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124 Other Research Areas This chapter covers additional areas of the research that were conducted to help meet the objective of the project. These additional areas of study were an on-road before-and-after speed study, evaluation of in-service RPM retroreflectivity measurement, and safety analysis on the effectiveness of RPMs. 6.1 Field Speed Study A speed study was conducted on roads where RPMs were to be replaced. The conclusions of that study are presented in the following paragraphs, while the full description of the work can be found in Appendix D. It was assumed by the research team that if only RPMs were replaced on a road segment, then the current RPMs were close to not visible or provided very minimal guidance to motorists. Researchers contacted different regions within TxDOT and conducted the study on four dif- ferent roads: two farm to market (FM) roads and two state highways (SH). The research study was interested in evaluating the effects of RPMs on driving speeds during nighttime conditions and wet pavement conditions, if possible. Rainfall data were observed for both periods of the study; however, SH 21 did not have any rain data recorded for the after-replacement period of the study. The study used an ANOVA to evaluate whether there was a statistically significant difference in average speeds before and after RPMs were replaced. Further analysis of the data included running a linear regression model to find the magnitude of the effects of RPMs. The ANOVA results are summarized in Table 86 and Table 87. The ANOVA showed that there was a significant difference in average speeds during night- time under dry weather conditions. The statistically significant conditions were observed on four-lane highways and on two-lane rural roads where the speed limit was 75 mph. The mag- nitude of the difference in average speed depended on direction of travel and road segment. A separate analysis was made for nighttime speeds under wet conditions. There was no statistically significant difference in the average speeds during rain events. Linear regression models were made for nighttime speeds during clear weather and rainy weather according to the number of lanes. The models showed that when the replacement of RPMs was statistically significant, the magnitude of the effect was around 1 mph. The limitations to this study include the limited number of study sites and the fact that only one curve was evaluated. The sites evaluated, however, were chosen so that the effect of RPMs could be isolated. At the curve site, there were no other delineation devices present. For all sites, there were no other improvements made to the road including repaving and restriping the road. That the magnitude of the effect of RPMs is around 1 mph shows that even though the influence C H A P T E R 6
Other Research Areas 125  is statistically significant, the influence of RPMs on speed may not be practically significant. The inclusion of rain data into the model showed speeds were influenced by the presence of rain. Other factors on the roadway and weather conditions may have a bigger factor on a driverâs speed than RPMs. 6.2 In-Service RPM Retroreflectivity Measurement The conclusions of a study comparing the capabilities of mobile retroreflectometers and portable retroreflectometers in terms of RPM measurement are described in the following paragraphs, while the full description of the work can be found in Appendix E. The mobile retroreflectometer has the ability to record retroreflectivity values for RPMs while traveling at highway speeds while simultaneously evaluating the pavement markings on the roadway. The research found that the mobile retroreflectivity values for the markers did not correlate well with the portable RPM retroreflectometer. There are many potential causes for this, including the different geometries the two devices use and the fact that the mobile retrore- flectometer was not specifically designed for evaluating markers. To account for what appeared to be an oversaturation of the mobile retroreflectometer detector, the researchers lowered the system output power to reduce the signal. This resulted in a larger range of retroreflectivity values, but the data still did not correlate with the portable RPM retroreflectometer. The lower laser strength also resulted in a decrease in marker detection rate from approximately 95% to 60%. When observing the portable retroreflectivity values of individual markers, researchers found a large RI difference when evaluating the two faces of the double-sided markers. Both new and in-service markers showed directional differences. The wet testing results indicated that the RI values for RPMs generally decrease when first exposed to water but later increase as the initial beading and pooling of water begins to flow off the face of the marker. One shortcoming of this testing was that the markers were degraded by Road Segment Mean Speed (before) Mean Speed (after) Difference (after â before) ANOVA p-value FM 1971 Tangent 56.79 57.97 1.18 0.499 FM 2517 Tangent 59.55 58.10 -1.45 0.423 FM 2517 Curve 64.06 65.42 1.36 0.104 SH 47 northbound Tangent 66.57 67.40 0.83 0.115 SH 47 southbound Tangent NA 66.35 NA NA SH 21 eastbound Tangent 71.48 NA NA NA SH 21 westbound Tangent 69.83 NA NA NA NA = no data available * = Statistically significant. Table 87. ANOVA for nighttime speeds during rain events. Road Segment Mean Speed (before) Mean Speed (after) Difference (after â before) ANOVA p-value FM 1971 Tangent 60.05 59.37 -0.68 0.22937 FM 2517 Tangent 61.60 61.44 -0.16 0.77349 FM 2517 Curve 66.45 67.34 0.89 0.00014* SH 47 northbound Tangent 71.09 71.99 0.90 0.00001* SH 47 southbound Tangent 71.86 70.13 -1.73 < 0.00001* SH 21 eastbound Tangent 70.94 69.86 -1.08 < 0.00001* SH 21 westbound Tangent 71.24 69.98 -1.26 < 0.00001* * = Statistically significant. Table 86. ANOVA for nighttime speeds under dry weather conditions.
126 Performance Criteria for Retroreflective Pavement Markers sanding. This may be unrealistic when evaluating the wet conditions. The water could have filled the grooves left by sanding, resulting in a more reflective surface. Future testing should be done with markers that are degraded from normal in-service conditions. The benefits of being able to evaluate the retroreflectivity levels of markers from a mobile platform necessitate further testing and development in this area. Additional modifications to the test setup and equipment are planned for future testing outside of this research project. 6.3 Safety Analyses The objective of the safety analyses was to determine the safety impact of RPMs. To determine the approach for estimating the safety impacts of RPMs, the project team contacted the states with RPMs to get answers to the following questions: ⢠Are RPMs implemented system-wide (say for a particular roadway type) or only for certain sections based on crash history or other parameters? Is the policy different in different parts of the state? ⢠What types of RPMs are used? Can you provide the specifications? ⢠How are they applied (spacing criteria)? ⢠For system-wide implementations, approximately when did the state start using RPMs? If they are implemented only on certain sections, are there records on when and where they were installed? ⢠How often are the RPMs replaced? Is the replacement cycle related to other maintenance? What rules or guidelines are available for conducting inspections (such as at least three in view or no more than two consecutive missing markers, etc.)? Many of the states that responded indicated that RPMs are used system-wide based on roadway class. However, there were some exceptions. For example, North Carolina indi- cated that there is no official policy regarding RPMs, but most divisions use them on roads where the AADT is higher than 4,000 or 6,000. In New Mexico, RPMs are installed based on the request of a district traffic engineer. Some states (e.g., Maryland and Pennsylvania) use recessed pavement markers in locations that may get more snow. In addition, some states (e.g., Mississippi, Tennessee, and Virginia) use closer spacing of RPMs on curves compared to tangent sections. Most of the states implemented RPMs in the 1990s, 1980s, or 1970s. Initially, the intent was to explore the possibility of conducting a before-after evaluation of the safety of RPMs (i.e., the before condition would be without RPMs, and the after condition would be with RPMs). In order to conduct such an evaluation, in addition to knowing when exactly the RPMs were installed, it is also important to know what other changes were made to individual segments during the before-and-after periods so that the effects of the RPMs can be isolated as much as possible. These other changes can include the addition or removal of lanes, changes to the width of lanes, changes to the shoulder and roadside, changes to roadway alignment, changes to pave- ment markings and other TCDs, and repaving of roads. Very few states have reliable records of these changes from the 1990s, 1980s, and 1970s. Thus, conducting a valid before-after safety evaluation to determine the safety effect of RPM installation would be extremely difficult. For this reason, the project proposed the following options: ⢠Option 1: Before-After Evaluation to Determine Safety Effect of Improved RPMs. If a state has records on when it replaces RPMs, then it would be possible to provide useful information regarding the safety aspects of improved RPMs by comparing the frequency and severity of target crashes before the RPMs are replaced to the frequency and severity of target crashes after the RPMs are replaced.
Other Research Areas 127  ⢠Option 2: Before-After Evaluation to Determine Safety Impact of Removed RPMs. As men- tioned earlier, at least seven states had RPMs in the past but decided to remove them, pri- marily because of concerns that RPMs get dislodged from the pavement and create a safety hazard. However, at this time, it is not clear if the removal of RPMs affected the frequency and severity of crashes. A before-after evaluation could provide insight into the safety effect associated with the removal of the RPMs. ⢠Option 3: Cross-Sectional Comparison to Determine the Safety Effect of RPMs. In some states (e.g., North Carolina and New Mexico), there are no statewide policies on RPMs. Thus, it may be possible to compare the crash rates of roads with RPMs in one district with similar roads without RPMs in another district. There is general consensus in the safety community that cross-sectional comparisons do not always provide reliable CMFs. For this reason, for the cross-sectional comparison to provide reliable and meaningful results, the roads with and with- out RPMs should have similar AADT values, roadway characteristics, and weather conditions. These options were documented as part of the interim report and presented to the NCHRP panel during the project interim meeting. The NCHRP panel was not interested in Option 1 since there was not an accepted and efficient way to document and measure the retroreflectivity of RPMs, and thus, any safety evaluation of improved RPMs would not provide useful results. The panel was not interested in Option 2 because it would have provided the safety impact of removing RPMs, which was not the focus of this project. The panel did indicate interest in pur- suing Option 3 and also instructed the project team to consider before-after studies as well. Since NCHRP Report 518 (Bahar et al. 2004) raised questions about the safety effects of RPMs on rural two-lane roads, the panel suggested that the safety evaluation should focus on rural two-lane roads in states where curvature data were available. Following the interim meeting, the project team investigated the possibility of implementing Option 3 with either cross-sectional or before-after studies. In order to conduct cross-sectional comparisons to determine the safety effect of RPMs, it was important to find roads with RPMs and roads without RPMs but otherwise similar in terms of AADT, roadway characteristics, and weather conditions. Since RPMs are typically used system-wide based on roadway class, it was difficult to find similar roads with and without RPMs. The project team did explore the possibility of using data from North Carolina where the policy on RPMs is left to the individual divisions. However, as mentioned earlier, divisions used RPMs when AADT was higher than 4,000 or 6,000. Therefore, it was not possible to find roads with and without RPMs with similar AADT values. The project team also explored the possibility of implementing Option 3 with a before-after study. For a valid and reliable before-after study, it is necessary to find locations where new RPMs are installed (if RPMs were replaced, then it becomes Option 1, which the panel was not interested in). A state would need to have a reliable record of RPM installations for this option to be successful. The other challenge was that RPMs were typically installed or replaced during resurfacing or in combination with other roadway improvements, like the installation of pave- ment markings such as lane markings. Unfortunately, the project team was unable to find a state that had newly installed RPMs in the recent past without implementing other changes (such as resurfacing and pavement markings) at the same time. Because of these issues, a crash-based safety evaluation could not be conducted as part of this project.
