a greater impact on vehicles near the end of the school zone. The speed reductions were greater near the end of the zone than they were upstream. In most cases, the standard deviation of speed also was significantly reduced after installation of the rear-facing beacon. These reductions in the mean and 85th-percentile speeds were small. Normally, such small reductions, even if statistically significant, would not have sufficient practical significance to justify implementation. However, the speed reduction should be considered within the context of the school speed limits.

Accordingly, Table 3 also presents the speed reductions as a percentage of the school speed limit. For the mean speeds, the reduction for the three sites where the treatment was expected to have the greatest benefit was about 5 to 7 percent of the school speed limit. For the 85th-percentile speeds, the speed reduction was about 6 to 12 percent of the school speed limit. Although the values of the speed reductions are small, they represent a noticeable reduction as a percentage of the speed limit, particularly because the school speed limits already present a 57to 66-percent reduction from the normal speed limit.

Researchers also examined the percentage of vehicles exceeding various speed thresholds between the before and after studies to see it the treatment had an effect on reducing the percentage of higher-speed traffic. The speed thresholds examined in this analysis included 0, 5, 10 and 15 miles per hour (0, 8, 16 and 24 kilometers per hour) over the school speed limit.

Table 4 shows the results of this analysis. Table 5 summarizes the changes in the percentage exceeding the speed thresholds. These data also demonstrate positive benefits for the rear-facing beacon, indicating an average reduction of 10 percent in the number of vehicles exceeding the school speed limit. At the two sites with a higher school speed limit, the impact on speeding was even greater, with a reduction of about 14 percent.

SUMMARY AND CONCLUSIONS

This study examined the effect of a rearfacing flashing amber beacon mounted at the end of a school speed /one on driver compliance with school speed limits. Four sites were evaluated-one with a signalized intersection within the boundary of the zone, one with a stop-controlled intersection within the zone, one that was considered a long school zone and one of typical length with no intermediate stops. The results found statistically significant speed reductions at the three sites where drivers are most likely to forget they are in a school zone (a long zone or a stop within the zone).

Although the magnitude of the speed reductions was small, the reductions represented between 5 and 12 percent of the school speed limits studied. The rearfacing beacon also reduced the percentage of speeding vehicles by about 10 percent.

Reducing the number of vehicles that exceed the speed limit can have valuable benefits for school safety, even if the actual reduction in the mean or 85th-percentile speed is not practically significant. Based on the results of the study, the researchers conclude that a rear-facing beacon mounted on the back of a school speed limit assembly is an effective alternative for improving compliance with the school speed limit at locations where drivers are prone to be distracted and forget they are within the school speed zone.

RECOMMENDATIONS

The study results indicate that a rearfacing beacon will improve driver compliance with school speed limits at sites where drivers may forget that they are in an active school zone (such long school zones and sites with an intermediate stop located within the school zone). The greatest benefit of the rear-facing beacon at these sites will be a reduction in the percentage of vehicles exceeding the speed limit, although there will also be a small reduction in mean and 85th-percentile speeds. Additionally, the use of a rear-facing beacon has no known detrimental effect and can be installed at minimal cost.

A rear-facing beacon is not a new device, but merely a different application of an existing device. This permits its use without requiring permission to experiment. When used, the rear-facing beacon should be accompanied by an End School Zone (S5-2) sign mounted below the rear-facing beacon to promote driver association of the beacon with the end of the school speed limit zone. Based on the results of this research, the Texas Department of Transportation (TxDOT) revised the Texas Manual on Uniform Traffic Control Devices to include the following statements regarding the use of rear-facing beacons as an option for the School Speed Limit Assembly:10

* A confirmation beacon or device to reinforce to the driver that the school speed limit is in operation may be considered for inclusion on the back of t he School Speed Limit assembly.

* If a confirmation beacon or device is used on the back of the School Speed Limit Assembly, it shall be a speed limit sign beacon (see section 4K.04).

ACKNOWLEDGMENTS

This feature is based on two research efforts conducted by the Texas Transportation Institute (TTI) of the Texas A&M University System. The pilot study was conducted as unsponsored research in cooperation with the City of College Station, TX. The expanded study was part of a research effort sponsored by TxDOT.

The author wishes to recognize the contributions of Elisabeth Rose, now with the Texas Engineering Experiment Station, in collecting, analyzing and reporting the data described in this feature.

The contents of this feature reflect the views of the author, who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration (FHWA) or TxDOT.

© 2007 Institute of Transportation Engineers Provided by ProQuest LLC. All Rights Reserved.

Source: Institute of Transportation Engineers. ITE Journal